Caelum See feature article

CAI See calcium-aluminium-rich inclusion

Calar Alto Observatory German-Spanish optical observatory situated in the Sierra de los Filabres in Andalucia, southern Spain, at an elevation of 2170 m (7120 ft). Dating from 1973, it is operated by the max-planck-institut fur astronomie in Heidelberg; the Madrid Observatory also has a 1.5-m (60-in.) telescope there. Principal instruments of the German-Spanish Astronomical Centre are the 3.5-m (138-in.) and 2.2-m (87-in.) reflectors, dating from 1984 and 1979 respectively. There is also a 1.2-m (48-in.) telescope and a small Schmidt that was formerly at hamburg observatory.

calcium-aluminium-rich inclusion (CAI) In a meteorite, an irregular-shaped refractory inclusion of oxide and silicate minerals such as spinel, hibonite and melilite. CAIs can be up to c.1 cm (about half an inch) in size and frequently exhibit complex mineralogical zoning, both in their rims and in their cores. They contain the daughter products from now-extinct short-lived radionuclides, indicating rapid solidification after production of the nuclides. CAIs are the oldest solid objects from the pre-solar nebula.

caldera Large, roughly circular volcanic depression formed by collapse over an evacuated magma chamber. Small calderas (less than 5 km/3 mi in diameter) are common at the crests of terrestrial basaltic and andesitic volcanoes. Calderas as large as 75 km (47 mi) across have formed on Earth during volcanic ash flow eruptions. The summit caldera of the Martian volcano olympus mons, the largest volcano in the Solar System, is about 80 km (50 mi) across.

caldera A Magellan radar image of Sacajawea Patera, in the western part of Ishtar Terra on Venus. With a depth of 12 km (0.61.2 mi), this caldera has a maximum diameter of 420 km (252 mi).

Caldwell Catalogue Listing of 109 deep-sky objects intended to complement those in the messier catalogue. Charles Messier omitted from his list several prominent objects, such as the Double Cluster in Perseus, and southern-hemisphere objects of which he was unaware. In 1993 English amateur astronomer Patrick moore, recognizing these omissions, gathered a further 109 deep-sky objects in a catalogue that takes its title from one of his middle names. The list enjoys some popularity among amateur observers who have observed all the Messier objects and are seeking further targets. See table

calendar System for measuring longer intervals of time by dividing it into periods of days, weeks, months and years. The length of the day is based on the average rotation period of the Earth, while a year is based on the orbital period of the Earth around the Sun.

The month was originally the period between successive full moons (29.5 days), giving a lunar year of 354 days, but in modern calendars it is now roughly one twelfth of a year. Because there are not a whole number of full moons in a year, the two cannot be simply reconciled into a common calendar, and while the modern civil calendar is based on the year, the dates of religious festivals (such as Easter) are still set by reference to the lunar month. The week is not based on any astronomical phenomena but instead derives from the Jewish and Christian tradition that every seventh day should be a day of rest.

The ancient Egyptians were the first to use a calendar based on a solar year, the Babylonians having used a lunar year of 12 months, adding extra months when it became necessary to keep the calendar in step with the seasons.

Our present calendar is based on that of the Romans, which originally had only ten months. Julius Caesar, on the advice of the Greek astronomer Sosigenes, introduced the Julian Calendar in 45 bc. This was a solar calendar based on a year of 365.25 days, but fixed at 365 days, with a leap year of 366 days every fourth year to compensate. The year commenced on March 25 and the Julian Calendar, which was completed by Caesar Augustus in 44 bc, also established the present-day names, lengths and order of the months.

The Julian year of 365.25 days is slightly different from the solar year of 365.2419 days (being 11m14s longer) but accumulates an error of almost eight days every 1000 years. By the 16th century this error had become noticeable, the vernal equinox, for example, occurring 10 days early in 1582. To correct this error, Pope Gregory XIII removed 10 days from the calendar so in 1582, October 15 followed October 4, and instigated the use of a new system known as the Gregorian calendar in which only century years divisible by 400 (for

CAELUM (gen. caeli, abbr. cae) Faint and obscure southern constellation, sandwiched between Columba and the southern end of Eridanus, introduced by Nicolas Lacaille in the 1750s. It represents a sculptor's chisel. Its brightest star, a Cae, is only mag. 4.4. example, 1600, 2000) should be leap years. The start of the year was also changed to January 1.

By the time this 'New Style' calendar, as it was known, was adopted in Great Britain in 1752 the error had increased to 11 days, so that September 14 followed September 2. This is why the British financial year ends on April 5 - the old New Year's Day (March 25) plus the 11 days lost in 1752.

Today the Gregorian calendar, which repeats every 400 years since this equates to an exact number of weeks, is used throughout the Western world and parts of Asia. It is also known as the Christian calendar since it uses the birth of Christ as a starting point, subsequent dates being designated anno domini (in the year of our Lord) and preceding dates being BC (before Christ). The accumulated error between the Gregorian year and the true solar year now amounts to just three days in 10,000 years.

The two other major calendars in use today are the Jewish and Muslim calendars, both of which are based on a lunar year. The Jewish is derived from the ancient Hebrew calendar and is based on lunar months of 29 and 30 days alternately, with an intercalary month inserted every three years. The length of the Muslim year is also 12 months of alternate lengths 30 and 29 days, except for the 12th month which can have either 29 or 30 days.

Caliban One of the several small outer satellites of URANUS. It was discovered in 1997 by Brett Gladman and others. Caliban is about 60 km (40 mi) in size. It takes 579 days to circuit the planet at an average distance of 7.23 million km (4.49 million mi). It has a RETROGRADE orbit (inclination near 141) with eccentricity 0.159. In origin Caliban is thought to be a captured body, previously a CENTAUR. Other recently discovered outer Uranian satellites with a similar suspected origin to Caliban are PROSPERO, SETEBOS, STEPHANO and SYCORAX; an object discovered in VOYAGER 2 images in 1999 and provisionally designated as S/1986 U10 awaits confirmation.

California Extremely Large Telescope (CELT) Proposed 30-m (100-ft) optical/infrared telescope, sponsored jointly by the University of California and the

CALIFORNIA INSTITUTE OF TECHNOLOGY. The instrument would build on the segmented-mirror technology used for the 10-m (33-ft) telescopes of the W.M. KECK OBSERVATORY, and would have a mirror composed of 1080 hexagonal segments. It would rely on ADAPTIVE OPTICS for high resolution, bringing the first generation of galaxies within its grasp when fully operational in 2010-15.

California Institute of Technology (Caltech) Small, independent university dedicated to exceptional instruction and research in engineering and science, located in Pasadena, California, USA. Its off-campus facilities include the JET PROPULSION LABORATORY, PALOMAR OBSERVATORY, OWENS VALLEY RADIO OBSERVATORY and the W.M. KECK OBSERVATORY.

California Nebula (NGC 1499) emission nebula in the eastern part of the constellation perseus (RA 04h 00m.7 dec. +3637'). The nebula is illuminated by the hot, young O-class star Persei. Although visually faint, the nebula shows up well in long-exposure photographs taken on red-sensitive films. It takes its name from its resemblance, in outline, to the American state. The California Nebula covers an area of 160'X 40', elongated roughly north-south.

Caltech Submillimeter Observatory (CSO) Enclosed 10.4-m (34-ft) radio dish with a hexagonally segmented mirror, at MAUNA KEA OBSERVATORY, Hawaii. The telescope is operated by the CALIFORNIA INSTITUTE OF TECHNOLOGY under contract from the National Science Foundation, and has been in regular use since 1988. The instrument operates at wavelengths between 350 jm and 1.3 mm, and can be linked with the nearby JAMES CLERK MAXWELL TELESCOPE for short-baseline interferometry.

Calypso Small satellite of SATURN, discovered in 1980 by Dan Pascu (1938- ) and others in images from the VOYAGER missions. It is irregular in shape, measuring about 30 X 16 X 16 km (19 X 10 X 10 mi). With a distance from the centre of the planet of 294,700 km (183,100 mi), it is co-orbital with TETHYS and TELESTO. Calypso and Telesto have circular, near-equatorial orbits, near the L5 and L4 LAGRANGIAN POINTS, respectively, of Tethys' orbit around Saturn, with a period of about 1.89 days.

Callisto Asgard, a multi-ring impact basin on Callisto, imaged by the Galileo spacecraft. Callisto's icy surface is heavily scarred by impact cratering.

Callisto Asgard, a multiring impact basin on Callisto, imaged by the Galileo spacecraft. Callistos icy surface is heavily scarred by impact cratering.

Caloris Planitia This vast (1300 km/800 mi diameter) multi-ring impact basin on Mercury was imaged during the Mariner 10 mission in 1974. Shockwaves from the impact, early in Mercury's history, have produced unusual geological features on the opposite hemisphere of the planet.

Caloris Planitia This vast (1300 km/800 mi diameter) multi-ring impact basin on Mercury was imaged during the Mariner 10 mission in 1974. Shockwaves from the impact, early in Mercurys history, have produced unusual geological features on the opposite hemisphere of the planet.

Callisto Outermost and darkest of the GALILEAN SATELLITES of JUPITER. It has an albedo of only 0.2. Callisto is heavily cratered and is the only member of the group to show no clear surface signs of current or past geological activity. Callisto's density of 1.9 g/cm3 suggests that it is composed of rock and ice in a 40:60 mixture. Gravity measurements made by the Galileo orbiter during close flybys of Callisto suggest that its interior is only weakly differentiated (see DIFFERENTIATION), and that the rock is not all concentrated into a core, in contrast to Callisto's neighbour GANYMEDE. Perplexingly, measurements made by the Galileo orbiter hinted at a magnetic field, apparently generated at a shallow depth within Callisto as a result of its orbital passage through Jupiter's MAGNETOSPHERE. One way to explain this would be if Callisto has a salty ocean no more than 100 km (60 mi) below its icy surface, but this is hard to reconcile with the great age of the surface implied by the density of CRATERS.

Although Callisto's craters are clearly produced by meteoroidal or cometary impact, they are rather different from those found on the Moon. For example, Callisto has very few craters less than about 60 km (40 mi) in diameter, showing that the satellite was bombarded by a different population of impactors from that responsible for lunar craters. The largest craters have a subdued shape, as if Callisto's LITHOSPHERE has not been strong enough to support their topography. There are also a few enormous impact basins marked by concentric rings of fractures; the largest of these, named VALHALLA, is about 4000 km (2500 mi) across. About a dozen linear chains of craters have been identified on Callisto, each of which was probably produced by the serial impact of fragments of a comet broken up by tidal forces during a close passage of Jupiter (see SHOEMAKER-LEVY 9). See data at JUPITER

Caloris Planitia ('Basin of Heat') Largest single feature on MERCURY imaged by MARINER 10. It is situated near one of the planet's hot poles and centred on 30.5N 189.8W. The Caloris Planitia is an enormous multi-ring impact structure 1300 km (800 mi) in diameter - a quarter that of the planet. Imaged half-lit from the departing spacecraft, it is defined by a ring of discontinuous mountains, the Montes Caloris, roughly 2 km (1.2 mi) high. The BASIN floor consists of smooth plains with quasi-concentric and other ridges, transected by younger crack-like GRABEN. The ejecta blanket extends to a distance of approximately 700 km (430 mi) beyond the Montes Caloris; it comprises tracts of uneven hummocky plains and lineated terrain. The whole structure is undoubtedly the modified scar left by the impact of an asteroid-sized body, the floor being the end result of refilling of the crater by the crusting-over, semi-molten ASTHENOSPHERE. On the other side of Mercury, antipodal to Caloris, is a region of 'weird terrain', an extraordinary place of hills and valleys that break into other landforms; it probably formed as a result of shockwaves from the impact that created Caloris.

CAMELOPARDALIS (gen. camelopardalis, abbr. cam) Cambridge Low Frequency Synthesis Telescope (CLFST) East-west aperture synthesis radio telescope consisting of 60 trackable Yagi antennae, operated by the mullard radio astronomy observatory and situated close to the ryle telescope. The individual antennae are located on a 4.6-km (2.9-mi) baseline, and have a working frequency of 151 MHz

Cambridge Optical Aperture Synthesis Telescope (COAST) Instrument built by the mullard radio astronomy observatory to extend the interferometric image-reconstruction techniques used in radio astronomy to optical and near-infrared wavelengths. It consists of an array of five 400-mm (16-in.) telescopes, of which up to four are in use at any one time. They can be used to synthesize a virtual telescope mirror 100 m (330 ft) in diameter, yielding images showing detail as fine as 0".001 (1 milliarc-second). COAST produced its first images in 1995.

Cambridge Radio Observatory See mullard radio astronomy observatory

Camelopardalis See feature article

Camilla main-belt asteroid; number 107. It is notable because it is accompanied by a small moon.

Campbell, (William) Wallace (1862-1938) American astronomer and mathematician who measured a large number of radial velocities. As director of Lick Observatory (1900-1930), he founded the observatory's southern-hemisphere station in Chile and designed important accessory instruments for Lick's telescopes, including the Mills spectrographs. Campbell led an eclipse expedition in 1922, which confirmed Einstein's general theory of relativity by showing that the Sun's mass was sufficient to deflect light waves from other stars. With Heber curtis, he undertook a huge photographic survey of stellar spectra to determine the radial velocities of stars. This project had two important results: it allowed Campbell to map the local Milky Way and our Sun's motion relative to other nearby stars, and it led to the discovery of over a thousand spectroscopic binaries.

Canada-France-Hawaii telescope Optical/infrared telescope of 3.6-m (142-in.) aperture located at mauna kea observatory and operated jointly by the National Research Council of Canada, the Centre National de la Recherche Scientifique of France and the University of Hawaii. The instrument began operation in 1979, and its suite of instruments includes high-resolution wide-field imagers.

canals Martian Elusive network of dark linear markings on the surface of Mars, reported by some observers from around 1870 until well into the 20th century. In 1877 Giovanni schiaparelli marked a number of very narrow features on his map of Mars which he referred to as canali, which in Italian means 'channels' or 'canals'. But when his findings were translated into English, it was the latter sense, with its implication of artificial construction, that found its way into reports. Controversy continued in the 1880s, some astronomers claiming they could see the 'canals', while others could not. In the 1890s, their existence was championed by Percival lowell, who founded Lowell Observatory in Flagstaff, Arizona, largely to study them. He became convinced that they were waterways constructed by intelligent beings to irrigate a desiccating planet. Only when close-up images were returned by the Mariner craft and later Mars probes was the existence of canals disproved with certainty.

Martian 'canals' make an intriguing episode in the annals of observational astronomy. No canal was ever convincingly photographed, yet highly detailed maps were prepared from sketches made at the eyepiece that showed canals in prodigious numbers. Their explanation is part psychological, part physiological. There is no denying the integrity of some who reported having observed canals,

Fairly large but faint northern constellation, representing a giraffe; it extends from the northern borders of Perseus and Auriga towards the north celestial pole. Camelopardalis was introduced by Petrus Plancius in 1613, supposedly to commemorate the Biblical animal that carried Rebecca to Isaac. Its brightest star, P Cam, is a wide double, mags. 4.0 and 8.6. NGC 1502 is a small open cluster from which a chain of stars called Kemble's Cascade runs for 2J towards neighbouring Cassiopeia.

CANCER (gen. cancri, abbr. cnc) Faintest constellation of the zodiac, lying between Gemini and Leo. Mythologically, it represents the crab that was crushed underfoot by Hercules during his fight with the Hydra. The brightest star is P Cnc, a K4 giant of mag. 3.52, distance 290 l.y. Cnc is a long-period binary divisible through small telescopes, mags. 5.0 and 6.2. Even easier to divide is i Cnc, mags. 4.0 and 6.6. The constellation's most celebrated feature is M44, also known as praesepe or the Manger, a large open star cluster. North and south of it lie y and 8 Cnc, known as the aselli ('asses'). In the south of the constellation, next to a Cnc, binoculars show M67, a smaller and fainter open cluster 2500 l.y. away.

Cancer See feature article

CANES VENATICI (gen. canum venaticorum, abbr. cvn) Northern constellation positioned beneath the tail of Ursa Major, representing a pair of hunting dogs, Asterion and Chara, held on a leash by neighbouring Bootes. Canes Venatici was introduced by Johannes Hevelius in 1687. a CVn, known as cor caroli, is an easy double, mags. 2.9 and 5.6. Y CVn is a red supergiant semiregular variable known as La Superba, with range 5.0-6.5 and period roughly 160 days. The constellation's most famous feature is the spiral galaxy M51, known as the whirlpool galaxy. Other spirals visible with small instruments are M63 and M94. One of the best globular clusters in northern skies is M3, just on the naked-eye limit at 6th magnitude.

CANIS MAJOR (gen. canis majoris, abbr. cma) Prominent constellation just south of the celestial equator, containing the brightest star in the sky, sirius. Canis Major represents the larger of the two dogs of Orion and is one of the constellations recognized since the time of the ancient Greeks. (adhara) and j CMa are difficult double stars with much fainter companions. UW CMa is an eclipsing binary, range 4.8-5.3, period 4.4 days. M41 is a naked-eye open cluster 4 south of Sirius, similar in apparent size to the full moon and containing some 80 stars of 7th magnitude and fainter. NGC 2362 is a small open cluster surrounding the mag. 4.4 blue supergiant t CMa, its brightest member. See also mirzam; wezen

cannibalism A Hubble Space Telescope image of the elliptical galaxy NGC 1316 in Fornax. The dark dust clouds and bluish star clusters are probably remnants of a collision 100 million years ago, during which NGC 1316 consumed a smaller galaxy.

Candy, Michael Philip (1928-94) English-born astronomer who worked at Royal Greenwich Observatory (1947-69) and Perth Observatory (1969-93). From Perth, he directed an observational programme that contributed greatly to the study of Halley's Comet at its 1986/87 apparition. Candy was an expert on the orbits of comets and asteroids, and has one of each type of object named after him.

Canes Venatici See feature article, page 69 Canis Major See feature article, page 69 Canis Minor See feature article

cannibalism Merging of a GALAXY into a much larger and more massive one, so that its content is incorporated without a major change in the structure of the larger galaxy. This process is thought to explain how CD GALAXIES at the centres of clusters have become so bright and massive. Cannibalism of dwarf satellites also seems to have played a role in the growth of the halos of SPIRAL GALAXIES, exemplified by the distinct streams of stars in the Milky Way and the ANDROMEDA GALAXY, which may be the assimilated remnants of former companions.

Cannon, Annie Jump (1863-1941) American astronomer who, working at the Harvard College Obser vatory under the direction of Edward C. PICKERING, refined the system for classifying stellar spectra. In 1896, after teaching physics at Wellesley College, Cannon joined Harvard's staff of 'Pickering's women', a group of computing assistants hired mainly to work on the henry draper catalogue of stellar spectra.

Taking the alphabetical spectral classification begun by Williamina FLEMING and Antonia C. MAURY, Cannon dropped several categories and rearranged the sequence to give O, B, A, F, G, K, M, from the hottest stars to the coolest. White or blue stars were classified as type O, B or A, yellow stars as F or G, orange stars were designated K, and red stars as M. This scheme, which is the basis for the present-day HARVARD SYSTEM of spectral classification, was used by Cannon in the first ever catalogue of stellar spectra, for the 1122 brightest stars (1901). Later, she added types R, N and S, plus ten subcategories based upon finer spectral features.

In the course of her work, Cannon visually examined and classified hundreds of thousands of spectra: the Henry Draper Catalogue, filling nine volumes of the Harvard Annals, ultimately contained almost 400,000 stars sorted by spectral class. Cannon also discovered five novae, 300 new variable stars and published extensive catalogues of these objects in 1903 and 1907. She was the first woman to be elected an officer of the American Astronomical Society, but because of a reluctance of the scientific community to accept women in astronomy, she did not receive a regular appointment at Harvard until 1938, just two years before she retired.

Canopus The star a Carinae, the second-brightest star in the entire sky, visual mag. 0.62, distance 313 l.y. It is a white supergiant of spectral type F0 Ib, more than 10,000 times as luminous as the Sun. The Hipparcos satellite detected variations of about 0.1 mag., but the period (if any) and cause of the variation are not known. Canopus is named after the helmsman of the Greek King Menelaus.


Capella The star a Aurigae, at visual mag. 0.08 the sixth-brightest star in the sky, distance 42 l.y. It is a spectroscopic binary, consisting of two yellow giants with an orbital period of 104 days. Various spectral types have been given for this pair, but they are likely to be near G6 III and G2 III. The star's name comes from the Latin meaning 'she-goat'.


Cape Photographic Durchmusterung (CPD) Catalogue compiled from the first large photographic survey of the southern sky, made at the Cape Observatory between 1885 and 1900 under the direction of David GILL. Data for the CPD were taken from Gill's photographic plates by Jacobus KAPTEYN. It contains 455,000 stars to 10th magnitude from dec. 18 to 90 and, achieved before the cordoba durchmusterung, complements the bonner durchmusterung. It was later revised by Robert INNES.

Capricornus See feature article

captured rotation See SYNCHRONOUS ROTATION

Carafe Galaxy (Cannon's Carafe Galaxy) SEYFERT GALAXY in the southern constellation of Caelum (RA 04h 28m.0 dec. 4724'), part of a group with NGC 1595 and NGC 1598. Long-exposure images show a curved jet of emerging material, possibly the result of gravitational interaction with NGC 1595.

carbon Sixth element, chemical symbol C; it is fourth in order of COSMIC ABUNDANCE. Its properties include:

CAPRICORNUS (gen. capricorni, abbr. cap) atomic number 6; atomic mass of the naturally occurring element 12.01115; melting point c.3820 K (it sublimes at 3640 K); boiling point 5100 K; valence 2, 3 or 4. Carbon has seven isotopes, with atomic masses from 10 to 16, two of which are stable (carbon-12 and carbon-13). Car-bon-12 is used to define the atomic mass unit, and so its atomic mass is exactly 12. The naturally occurring element contains carbon-12 (98.89%) and carbon-13 (1.11%) plus variable but small amounts of carbon-14. In its free state, carbon exists as amorphous carbon, graphite and diamond.

Carbon is unique in the vast number and variety of compounds that it can form. The study of its reactions forms the entire discipline of organic chemistry. Carbon's property of forming hexagonal rings and long-chain molecules and of linking with hydrogen, nitrogen and oxygen makes it the basis of life. In the form of carbon dioxide (CO2) and methane (CH4) it produces the two most significant greenhouse gases.

Carbon-14 is radioactive, with a half-life of 5730 years, and forms the basis for carbon-dating archaeological remains. It is continually produced in the Earth's atmosphere from nitrogen-14 by cosmic rays. Once produced, it is incorporated into living material. After the death of the organism, the decay of the carbon-14 slowly reduces its abundance relative to carbon-12, and so allows determination of the age.

Carbon plays an important role in the carbon-nitrogen-oxygen cycle, which is the major helium-producing nuclear reaction in massive stars. It is unusually abundant in some types of peculiar star (see astrochem-istry), and may comprise the bulk of the material forming some white dwarfs.

carbonaceous chondrite Chondritic meteorite with atomic magnesium to silicon ratio greater than 1.02. Carbonaceous chondrites are subdivided, on chemical or tex-tural grounds, into seven groups, each (apart from the CH group) named after its type specimen. The CI (for Ivuna) group has six members, including tagish lake. These meteorites have a composition very close to that of the Sun, without the volatiles. They are rich in water (up to c.20% by weight) and carbon (up to c.7% by weight); carbon is present mainly as organic compounds, including amino acids. Members of the CM (for Mighei) group are similar to CI chondrites, but they contain chondrules and slightly less carbon (up to c.3% by weight). This group includes murchison. micrometeorites show many similarities to CM chondrites. Meteorites in the CV (for Vigarano) group have large chondrules and are rich in refractory elements, such as aluminium and iridium. CV chondrites contain centimetre-sized calcium-aluminium-rich inclusions (CAIs), but little carbon or water. allende is a member of this group. The CO (for Ornans) group has smaller chondrules and CAIs. It is poorer in refractories than CV chondrites. CO chondrites also contain little water or carbon. The CR (for Renazzo) group of chondrites is characterized by abundant, well-defined chondrules and high metal content. The CK (for Karoonda) group is mostly thermally metamorphosed and re-crystallized chondrites. Members of the CH group (H for high iron content) have very small chondrules and high metal contents.

Smallest constellation of the zodiac, lying in the southern celestial hemisphere between Sagittarius and Aquarius. In Greek mythology, it was said to represent the god Pan after he jumped into a river to escape from the monster Typhon. The constellation's brightest star is 8 Cap, known as Deneb Algedi ('kid's tail'). It is a beta lyrae star, an eclipsing binary with a range of 0.2 mag. and a period of 1.02 days. a Cap (known as Algedi or Giedi, from the Arabic meaning 'kid') is of a wide unrelated pair of yellow stars, mags. 3.6 and 4.3, spectral types G9 and G3. p Cap is a wide double, mags. 3.1 and 6.1, colours golden yellow and blue-white.

The presence of carbon as one of the reactants is essential, but it behaves like a catalyst. Carbon-12 is used in the first reaction, but after a series of reactions, during which four hydrogen nuclei are absorbed and a helium nucleus is formed, carbon-12 is reproduced. Isotopes of carbon, oxygen and nitrogen occur as transient intermediate products during the reactions.

carbon star (C star) Classically, a cool giant star, between about 5800 and 2000 K, that exhibits strong spectral absorptions of C2, CN and CH and that has an atmosphere containing more carbon than oxygen. The original classes, rstars (comparable in temperature with gstars and kstars) and nstars (comparable with m stars), are combined into class C. N stars are on the asymptotic giant branch (AGB) of the hertzsprung-russell diagram (HR diagram). As main-sequence stars evolve on to the upper AGB, those of intermediate mass can dredge up carbon made by the triple-a process into their atmospheres, turning first into carbon-rich sstars and then into genuine carbon stars. When carbon exceeds oxygen, the two combine to make molecules (particularly CO), leaving no oxygen to make metallic oxides. The rest of the carbon then combines with itself and other atoms.

Most N-type carbon stars are irregular, semi-regular, or Mira variables. Carbon dust forming in strong stellar winds can surround, or even bury, the stars in molecule-rich shrouds, the stars being visible only in the infrared. Such carbon stars are also rich in elements created by the s process. Carbon stars are a major source of galactic dust and a significant source of interstellar carbon.

The warmer R stars appear to be core-helium-burning

carbon-nitrogen-oxygen cycle (CNO cycle, CN cycle, Bethe-Weizsacker cycle) Cycle of nuclear reactions that accounts for the energy production inside main-sequence stars of mass greater than the Sun. The cycle was first described, independently, by Hans bethe and Carl von weizsacker in 1938.

The reactions involve the fusion of four hydrogen nuclei (protons) into one helium nucleus at temperatures in excess of 4 million K. The cycle is temperature dependent, and at temperatures greater than 20 million K it becomes the dominant energy-producing mechanism in stellar cores. The mass of one helium nucleus (4.0027 atomic mass units) is less than the total mass of four pro-

carbonaceous chondrite A thin section of the Cold Bokkeveld meteorite, which fell in Cape Province, South Africa, in 1838. Taken with a petrological microscope, this photograph shows a near-circular chondrule about 1 mm in diameter.

CARINA (gen. carinae, abbr. car) Prominent southern constellation, part of the old Greek figure of argo navis, the ship of the Argonauts; Carina represents the ship's keel. Its leading star is canopus, the second-brightest in the entire sky. Carina lies on the edge of a rich region of the Milky Way. eta carinae is a violently variable star within the large and impressive eta carinae nebula, NGC 3372. v Car is an easy double for small telescopes, mags. 3.0 and 6.0. The variables R and S Car are mira stars, reaching respectively 4th and 5th magnitude at maximum and with periods of 309 and 150 days, while l ('ell') Carinae is a bright Cepheid, range 3.31.2, period 35.5 days. IC 2602 is a prominent open cluster known as the Southern Pleiades, 480 l.y. away; its brightest member is 0 Car, mag. 2.7. Other notable open clusters are NGC 2516, 3114 and 3532.

Carina See feature article

Carina arm Nearby spiral arm of the Milky Way galaxy. It extends in the sky from Carina to Centaurus. It may be an extension of the sagittarius arm, rather than a complete individual spiral arm. It may best be traced from its twenty-one centimetre radio emissions from hydrogen and by the presence of young blue stars.

Carme Outer moon of jupiter, about 40 km (25 mi) in size. Carme was discovered in 1938 by Seth Nicholson. It takes 692 days to orbit Jupiter, at an average distance of 22.6 million km (14.0 million mi) in an orbit of eccentricity 0.253. It has a retrograde path (inclination 165), in common with other members of its group. See also ananke

Carrington rotation Mean period of 25.38 days introduced in the 19th century by the English astronomer Richard C. carrington from observations of sunspots as the mean length of time taken for the Sun to rotate on its axis at the equator with respect to the fixed stars. Since the Earth revolves about the Sun in the same direction that the Sun rotates, this sidereal period is lengthened to a synodic period of 27.28 days when observed from Earth. Beginning with that which commenced on 1853 November 9, the Sun's synodic rotations are numbered sequentially; Carrington defined the zero of solar longitude as the central solar meridian on this day. For example, Carring-ton Rotation 1985 commenced on 2002 January 7.

Carte du Ciel Programme initiated in 1887 by the brothers henry and others to map the whole sky in 22,000 photographs taken from 18 observatories, using identical 'astrograph' telescopes. One of the first large international astronomical projects, the Carte du Ciel took about 60 years to complete. The project also included the generation of the Astrographic Catalogue, listing several million stars down to 11th magnitude, which has since proved invaluable for the derivation of accurate proper motions. See also hipparcos catalogue

Carter Observatory National observatory of New Zealand, established in Wellington's Botanic Garden in 1941. It now functions as a centre for public astronomy, for which 230-mm (9-in.) and 150-mm (6-in.) refractors are used, together with a planetarium. The observatory also serves as a national repository for New Zealand's astronomical heritage.

Cartwheel Galaxy (MCG-06-02-022a) starburst galaxy in the southern constellation Sculptor (RA 00h 37m.4 dec. -3344'). It lies about 500 million l.y. away. The galaxy's structure has been disrupted by the recent passage through it of a second, dwarf galaxy, which triggered an episode of star formation in the ring-shaped rim. The rim has a diameter of 150,000 l.y., and spokes of material extend towards it from the galaxy's centre.

cascade image tube Type of image intensifier used to amplify the brightness of faint optical images through a multistage process. In a simple image tube, the incident light beam falls on to a photocathode, liberating a stream of electrons. These electrons are accelerated by an electric field of around 40 kV and are focused by either a magnetic or electrostatic field on to a phosphor screen to image. A cascade image tube incorporates a number of photocathode stages; the output image from one section serves as the input for the next, resulting in increased amplification of the signal, and hence greater image brightness. The final detector may be a television camera or a ccd (charge-coupled device).

Carnegie Observatories Observatories of the Carnegie Institution of Washington, founded in 1904. The main offices in Pasadena, California, house the observatories' scientific and technical staff. Until 1980 the Carnegie Observatories operated the mount wilson and palomar Observatories under the name 'Hale Observatories' in partnership with the california institute of technology. Today, their main observing site is las campanas observatory.

Cartwheel Galaxy An Anglo-Australian Telescope photograph of this irregular galaxy, whose ring and spoke configuration is a result of a comparatively recent encounter with another galaxy.

Carrington, Richard Christopher (1826-75) English brewer and amateur astronomer who built his own observatory at Redhill, Surrey. By making daily measurements of sunspot positions during the years 1853-61, he discovered, independently of Gustav sporer, that the Sun's equatorial regions rotate faster than its more extreme latitudes. From a sunspot's heliographic latitude, he was able to predict how quickly the spot would move across the solar disk. In 1859 Carrington became the first person to observe a solar flare, one sufficiently energetic to be visible to the naked eye, and noted that it was followed by auroral displays. He also compiled an important catalogue (1857) of almost 4000 circumpolar stars.

Cassegrain telescope reflecting telescope with a concave paraboloidal primary and a convex hyper-boloidal secondary. Light is gathered by the primary and reflected to the secondary, which is placed in the light path on the optical axis. The convex secondary increases the focal length by enough for the light to pass through a hole in the primary before coming to a focus behind it. This 'folding' of the light path results in an instrument that is much more compact than a refractor or Newtonian of the same focal length. The Cassegrain has no chromatic or spherical aberration, but does suffer from slight astigmatism, moderate coma and strong field curvature.

Cassegrain telescope A cutaway showing the light path in a Cassegrain telescope. Light collected by the primary mirror is reflected from a hyperboloidal secondary to the eyepiece through a hole in the primarys centre.

The design is credited to the French priest Laurent Cassegrain (1629-93), but did not become popular until the late 19th century because of the difficulty of accurately figuring the convex hyperboloidal secondary mirror. However, this eventually became the commonest design for large professional telescopes, and popular for compact amateur instruments, leading to the development of many variations, such as the dall-kirkham telescope, schmidtcassegrain telescope and ritcheychretien telescope.

Cassini NASA spacecraft launched to saturn in 1997 November. In 2004 November, it will become the first spacecraft to orbit Saturn and will also deploy the European Space Agency craft huygens, which is scheduled to land on the surface of the Saturnian moon titan. Cassini flew towards Saturn via two gravity assist flybys of Venus and one of the Earth, and in 20002001 it conducted, with the orbiting galileo, the first dual-spacecraft Jupiter science mission.

Cassini En route to Saturn, the Cassini spacecraft passed through the Jupiter system. This image of Jupiters Great Red Spot and its environs, together with Io and its shadow on the cloud-tops, was obtained on 2000 December 12 from a distance of 19.5 million km (12.1 million mi).

Cassini family French family of Italian origin that produced four astronomers and cartographers, all associated with paris observatory, which was set up by Giovanni Domenico cassini (known to historians as Cassini I) at the invitation of Louis XIV of France. The elder Cassini's son, Jacques (or Giacomo) Cassini (16771756, Cassini II), assumed directorship of Paris Observatory in 1700. Jacques' major contributions were in geodesy and cartography: he established the definitive arc of the Paris Meridian, and conceived a method for finding longitudes by observing lunar occultations of stars and planets.

In 1771 Jacques' son, Cesar-Francois (or Cesare Francesco) Cassini de Thury (171484, Cassini III), became the observatory's director. Cesar-Francois was responsible for producing the first truly reliable map of France, which took half a century to complete. Despite this work, the fortunes of Paris Observatory declined during his directorship, due to lack of government support and competition from better-funded private observatories. After his death in 1784, his son Jacques Dominique Cassini (17481845, Cassini IV) took over the observatory and restored its former prestige.

Cassini, Giovanni Domenico (16251712) Italian astronomer and geodesist (also known as Jean Dominique after his move to France, and to historians as Cassini I) who founded paris observatory and helped to establish the scale of the Solar System. As a result of his important studies of the Sun and Venus, Mars and Jupiter carried out at Bologna University, he was invited to plan the new Paris Observatory, built between 1667 and 1672 by order of Louis XIV. Cassini became the observatory's director and used its very long focal length, tubeless refracting telescopes to discover, in 1675, the division in Saturn's ring system that today bears his name. He also discovered four Saturnian moons (167184). He constructed a lunar map from eight years of original observations; this map served as the standard for the next 100 years.

Cassini accurately determined the rotation periods of Mars and Venus, and his studies of Jupiter's satellites enabled Ole romer to calculate the velocity of light by observing the delay in satellite eclipse intervals caused by variations of Jupiter's distance from the Earth. Most importantly, Cassini established the scale of the Solar System. He derived a measure of Mars' parallax from his own Paris observations and those made by Jean richer from Cayenne, South America, that corresponded to a value of 140 million km (85 million mi) for the astronomical unit. Although 10 million km (6 million mi) short of the true figure, this was a huge improvement on previous values (for example, Johannes Kepler's estimate of 15 million km/10 million mi). After Cassini's health failed in 1700, his son Jacques assumed the directorship of Paris Observatory (see cassini family).

Cassini Division Main division in saturn's ring system, separating the bright aringand b ring at a distance of 117,600 km (73,100 mi) from the planet's centre. The Cassini Division is not empty: voyager results show that it contains several narrow rings and that there are particles in the gaps between these rings.

Cassini En route to Saturn, the Cassini spacecraft passed through the Jupiter system. This image of Jupiter's Great Red Spot and its environs, together with lo and its shadow on the cloud-tops, was obtained on 2000 December 12 from a distance of 19.5 million km (12.1 million mi).

Cassiopeia See feature article

Cassiopeia A Strongest radio source in the sky, apart from the Sun. It appears optically as a faint nebula. It is a SUPERNOVA REMNANT from an unrecorded supernova explosion in 1660.

Cassiopeia A Chandra X-ray image of this supernova remnant, which is extremely faint at visual wavelengths. The image reveals a shell of material 10 l.y. in diameter.

Castalia APOLLO ASTEROID discovered in 1989; number 4769. Radar images have shown Castalia to have a twin-lobed shape. It is c.2.4 km (c.1.5 mi) long.

Castalia This series of radar images of the near-Earth asteroid (4769) Castalia was obtained during its close approach in 1989 using the Arecibo telescope. [S. Ostro (JPL/NASA)]

Castor The star a Geminorum, visual mag. 1.58, distance 52 l.y. Castor is a remarkable multiple star. Small telescopes show it as a double star, with blue-white components of mags. 2.0 and 2.9, spectral types A2 V and A5 V; these form a genuine binary with an orbital period of about 450 years, separation currently increasing. Each component is also a spectroscopic binary, with periods of 9.2 and 2.9 days respectively. There is also a third member of the Castor system, known as YY Geminorum, an ALGOL STAR eclipsing binary consisting of two red dwarfs, both of spectral type M1. During eclipses, which occur every 19.5 hours, their total visual magnitude drops from 9.3 to 9.8. Castor is named after one of the celestial twins commemorated by the constellation Gemini, the other being POLLUX.

cataclysmic variable (CV) Term given to a diverse group of stars that undergo eruptions, irrespective of the cause of the outburst. The term may describe a SUPERNOVA, NOVA, RECURRENT NOVA, NOVA-LIKE VARIABLE, FLARE STAR, DWARF NOVA, some X-ray objects, and other erupting stars. CVs are very close binary systems, with outbursts caused by interaction between the two components. A typical system of this type has a low mass secondary which fills its ROCHE LOBE, so that material is transferred through its LAGRANGIAN POINT on to the primary, which is usually a WHITE DWARF. The transferred material has too much angular momentum to fall directly on to the primary; instead, it forms an ACCRETION DISK, on which a hot spot is formed where the infalling material impacts on its outer edge. For any particular star, outbursts occur at irregular intervals from about ten days to weeks, months or many years.

Most dwarf novae and recurrent novae show a relationship between maximum brightness and length of the mean cycle: the shorter the cycle, the fainter the maximum magnitude. Nova-like variables are a less homogeneous group and are also less well-studied. Some have bursts of limited amplitude; others have had no observed outburst but have spectra resembling old novae. There are many other objects that show some if not all of the characteristics of CVs, for example old novae, some X-ray objects, AM HERCULIS STARS and others.

Many models have been suggested to explain the observed outbursts. The two most probable theories are that they are caused either by variations in the rate of MASS TRANSFER or by instabilities in the accretion disk. Both models require transfer of mass from the red secondary. A few systems are so aligned that we see them undergoing eclipses, enabling the main light source to be studied in detail. Typical examples of such systems are Z Chamaeleontis and OY Carinae; these stars may be examined both at outburst and minimum. It appears that during outbursts the disk increases in brightness. The intervals between consecutive outbursts of the same type vary widely, as they do for most CVs. Z Chamaeleontis has a mean cycle for normal outbursts of 82 days; for super-outbursts the mean cycle is 287 days. For OY Cari-nae the respective values are about 50 and 318 days. Their semiperiodic oscillations, timed in seconds, are small.

The advantage of studying eclipsing systems is that both the primary and the hot spot may be eclipsed, indicating the probable sizes of the components. It is generally agreed that a variable mass-flow rate can account for many observations. For the mass-flow rate to vary, there must be instability in the red star, but no generally accepted theory has been advanced. The red star must relax after a burst until another surge again overflows, discharging another burst of gas, but exactly what causes this ebb and flow is a mystery. The angular momentum of the mass flow prevents it from falling directly on to the primary. Instead a disk is formed around it on which a hot spot is formed at the point of impact. Some of this matter must be carried away, and it is not known whether this material is lost to the system or whether it splashes back on to the disk. When an eclipsing system is at minimum, the main light source at primary eclipse comes from the red star. The disk is then in a steady state.

Theories stipulating that the cause of the outburst is instabilities in the disk, while accepting variable mass transfer as the origin of the subsequent observed phenomena, contend that it is what happens on the disk that gives rise to outbursts. A number of disk models have been proposed, but none appears to fit the observed facts. For example, the disk increases in brightness during outbursts, at least in the eclipsing systems, and presumably in other dwarf novae. Disk instability models differ on how this released energy spreads through the disk, which mainly radiates in the ultraviolet. This ultraviolet radiation does not behave in the same way as the visual radiation and appears to contradict these models. If the disk dumps energy on to the primary, nuclear burning would be expected, but evidence on this is unclear.

catadioptric system Telescope or other optical system that includes both mirrors and lenses. To be considered catadioptric, at least one lens and mirror must be optically active (it must converge, diverge or correct the optical path) - flat mirrors and optical 'windows' do not count. So, for instance, the SCHMIDT-CASSEGRAIN TELESCOPE and MAKSUTOV TELESCOPE are both catadioptric, the light passing through an optically figured corrector plate before striking the primary mirror, but a folded refractor or a Cassegrain telescope with a flat optical window is not. The catadioptric design is compact, and is often used for portable instruments.

catalogue, astronomical Tabular compilation of stars or other deep-sky objects, giving their CELESTIAL COORDINATES and other parameters, such as magnitude. The various types of catalogue are distinguished by their content (the kind of sources covered and the parameters given), their context (when, why and how they were created) and their scope (the LIMITING MAGNITUDE). Thousands of astronomical catalogues have been produced, from the one in Ptolemy's Almagest (c.ad 150), which gave positions (to an accuracy of around 1) and magnitudes of about 1000 stars, to modern catalogues - usually accessible only as electronic files - which may contain the positions (accurate to 0".1 or better) and magnitudes of up to several hundred million sources. A list of catalogues is given in the Dictionary of Nomenclature of Celestial Objects, maintained under the auspices of the International Astronomical Union.

Before the modern era, cataloguers such as al-sufi were concerned mainly with refining the positions and magnitudes given in the Almagest. The need for improved navigation that came with European expansion after the Renaissance provided the impetus for early modern catalogues such as John flamsteed's Historia coelestis brittanica. As the power of telescopes increased, more and more stars were revealed for the cataloguers to measure. In the 19th and early 20th centuries, photographic methods supplanted visual observation as a means of gathering data, and catalogues swelled to contain hundreds of thousands of entries. Stars are known by their designations in catalogues, preceded by the catalogue's initials (see stellar nomenclature).

Three great Durchmusterungen ('surveys' in German) were compiled between 1855 and 1932 at Bonn, Cordoba and Cape Town, gathering together 1.5 million positions and magnitudes for about 1 million stars over the whole sky: the bonner durchmusterung (BD), the cordoba durchmusterung (CD) and the cape photographic durchmusterung (CPD). The first major catalogue of stellar spectra was the henry draper catalogue (HD), classifying 360,000 stars by spectral type, compiled by Annie cannon and published between 1918 and 1936. The first major astrometric and photometric catalogues were produced by, respectively, Lewis and Benjamin boss and by Friedrich zollner. Other notable star catalogues include the Astrographic Catalogue from the carte du ciel project; the bright star catalogue (BS), with accurate values of the positions, parallaxes, proper motion, magnitudes, colours, spectral types and other parameters for the 9110 stars brighter than magnitude 6.5; the 1966 Smithsonian astrophysical observatory star catalog (SAO), the first large 'synthetic' catalogue created on a computer by combining data from several astrometric catalogues; the guide star catalogue (GSC), created for the Hubble Space Telescope in 1990; and the 1997 hipparcos catalogue (HIP) and Tycho Catalogue (TYC), the products of the Hipparcos astrometry mission.

Many other star catalogues have been compiled for specific categories of stars, such as the Catalogue of Nearby Stars (Gl, after its compiler, Wilhelm Gliese, 1915-93). Important early catalogues of double stars were prepared by Wilhelm struve (S), Otto struve (OS), Sherburne burnham (BPS) and Robert aitken (ADS); the principal modern source is the 1996 Washington Visual Double Star Catalog (WDS) by C.E. Worley and G.C. Douglass. For variable stars the current reference is the general catalogue of variable stars. Stars with high proper motion are listed in William luyten's Two-Tenths Catalog of Proper Motions (1979-80). fundamental catalogues give highly accurate positions and proper motions for selected stars against which the relative positions of other celestial objects may be measured.

There are also many catalogues of non-stellar objects. The earliest is the famous messier catalogue (M); there followed John herschel's General Catalogue of Nebulae and Clusters, and J.L.E. Dreyer's new general catalogue (NGC) and index catalogues (IC). The NGC and the two ICs together represent the last attempt to include all known non-stellar objects in a single listing before the advent of today's powerful survey capabilities. Subsequent catalogues dealing with specific objects included: Edward Emerson barnard's 1927 list of dark nebulae, Catalogue of 349 Dark Objects in the Sky (B); S. Sharpless' 1959 listing of emission nebulae, A Catalogue of H II Regions (Sh-2); and the Catalog of Galactic Planetary Nebulae by LubosPerek (1919- ) and Lubos Kohoutek (1935- ) published in 1967 (PK). Catalogues of galaxies include the 1962-68 Morphological Catalogue of Galaxies (MCG or VV, from author Boris Alexandrovich Vorontsov-Vel'iaminov, 1904-94) and 1973 Uppsala General Catalogue of Galaxies (UGC) by Peter Nilson (1937-98). Peculiar galaxies have been catalogued by Halton arp, and clusters of galaxies by George abell.

Catalogues have also been compiled for sources of radiation outside the optical region. The first Cambridge catalogue, of the 50 brightest radio sources, was published in 1950 (see third cambridge catalogue). A recent radio catalogue, The NRAO VLA Sky Survey, nearing completion, has yielded a catalogue of over 1.7 million radio sources north of dec. 40. At the other end of the electromagnetic spectrum, the Rosat catalogues (RX) are currently the most frequently used sources for X-ray emitters.

They are typically tens to hundreds of kilometres long and consist of craters of less than one kilometre to tens of kilometres in diameter. Catenae are most frequently chains of impact craters. Some catenae, such as those found on Io, may be chains of volcanic craters. Drainage of surface material into tectonic fault openings or collapsed lava tubes may also form small catenae.

Catharina Lunar crater (18S 24E), 88 km (55 mi) in diameter. With THEOPHILUS and CYRILLUS, it forms a trio of similar large craters. An older formation, Catharina shows much erosion, which gives its walls a highly irregular outline. Later impacts have almost completely destroyed its north-east wall. No terracing is visible on Catharina's inner slopes, and its floor is devoid of a central peak.

Cat's Eye Nebula (NGC 6543) Comparatively bright (magnitude +8.1) PLANETARY NEBULA; it shows a greenish tint when observed visually. Located in the northern constellation Draco, roughly midway between and 8 (RA 17h 58m.6 dec. +6638'), the Cat's Eye has an apparent diameter of 350". It has an 11th-magnitude central star.

Caucasus Montes Lunar mountain range (36N 8E), dividing Mare IMBRIUM from Mare SERENITATIS. Some of its peaks rise to altitudes of 3650 m (12,000 ft). Material ejected by the Imbrium impact has scoured the Caucasus Mountains, which run roughly north-south. The most southerly part of the range consists mainly of isolated mountain peaks.

CCD (charge-coupled device) Small electronic imaging device, widely used in astronomy, which is highly efficient in its response to light and therefore able to detect very faint objects over a broad range of the spectrum.

CCDs consist of an oxide-covered silicon substrate with a two-dimensional rectangular array of light-sensitive electrodes on the surface. These electrodes form a matrix of pixels, or picture elements, each less than 0.03 nm in size and capable of storing electronic charges created by the absorption of light.

Photons imaged on to the surface of the CCD penetrate the electrode structure and enter the substrate, where electron-hole pairs are generated via the photoelectric effect, in numbers precisely proportional to the number of incident photons. The holes are conveniently lost by diffusion down into the depths of the substrate, while the electrons migrate rapidly to the nearest biased electrode, where they collect as a single charge packet in a 'potential well'.

Since it is impractical to wire the output from each individual electrode, the signal charge is transferred through the array, from one pixel to the next, by changing the voltages on each one. Pixels in adjacent rows are said to be 'charge-coupled' and the signal can move in parallel from row to row. As each row of signals reaches the end of the CCD it is read off into a serial register from which it can be stored in a computer, processed and the final image displayed.

Because the charge in each pixel is in proportion to the number of photons that have fallen on it, the output from a CCD is linear, which in turn means that the brightness of the final image produced is directly related to the brightness of object being observed, something which is not true for photographic emulsions.

In order to generate electron-hole pairs, photons have to pass through the electrode structure, and some are inevitably absorbed in this layer. Because of this, the quantum efficiency of front-illuminated CCDs is poor in the blue and UV regions of the electromagnetic spectrum. To overcome this, the silicon substrate can be thinned to around 15 |jum and the CCD back-illuminated, thus eliminating the need for the photons to negotiate the electrode structure. This process can increase the sensitivity of a CCD by a factor of two and the use of anti-reflective coatings can also improve quantum efficiency.

One of the other problems that has to be overcome when using CCDs is the generation of 'dark current'. This is the unwanted charge that is created by the natural random generation and recombination of electron-hole pairs that occurs at temperatures above absolute zero. This thermally induced charge can mask the signal when observing very faint objects over a long period of time but can be effectively eliminated by cooling the CCD using liquid nitrogen. A low dark current makes it possible to store the signal charge for long periods of time, thus allowing exposures from a few tens of seconds to several hours to be achieved.

CCDs can thus be used to accumulate signals from very faint light sources. They are more sensitive than photographic emulsion, are linear, spatially uniform and stable with time. They are also sensitive over a broad range of the spectrum, have low noise levels and a large dynamic range so that they can be used to detect both bright and faint objects at the same time. They can be used for direct imaging applications, surveying large areas of sky, for photometry or as spectroscopic detectors. Because of their versatility, they have almost completely replaced photographic plates.

CD Abbreviation of cordoba durchmusterung cD galaxy Largest, most luminous kind of normal GALAXY. Often found at the centres of rich clusters, they resemble giant ELLIPTICAL GALAXIES except for having a more extensive outer envelope of stars. Some cD galaxies can be traced over spans exceeding a million light-years. They may grow as a result of attracting stars that were stripped from surrounding galaxies by tidal encounters, either within the cluster as a whole or with the cD galaxy. This class was recognized by William W. MORGAN as part of the Yerkes system of galaxy classification, along with N GALAXIES.


celestial coordinates Reference system used to define the positions of points or celestial objects on the celestial sphere. A number of systems are in use, depending on the application.

EQUATORIAL COORDINATES are the most commonly used and are the equivalent of latitude and longitude on the Earth's surface. DECLINATION is a measure of an object's angular distance north or south of the CELESTIAL EQUATOR, values north being positive and those south negative. RIGHT ASCENSION, or RA, equates to longitude and is measured in hours, minutes and seconds eastwards from the FIRST POINT OF ARIES, the intersection of the celestial equator with the ECLIPTIC. HOUR ANGLE and POLAR DISTANCE can also be used as alternative measures.

The HORIZONTAL COORDINATE system uses the observer's horizon as the plane of reference, measuring the ALTITUDE (angular measure of an object above the horizon) and AZIMUTH (bearing measured westward around the horizon from north).

ECLIPTIC COORDINATES are based upon the plane of the ECLIPTIC and use the measures of CELESTIAL LATITUDE and CELESTIAL LONGITUDE (also known as ecliptic latitude and ecliptic longitude). Celestial latitude is measured in degrees north and south of the ecliptic, while celestial longitude is measured in degrees eastwards along the ecliptic from the First Point of Aries.

The GALACTIC COORDINATE system takes the plane of the Galaxy and the galactic centre (RA 17h 46m dec. 2856') as its reference points. Galactic latitude is measured from 0 at the galactic equator to 90 at the galactic pole, while galactic longitude is measured from 0 to 360 eastwards along the galactic equator.

celestial equator Projection on to the CELESTIAL

SPHERE of the Earth's equatorial plane. Just as its terrestrial counterpart marks the boundary between the northern and southern hemispheres of the Earth and is the point from which latitude is measured, so the celestial equator delineates between the northern and southern hemispheres of the sky and is used as the zero point for measuring the celestial coordinate DECLINATION.

celestial latitude (symbol p, ecliptic latitude) Angular distance north or south of the ECLIPTIC and one coordinate of the ecliptic coordinate system. Designated by the Greek letter p, celestial latitude is measured in degrees from the ecliptic (0) in a positive direction to the north ecliptic pole ( + 90) and a negative direction to the south ecliptic pole (-90). See also CELESTIAL LONGITUDE

celestial longitude (symbol X, ecliptic longitude) Angular distance along the ecliptic from the FIRST POINT OF ARIES and one coordinate of the ecliptic coordinate system. Designated by the Greek letter X, celestial longitude is measured from 0 to 360, eastwards of the First Point of Aries. See also CELESTIAL LATITUDE

celestial mechanics (dynamical astronomy) Discipline concerned with using the laws of physics to explain and predict the orbits of the planets, satellites and other celestial bodies. For many years the main effort in the subject was the development of mathematical methods to generate the lengthy series of perturbation terms that are needed to calculate an accurate position of a planet. The advent of computers has eased this task considerably, with the result that the main emphasis of the subject has changed dramatically. Much effort is now devoted to the study of the origin, evolution and stability of various dynamical features of the Solar System, and in particular of the numerous intricate details of rings and satellite orbits revealed by the VOYAGER spacecraft.

The subject can be said to have started with the publication of Newton's Principia in 1687, in which are stated his law of gravitation, which describes the forces acting on the bodies, and his three laws of motion, which describe how these forces cause accelerations of the motions of the bodies. Then the techniques of celestial mechanics are used to determine the orbits of the bodies resulting from these accelerations. One of the first results achieved by Newton was to give an explanation of KEPLER'SLAWS. These laws are descriptions deduced from observations of the motions of the planets as being elliptical orbits around the Sun, but until the work of Newton no satisfactory explanation of these empirical laws had been given. However, Kepler's laws are true only for an isolated system of two bodies; in the real Solar System the attractions of the other planets and satellites cause the orbits to depart significantly from elliptical motion and, as observational accuracy improved, these perturbations became apparent. The greatest mathematicians of the 18th and 19th centuries were involved in the effort of calculating and predicting the perturbations of the orbits, in order to match the ever-increasing accuracy and time span of the observations. The orbit of the Moon was the major problem, partly because the Moon is nearby, and so the accuracy of observation is high, but also because its orbit around the Earth is very highly perturbed by the Sun.

Various techniques were developed for calculating the perturbations of an orbit. An important technique is the method of the 'variation of arbitrary constants', developed by Leonhard EULER and Joseph LAGRANGE. An unperturbed orbit can be described by the six ORBITAL ELEMENTS, or arbitrary constants, of the ellipse. The effect of perturbations can be described by allowing these 'constants' to vary with time. Thus, for example, the eccentricity of the orbit may be described as a constant plus a number of periodic terms. The resulting expressions are called the 'theory of the motion of the body', and can be very lengthy in order to achieve the desired accuracy, perhaps hundreds of periodic terms. Around the middle of the 19th century an alternative method was developed in which the perturbations of the three coordinates of the body (for example, longitude, latitude and distance from the Sun) are calculated instead of the perturbations of the six elements. Variants of this method have been used ever since for lunar and planetary theories, but the variation of constants is still more suitable for many of the satellites. The latest theories used to calculate the positions of the Moon and planets in the almanacs used by navigators and astronomers are those derived by: Simon NEWCOMB for the five inner planets, Uranus and Neptune; by Ernest BROWN for the Moon; and by George HILL for Jupiter and Saturn.

Overall, Newton's four laws and the techniques of celestial mechanics have proved successful at explaining the motions of the planets and satellites. Problems with the orbit of the Moon were eventually resolved by improved theories, and anomalous perturbations of the orbit of Uranus led John Couch ADAMS and Urbain LE VERRIER to suspect the existence of a further planet, which resulted in the discovery of Neptune in 1846 close to the predicted position. Problems with the seemingly unaccountably rapid advance of Mercury's perihelion were eventually solved in 1915 with the publication of Einstein's general theory of relativity (see VULCAN). This is a more accurate representation of the laws of motion under the action of gravitation, but the differences from using Newton's laws are small, and only become noticeable in strong gravity fields.

By the early 20th century fairly good theories had been developed for the Moon and the planets, but these had just about reached the limit of what was possible with the means then available, of mechanical calculators, mathematical tables, and masses of hand algebra. There was some further effort on theories for other satellites and some asteroids, but much effort in the subject moved into other areas, particularly into theoretical studies such as the idealized THREE-BODY PROBLEM. With the advent of computers, renewed effort on the development of lunar and planetary theories became feasible. The initial idea was to use the same techniques as before, but to do the vast amounts of algebraic manipulation involved on a computer. Excellent theories of the Moon and some of the planets have been produced in this way. However it has proved to be more effective to use the less elegant but much simpler and more accurate method of integrating the equations of motion by numerical techniques. The positions of the Moon and planets in the almanacs are now all computed in this way, but the old algebraic methods are still the most effective for most of the natural satellites.

With the task of planetary theory development that had occupied celestial mechanics for 300 years now effectively solved, the main emphasis of the subject has changed dramatically, with many problems of origin, evolution and stability now being studied. The stability of the Solar System has long been of considerable interest, and recent numerical integrations suggest that the orbits of the major planets are stable for at least 845 million years (about 20% of the age of the Solar System), but the orbit of Pluto may be unstable over that time scale. Another problem that is now close to a full solution after more than a century of effort is to explain the origin of the KIRKWOOD GAPS in the ASTEROID BELT, which occur at certain distances from the Sun that correspond to commensurabilities of period with Jupiter. The explanation that was found for the gap at the 3:1 COMMENSURABILITY required the introduction of the concept of CHAOS into Solar System dynamics. Using a sophisticated combination of analytical and numerical methods, it was found that over a long period of time the orbital elements of an object at the commensurability could have occasional large departures (termed chaotic motion) from the small range of values expected from conventional analytical modelling of the motion. Subsequent work suggests that the same mechanism can explain the widest gap at the 2:1 commensurability. Many other instances of chaos have been identified, and with them is the realization that the Solar System is not such a deterministic dynamical system as had been supposed.

It is now realized that TIDAL EVOLUTION has been a major factor in adjusting the orbits of satellites, and modifying the spin rates of satellites and planets. There are many more occurrences of commensurabilities in the Solar System, particularly among the satellites, than would be expected by chance. It has been shown that some of these could have been caused by orbital evolution due to tidal action, which would continue until a com-mensurability was encountered, whereupon the satellites would become trapped.

A theory of shepherding satellites was proposed to explain the confinement of the narrow rings of Uranus, which were discovered by ground-based occultation observations. Subsequent images from the Voyager spacecraft have discovered small satellites close to one of the rings, and similar satellites close to one of Saturn's narrow rings, thus at least partially supporting the shepherding mechanism. Many other interesting problems of dynamics have arisen following the Voyager observations, such as the cause of the intricate structure of Saturn's rings, which consist of hundreds of individual ringlets. Some of the features have been explained by resonances with the satellites, which can cause various effects such as clumping around a ring, and radial variations of density. Other features are possibly caused by small unseen satellites orbiting within the rings, but there are many features still to be explained, and no doubt many new dynamical mechanisms still to be discovered.

CENTAURUS (gen. centauri, abbr. cen) Large, prominent southern constellation, the ninth-largest in the sky, representing a centaur, a mythological beast with the legs and body of a horse and the upper torso of a man. This particular centaur was said to be Chiron, who taught the princes and heroes of Greek mythology. The constellation's brightest star is alpha centauri, a triple system that includes the red dwarf proxima centauri, the closest star to the Sun. a and p Cen act as pointers to Crux, the Southern Cross. An easy double is 3 Cen, with components of 4.6 and 6.1 divisible with small apertures. R Cen is a variable mira star, ranging between 5th and 12th magnitude in about 18 months. co Cen is the brightest globular cluster in the sky, so prominent that it was given a stellar designation by early observers. NGC 3766 and 5460 are open clusters for binoculars. NGC 5128 is a 7th-magnitude peculiar galaxy, also known as the radio source centaurus a. NGC 3918 is a planetary nebula known as the Blue Planetary, resembling the planet Uranus in small telescopes.

celestial meridian Great circle on the celestial sphere that passes through the north and south celestial poles, together with the zenith and the nadir. The term is usually used to refer to that part of the circle which is above the observer's horizon, intersecting it due north and due south.

celestial pole Either of the two points on the celestial sphere that intersect with a projection of the Earth's axis of rotation into space, and about which the sky appears to rotate. Because their position is dictated by the orientation of the Earth's axis, the celestial poles are subject to the effects of precession. This causes them to slowly drift, describing a circle in the sky of radius 23.5 (the inclination of the Earth's axis) over a period of some 25,800 years, the effect only being noticeable over a few decades. At present, the north celestial pole is within 1 of the star Polaris, which is known as the pole star, but in 3000 bc the north Pole Star was thuban, in Draco and by ad 10,000 it will be deneb. For an observer, the altitude of the celestial pole is always equal to their latitude.

Celestial Police Name given themselves by the 24 astronomers who collaborated to search for a planet between the orbits of Mars and Jupiter, as predicted by bode'slaw. They were first convened in 1800 by Franz von zach, at Johann schroter's observatory. Members of this international group, who also included Johann bode, William herschel, Nevil maskelyne, Charles messier and Wilhelm olbers, discovered three asteroids (Pallas, Juno and Vesta) between 1802 and 1807, after Giuseppe piazzi had found the first, Ceres, in 1801.

celestial sphere Inside of an imaginary sphere, with the Earth at its centre, upon which all celestial bodies are assumed to be projected.

celestial sphere A useful concept for describing positions of astronomical bodies. As shown, key points of reference the celestial pole and celestial equator are projections on to the sphere of their terrestrial equivalents. An objects position can be defined in terms of right ascension and declination (equivalent, respectively, to longitude and latitude).

The stars and planets are so far away that everything we see in the sky appears to be projected on to an enormous screen extending all around us, as if we were inside a gigantic planetarium. This is the illusion of the celestial sphere, half of which is always hidden from an observer on the Earth's surface, but upon which we base our charts of the sky and against which we make our measurements. The illusion is so strong that the early astronomers postulated the existence of a crystal sphere of very great radius to which the stars were fixed.

The most obvious behaviour of the celestial sphere is its apparent daily east to west rotation, due to the axial spin of the Earth. In the northern hemisphere we see some stars that never set - the circumpolar stars; they appear to turn about the polar point near to the bright star Polaris. There is, of course, a similar polar point in the southern hemisphere marked by the fifth magnitude star, Octantis. The direction of the polar axis seems fixed in space, but it is in fact slowly drifting because of the effects of precession.

Having recognized one easily observed direction within the celestial sphere, it is possible to define another - the celestial equator. This is a projection of the Earth's equator on to the celestial sphere, dividing the sky into two hemispheres and enabling us to visualize a set of small circles similar to those of latitude on Earth, known as declination, which allow us to specify the position of an object in terms of angle above or below the celestial equator.

Right ascension, the celestial equivalent of longitude, is based upon the Earth's orbital motion around the Sun and is measured eastwards along the ecliptic, the apparent path of the Sun through the constellations. For half of each year the Sun is in the northern hemisphere of the sky and for the other half it is in the southern hemisphere. On two dates each year, known as the equinoxes, it crosses the equator and it is the point at which it crosses into the northern hemisphere, at the spring or VERNAL EQUINOX, that is chosen as the zero of right ascension. This point on the celestial sphere, where the ecliptic and the celestial equator intersect, is called the FIRST POINT OF ARIES. This was an accurate description of its position thousands of years ago, but precession has now carried it into the constellation Pisces.

Right ascension can be measured in angular terms. However, it is more common to use units of time (hours, minutes, seconds or hms). Astronomers measure time by the rotation of the Earth relative to the stars, that is exactly 360 of axial rotation, rather than to the Sun. Their 'sidereal' timescale has a day of 24 sidereal hours, which in terms of civil time, is 23 hours, 56 minutes and 4 seconds long.

The beginning of the sidereal day is when the First Point of Aries lies on the MERIDIAN, the great circle linking the north and south points and passing directly overhead. After 24 sidereal hours it will be in that position again. The right ascension figures work like the face of a clock so that when the first point of Aries is on the meridian the sidereal time is 0h. At 1h sidereal time the sky will have rotated 15 and stars this angle east of the First Point of Aries will then lie on the meridian.

Apart from right ascension and declination, astronomers also use other systems of CELESTIAL COORDINATES to locate objects or points on the celestial sphere, dependent upon the particular application.

Although the stars seem relatively fixed in position, the Sun, Moon and planets (apart from Pluto) move across the celestial sphere in a band of sky about 8 either side of the ecliptic. This belt of sky is known as the ZODIAC and is divided into 12 signs named after the constellations they contained at the time of the ancient Greeks.

Celsius, Anders (170144) Swedish astronomer, best known for inventing the Centigrade temperature scale, now known as the CELSIUS scale. He took part in the 1736 expedition to Lapland organized by the French astronomer Pierre Louis Moreau de Maupertuis (1698-1759) to measure the length of a meridian arc. The results of this and a second expedition to Peru showed that the Earth is oblate. Celsius was a pioneer of stellar photometry, using a series of glass filters to measure the relative intensity of light from stars of different magnitudes, and he was one of the first to realize that the auro-rae were related to Earth's magnetic field.

Celsius scale Temperature scale on which the freezing point of water is 0C and the boiling point of water is 100C. It is named after Anders CELSIUS. The magnitude of 1C is the same as 1 K. It is also known as the Centigrade scale, although this name was officially abandoned in 1948. See ABSOLUTE TEMPERATURE size (generally bigger than 50 km/30 mi). It is likely, however, that these are only the largest members of the overall population, with a much greater number of smaller objects awaiting identification.

The origin of the Centaurs is suspected to be as members of the EDGEWORTH-KUIPER BELT, having coalesced there when the rest of the Solar System formed over 4.5 billion years ago; they are thought to have been inserted into their present unstable orbits during just the last million years or so. The dynamical instability of the Centaurs derives from the fact that they will inevitably make close approaches to one or another of the giant planets, and the severe gravitational perturbations that result will divert the objects in question on to different orbits. Some Centaurs will be ejected from the Solar System on hyperbolic paths; others may fall into the inner planetary region and so become extremely bright active comets.

The fact that Centaurs show characteristics of both asteroids and comets leads to the convention for their naming. In Greek mythology the Centaurs were hybrid beasts, half-man and half-horse. The first Centaur object to be discovered was CHIRON, in 1977. The next was PHOLUS, in 1992. Others added since include (7066) Nessus, (8405) Asbolus, (10199) Chariklo and (10370) Hylonome. See also DAMOCLES; HIDALGO; JUPITER-CROSSING ASTEROID; LONG-PERIOD ASTEROID

Centaurus See feature article

Centaurus A (NGC 5128) Galaxy with powerful radio emission at a distance of around 3 Mpc (10 million l.y.). Optically it appears to be a normal elliptical galaxy, but it is crossed by a dark and very prominent dust lane. Very deep photographs reveal the galaxy to be more than 1 across, and the radio image is much bigger. This enigmatic object appears to be two galaxies in collision, a massive elliptical galaxy and a smaller dusty spiral (which can be seen in the infrared). Centaurus A is emitting a huge amount of energy at X-ray and optical wavelengths as well as at radio wavelengths.

Centaurus A Crossed by a prominent dark lane, this elliptical galaxy (also designated NGC 5128) is a powerful radio and X-ray source.

Central Bureau for Astronomical Telegrams Bureau that rapidly disseminates information on transient astronomical events. It operates from the SMITHSONIAN ASTRO-PHYSICAL OBSERVATORY under the auspices of the INTERNATIONAL ASTRONOMICAL UNION. The bureau issues announcements concerning comets, asteroids, variable stars, novae and supernovae in the form of IAU Circulars, both electronically and in printed form


Censorinus Small (5 km/3 mi) but brilliant lunar crater (0 32E); it is located on a bluff near the south-east border of Mare TRANQUILLITATIS.

Centaur Any of a group of planet-crossing objects in the outer planetary region that are classified as being ASTEROIDS, although it is likely that in nature they are actually large COMETS. Through to late 2001 more than 35 Centaurs had been discovered, taking the criterion for membership as a Neptune-crossing orbit (that is, perihelion distance less than 30 AU), implying that the Centaurs could not be classed as TRANS-NEPTUNIAN OBJECTS. At such great distances from the Sun, the temperature is extremely low and only the most volatile chemical constituents sublimate, making comet-like activity either totally absent or at least difficult to detect using Earth-based telescopes. Their faintness, coupled with their considerable helio- and geocentric distances, implies that all known Centaurs must be of substantial

central meridian Imaginary north-south line bisecting the disk of a planet, the Moon or the Sun. The central meridian passes through the poles of rotation of the object in question and is used as a reference point from which to determine the longitude of features on the disk as the body rotates. It is independent of any phase that may be present.

Centre de Donnees Astronomiques de Strasbourg (CDS) World's main astronomical data centre dedicated to the collection and distribution of computerized astronomical data and related information from both ground-and space-based observatories. It hosts the SIMBAD (in full, Set of Identifications, Measurements and Bibliography for Astronomical Data) astronomical database, the world reference database for the identification of astronomical objects. The CDS is located at the strasbourg astronomical observatory in France, and is a laboratory of the Institut National des Sciences de l'Univers.

The CDS was founded in 1972 as the Centre de Donnees Stellaires, its main aim being to cross-identify star designations in different catalogues - often the same object had a host of different catalogue identifications. Bibliographic references to objects were added, and SIMBAD was the result. By 2001 the database contained data for more than 2.25 million objects, and over 5 million references.

CEPHEUS (gen. cephei, abbr. cep)

Constellation of the northern polar region, between Cassiopeia and Draco. It represents a mythological king, husband of Cassiopeia and father of andromeda. Its most celebrated star is 8 Cep, the prototype cepheid variable, a pulsating yellow supergiant varying between 3.5 and 4.4 with a period of 5.4 days; its variability was discovered in 1784 by John goodricke. A wide bluish companion of mag. 6.3 makes it an attractive double for small telescopes or even binoculars. p Cep is another pulsating variable, though of much smaller amplitude (0.1 mag.) and far shorter period (4.6 hours); it is the prototype beta cephei star. j Cep is a pulsating red supergiant known as the garnet star, range 3.4-5.1, period about 2 years. Another variable red supergiant, VV Cep, is one of the largest stars known, with an estimated diameter about that of Jupiter's orbit. It varies semiregularly between mags. 4.8 and 5.4.

centre of mass Position in space for an object or collection of objects at which the various masses involved act as though they were a single mass concentrated at that point. For an object in a uniform gravitational field it is the same as the centre of gravity. When two masses are linked by gravity, the centre of mass occurs on the line joining them at a distance from each object that is inversely proportional to the mass of that object. In this case the centre of mass is also called the barycentre.

centrifugal force Apparent force that appears when an object is forced to move along a circular or curved path. The force is actually the result of the inertia of the object attempting to keep the object moving in a straight line. It is the reaction to the centripetal force.

centripetal force Force acting on an object that causes it to move along a circular or curved path. It produces an acceleration towards the centre of curvature of the path, and the reaction to this acceleration is experienced as the centrifugal force. Gravity provides the centripetal force on an orbiting body, and the magnetic field on electrons producing synchrotron radiation.

Cepheid instability strip See instability strip

Cepheid variable Yellow giant or supergiant pulsating variable star, so called because the first variable of the type to be discovered was delta cephei. Cepheids pulsate in a particularly regular manner. These stars have left the main sequence and occupy, on the hertzsprung-russell diagram, a position to the right of the upper main sequence and to the left of the red giants, termed the Cepheid instability strip. Cepheids are passing through the first Instability Transition after leaving the main sequence.

Cepheid variable Their extremely regular light variations make Cepheids valuable standard candles: the longer the period from one maximum to the next, the greater the stars intrinsic luminosity. As shown, the typical Cepheid light-curve shows a rapid rise to peak brightness, followed by a slower decline to minimum.

During this brief period in their lives, these stars oscillate, alternately expanding and contracting so that in each cycle a star may change in size by as much as 30%. These regular, rhythmic changes in size are accompanied by changes in luminosity. The surface temperature also changes in the course of each cycle of variations in brightness, being at its lowest when the star is at minimum and at its highest when the star is brightest. This temperature change may equal 1500 K for a typical Cepheid. A change in temperature also means a change in spectral type, so that the star may be F2 at maximum, becoming the later type, G2, at minimum, changing in a regular manner as the temperature falls or rises. A Cepheid may continue to pulsate in this manner for a million years, which is a comparatively short time compared to the life span of a star.

Most massive stars spend at least some time as Cepheid variables. Stars like Delta Cephei have amplitudes of around 0.5 mag. and periods usually not longer than 7 days; there are, however, Cepheids with larger amplitudes and longer periods, which form a separate subtype. This subtype includes the naked-eye stars 1 Carinae, p Doradus and k Pavonis. The period of light changes is related to the average luminosity of the star. This means that the absolute magnitude of a Cepheid variable may be found by measuring the period of the light cycle. The apparent magnitude may be obtained directly. Once period, apparent and absolute magnitude are known, it becomes possible to determine the distance to the star.

Cepheids are visible in external galaxies, but their value as distance indicators is compounded by the fact that there are two types. Both follow a period-luminosity relationship, but their light-curves are different. First, there are the classical Cepheids, such as Delta Cephei itself, which are yellow supergiants of Population I. The second type, the w virginis stars, are Population II stars found in globular clusters and in the centre of the Galaxy. In using Cepheids to determine distances it is necessary to know which type is being observed. At the time Cepheids were first used to determine distances it was not known that there were Cepheids with different period-luminosity values. This resulted in erroneously applying the value for type II to classical Cepheids. When this error was found, in 1952, the result was to double the size of the Universe. For periods of 3-10 days the light-curves of the two types closely resemble one another and classification is based on spectral differences. In particular, at certain phases, classical Cepheids exhibit calcium emission, whereas W Virginis stars show hydrogen emission.

The period-luminosity relationship means that the longer the period, the brighter the visual absolute magnitude. A comparison of the curves shows that classical Cepheids are about one magnitude brighter than type II Cepheids. The light-curves may be arranged in groups, according to their shapes, which progressively become more pronounced in each group as the period lengthens. Most Cepheid light-curves fall into one of about 15 such divisions, each with a longer average period. They all follow a period-luminosity relationship, which commences with the RR LYRAE stars of very short period and, after a break, is continued by the MIRA STARS. This regular progression - the longer the period, the later the spectral type - is called the Great Sequence. A typical Cepheid would have a surface temperature varying between 6000 and 7500 K and an absolute luminosity that is ten thousand times that of the Sun.

Since Cepheids are in a part of the Hertzsprung-Rus-sell diagram where changes occur, observations are directed towards detecting changes in periods. Such changes are small but give information as to how stars progress through the instability strip; they can be detected by making series of observations separated by a few years.

Cepheus See feature article

Cerenkov radiation Electromagnetic radiation emitted when a charged particle passes through a transparent medium at a speed greater than the local speed of light in that medium (the speed of light in air or water is less than that in a vacuum). Radiation is emitted in a cone along the track of the particle. Cosmic rays ploughing into the Earth's atmosphere produce (Cerenkov radiation, which can be detected at ground level. This type of radiation was discovered in 1934 by the Russian physicist Pavel Cerenkov (1904-90).

Fourth-largest constellation, lying on the celestial equator south of Aries and Pisces. It is not particularly prominent - its brightest star is p Cet, mag. 2.04, known as DENEB KAITOS; a Cet is known as MENKAR. Cetus represents the sea-monster from which ANDROMEDA was saved by Perseus. The constellation's most famous star is MIRA (o Cet), the prototype long-period variable. t Cet is the most Sun-like of all the nearby single stars. M77, a 9th-magnitude face-on spiral some 50 million l.y. away, is the brightest of all the Seyfert galaxies. Because Cetus lies close to the ecliptic, planets can sometimes be found within its boundaries was not large enough to be considered a major planet, and the rapid discovery of several other such objects in the following years, namely PALLAS, JUNO and VESTA, added to this view. William HERSCHEL coined the term asteroid for these new objects; they are also termed minor planets.

Cerro Tololo Inter-American Observatory (CTIO) Major optical observatory situated approximately 80 km (50 mi) east of La Serena, Chile, at an elevation of 2200 m (7200 ft). It is operated jointly by the ASSOCIATION OF UNIVERSITIES FOR RESEARCH IN ASTRONOMY and the National Science Foundation as part of the NATIONAL OPTICAL ASTRONOMY OBSERVATORIES; it is a sister observatory to KITT PEAK NATIONAL OBSERVATORY. The largest telescope on the site is the 4-m (158-in.) Victor M. Blanco Telescope, commissioned in 1974 to complement its northern-hemisphere twin, the MAYALL TELESCOPE, but now being developed to complement SOAR, the SOUTHERN ASTROPHYSICAL RESEARCH TELESCOPE, scheduled to begin operation in 2003. CTIO is also home to 1.5-m (60-in.), 1.3-m (51-in.) and 1.0-m (40-in.) reflectors.

Ceres First ASTEROID to be discovered, on the opening day of the 19th century, hence it is numbered 1. Ceres, a MAIN-BELT ASTEROID, has a diameter of c.933 km (c.580 mi), although it is not precisely spherical; it is the largest asteroid. Ceres' mass, 8.7 X 1020 kg, represents about 30% of the bulk of the entire main belt, or about 1.2% of the mass of the Moon. Its average density, about 1.98 g/cm3, is less than that of most meteorites. Ceres rotates in about nine hours, its brightness showing little variation, which is indicative of a fairly uniform surface, thought to be powdery in nature. It lies close to the middle of the main belt, at an average heliocentric distance of 2.77 AU.

The possibility of a planet between Mars and Jupiter had been suggested in the early 1600s by Johannes KEPLER, and in the late 18th century BODE'SLAWwas interpreted as implying the likely existence of such a body. In 1800 a group of European astronomers formed the so-called CELESTIAL POLICE, having the aim of discovering this purported planet. Before they could begin their search, however, Ceres was discovered by Giuseppe PIAZZI from Palermo, Sicily. Piazzi was checking Nicolas LACAILLE's catalogue of zodiacal stars when he found an uncharted body that moved over the subsequent nights. Piazzi wanted to call the object Ceres Ferdinandea (Ceres is the goddess of fertility, the patron of Sicily, while Ferdinand was the name of the Italian king), but only the first part of that name was accepted by astronomers in other countries. Although he was prevented by illness from following it for an extended period, Piazzi's observations allowed Ceres to be recovered late in 1801. It soon became apparent that it

Cetus See feature article



Challis, James (1803-82) English clergyman and astronomer, now chiefly remembered for his part in the search for Neptune. He succeeded George Biddell AIRY as Cambridge's Plumian Professor (1836), and served until 1861 as director of Cambridge Observatory. There in 1846 July, at the urging of John Couch ADAMS, who had calculated the theoretical position of a new planet from perturbations of Uranus, Challis initiated a rigorous search. He actually discovered Neptune on August 4, though he failed to recognize it as a planet before Johann GALLE and Heinrich D'ARREST of the Berlin Observatory announced the planet's discovery, on 1846 September 23.

Chamaeleon See feature article

Chamaeleon This complex of hot stars and nebulosity was photographed during testing of the Very Large Telescope (VLT). Designated Chamaeleon I, it lies close to the south celestial pole.

CHAMAELEON (gen. chamaeleontis, abbr. cha) Small and unremarkable constellation near the south celestial pole, introduced at the end of the 16th century by Keyser and de Houtman, representing the colour-changing lizard. Its brightest star is a Cha, mag. 4.1. 8 Cha is a binocular double, mags. 4.4 and 5.5.

Chamberlin, Thomas Chrowder (1843-1928) American geologist who, with Forest R. MOULTON, conceived the planetesimal hypothesis for the origin of the Solar System, according to which a star passing through the solar neighbourhood caused the Sun to eject filaments of material that condensed into planetesimals, which in turn accreted to form the major planets. Chamberlin set out this theory, which arose in part from his research into glaciation and Earth's geologic history, in his 1928 book The Two Solar Families.

Chandler period 430-day period of variation in position of the Earth's axis of rotation. This small movement of the geographic poles is known as the 'Chandler wobble' after the American who first discovered it. It is thought to be caused by seasonal changes in the distribution of mass within the Earth.

Chandrasekhar, Subrahmanyan (1910-95) Indian-American astrophysicist who was awarded the 1983 Nobel Prize for Physics for his mathematical theory of black holes. At Cambridge University, he developed a theoretical model explaining the structure of white dwarf stars that took into account the relativistic variation of mass with the velocities of electrons that comprise their degenerate matter. He showed that the mass of a white dwarf could not exceed 1.44 times that of the Sun - the CHANDRASEKHAR LIMIT. Stars more massive than this must end their lives as a NEUTRON STAR or BLACK HOLE. He spent most of his career at the University of Chicago and its Yerkes Observatory, where he was on the faculty from 1937 to 1995, and he served as editor of the prestigious AstrophysicalJournal (1952-71).

Chandrasekhar revised the models of stellar dynamics originated by Jan OORT and others by considering the effects of fluctuating gravitational fields within the Milky Way on stars rotating about the galactic centre. His solution to this complex dynamical problem involved a set of twenty partial differential equations, describing a new quantity he termed 'dynamical friction', which has the dual effects of decelerating the star and helping to stabilize clusters of stars. Chandrasekhar extended this analysis to the interstellar medium, showing that clouds of galactic gas and dust are distributed very unevenly. He also studied general relativity and black holes, which he modelled using two parameters - mass and angular momentum.

Chandrasekhar limit Maximum possible mass for a WHITE DWARF. It was first computed, in 1931, by the Indian astrophysicist Subrahmanyan CHANDRASEKHAR. The value computed by Chandrasekhar applies to a slowly rotating star composed primarily of helium nuclei and is about 1.44 solar masses.

A white dwarf is supported against its own gravitational attraction by electron degeneracy pressure (see DEGENER-ATEMATTER). The PAULI EXCLUSION PRINCIPLE states that no two electrons can occupy exactly the same state so that when all the low energy states have been filled, electrons are forced to take up higher energy states. With white dwarfs of progressively higher mass, as gravity attempts to squeeze the star into a smaller volume, so the electrons are forced into higher and higher energy states. They therefore move around with progressively higher speeds, exerting progressively higher pressures.

The greater the mass, the smaller the radius and the higher the density attained by a white dwarf before electron degeneracy pressure stabilizes it against gravity. As the mass approaches the Chandrasekhar limit, electrons are eventually forced to acquire velocities close to the speed of light (that is, they become 'relativistic'). As the limit is reached, the pressure exerted by relativistic electrons in a shrinking star cannot increase fast enough to counteract gravity. Gravity overwhelms electron degeneracy pressure, and a star that exceeds the Chandrasekhar limit collapses to a much denser state. Electrons combine with protons to form neutrons, and the collapse is eventually halted by neutron degeneracy pressure, by which time the star has become a NEUTRON STAR.

Chandrasekhar-Schonberg limit See SCHONBERG

Chandra X-Ray Observatory (CXO) Third of NASA's GREAT OBSERVATORY series. The former Advanced X-Ray Astrophysics Facility, it was deployed into orbit by the Space Shuttle Columbia in 1999 July. The 5.2-tonne, 14-m-long (45-ft-long) spacecraft was renamed the Chandra X-Ray Observatory. It followed the HUBBLE SPACE TELESCOPE and the COMPTON GAMMA RAY OBSERVATORY in the Great Observatory series. Chandra is equipped with four science instruments - an imaging spectrometer, a high-resolution camera, and high- and low-energy spectrometers. With the European Space Agency's NEWTON X-Ray Telescope, which was also launched in 1999, the Chandra X-ray Observatory is providing astronomers with a wealth of data, including images showing a PULSAR inside a PLANETARY NEBULA and material that seems to be disappearing down a BLACK HOLE.

Chandra X-Ray Observatory Among many valuable images obtained from Chandra has been this view showing a huge X-ray flare associated with the supermassive black hole at the centre of our Galaxy.

chaos Property of a mathematical model of a physical system that is akin to indeterminacy or instability, in which the final state of the system is very sensitively dependent on its initial state. The usual example quoted

Charon Only known satellite of pluto. It was discovered on telescopic images by the American astronomer James Christy (1938- ) in 1978. Charon is named after the ferryman who, in classical mythology, transported the ghosts of the dead across the river Styx into the underworld domain of the god Pluto. It is a mysterious body, never having been visited by any spacecraft.

The determination of Charon's orbit gave the first reliable measurement of Pluto's mass. Between 1985 and 1990, a fortunate series of mutual occultations between Pluto and Charon, when the plane of Charon's orbit lay in the line of sight from Earth, enabled the sizes of both bodies to be determined. Charon orbits exactly in Pluto's equatorial plane, and the rotations of the two bodies are mutually tidally locked so that they permanently keep the same faces toward each other.

Spectroscopic studies of Charon have revealed only water-ice, contaminated by rock or soot, with none of the more exotic ices found on Pluto. Charon's gravity is too weak to retain any kind of atmosphere, even in the cold outer reaches of the Solar System, where the surface temperature is only about 40 K. Competing tidal pulls on Charon from Pluto and the Sun could be responsible for sufficient tidal heating to have allowed Charon to differentiate (see differentiation), forming a rocky core, and even to maintain an ocean below the outer carapace of ice. See also europa is for mathematical models of meteorology, where a small change of some apparently insignificant parameter of the system can cause a major change in the outcome of the model after running it for a time span of several days. It is important to note that the term chaos describes a property of the mathematical model, not of the outcome of the model, which in the meteorological case is still just normal weather.

A Chandra X-Ray Observatory Among many valuable images obtained from Chandra has been this view showing a huge X-ray flare associated with the supermassive black hole at the centre of our Galaxy.

In celestial mechanics the term has become very popular, and is used with various shades of meaning in different contexts. One of these is the problem of predicting the orbit of an object, such as an asteroid, that has a very close approach or repeated close approaches to a planet. The initial orbit has inevitable uncertainties due to limitations of the accuracy and coverage of positional observations, and these uncertainties are magnified greatly following a close approach to a planet, to the extent that the orbit after the close approach can be completely uncertain, or more likely that it is sufficiently uncertain that it cannot be used for predicting subsequent close approaches.

Another context in which the term arises is in the study of orbits at a commensurability. There are usually multiple resonant terms at a commensurability, associated with the eccentricity and inclination of each object. In analytical studies it is necessary to simplify the problem and to pick out just the single dominant term. However numerical integrations over long time spans show that in some cases the effects of the overlapping resonances can result in changes to the orbit that cannot be predicted by the analytical study. The nature of these changes is usually an increase in the eccentricity of the orbit, which could perhaps lead to a close approach to a planet causing the ejection of the body from the com-mensurability. This mechanism is a likely explanation of the kirkwood gaps at the 3:1 and 2:1 commensurabili-ties with Jupiter.

charge-coupled device See ccd

Charlier, Carl Vilhelm Ludvig (1862-1934) Swedish astronomer who worked at the University of Uppsala, Stockholm Observatory and the University of Lund, where he directed the observatory for thirty years. He made many detailed studies of the distribution and motions of stars and star clusters near the Sun, finding that the Milky Way was shaped like a disk, and rotated. Charlier proposed a hierarchical grouping of galaxies in an infinite universe.

Chassigny meteorite that fell in Haute Marne, France, in 1815 October; approximately 4 kg of material was recovered. Chassigny is the sole member of its subgroup of martian meteorites (SNCs). It is an unshocked, olivine-rich rock. It crystallized below the Martian surface c.1.3 billion years ago.

chemically peculiar stars Class A, B and F stars with odd chemical compositions caused by diffusion that enriches some chemical elements while depleting others. Chemically peculiar (CP) stars are subdivided into CP1 (metallic-line am stars), CP2 (magnetic ap stars), CP3 (class B mercury-manganese stars) and CP4 (helium-weak b stars). The CP1 Am stars extend to class Fm, and the CP2 Ap stars to Fp and Bp. lambda bootis stars, not caused by diffusion, are sometimes included as well.

Chicxulub Impact site in the Yucatan Peninsula, Gulf of Mexico, where a huge meteorite collided with the Earth. The crater is now buried, but geophysical surveys estimate its diameter to be between 180 and 320 km (110 and 200 mi). The collision is linked with a period of mass extinction, which marks the end of the Cretaceous Period, approximately 65 million years ago. There was a dramatic drop in the number of species present on the Earth: about 60% of all species suddenly disappeared.

Chi Cygni mira star of spectral type S7. It has the largest visual range of any known Mira star (at least 10 magnitudes) and is very red at minimum, corresponding to a temperature of c.2000 K. Its period is 407 days. It has a maximum observed magnitude of 3.3 and minimum 14.2; its distance is 228 l.y. There is strong infrared excess and circumstellar emission from molecules such as CO and SiO. The gas lost from the star's surface cools to form molecules and silicate dust grains in the circumstellar envelope. The dust then absorbs some of the star's radiation, is itself heated, and radiates in the infrared, producing the infrared excess.

Chinese astronomy Astronomy as practised in the Chinese empire from ancient times until the overthrow of the last imperial dynasty in 1911. Ancient Chinese astronomical records of eclipses and other unusual events in the sky date from as early as 1500 BC and formed part of the royal archives. Rulers could be made or unmade by portents from heaven, which explains their concern with celestial matters. Of particular concern were solar and lunar eclipses, and other transient phenomena such as novae and supernovae ('guest stars'), comets, auroras and conjunctions of the Moon and planets. Official astrologers or tianwen had to explain any unusual celestial event and, if possible, forewarn of its occurrence. Other officials, called lifa, dealt with the predictable parts of astronomy needed for cal-endrical studies. The latter made observations, kept records and developed mathematical models.

China experienced a violently unsettled period in the 3rd century BC and many early records were destroyed. However, by about 206 BC, the commencement of the Han Dynasty, a single calendar was in use everywhere. A luni-solar scheme was in use, with a correction of seven extra months in19 years, according to the METONIC CYCLE, to keep pace with the seasons. Refinements were made from time to time, and the predictive powers of the almanacs were continuously improved. An interesting achievement of Han astronomers was to record in detail the passage of Halley's Comet through the constellations during its 12 BC apparition.

In contrast to early Western astronomy, the Chinese astronomers did not emphasize the zodiac. Instead, there were 28 unequally spaced 'lunar mansions' close to the celestial equator. Their constellations were small and very numerous, and almost completely different from the Western ones.

Observations making use of an instrument - the ARMILLARY SPHERE - were made as early as 52 BC. The imperial bureaucracy later encouraged the construction of observatories and the compilation of star catalogues. An engraved Song Dynasty (960-1277) star map survives from AD 1247 in a temple in Jiangsu province, containing 1440 stars with a typical positional accuracy of 1.

In the early 17th century, Western influences began to arrive with Jesuit missionaries such as Matteo Ricci (1552-1610), and after the fall of the last imperial dynasty Chinese astronomy became aligned with that of the outside world. The ancient Chinese records have proved useful for, among other things, establishing the ages of supernovae that were not recorded in the West; the supernova of AD 1054 that led to the formation of the Crab Pulsar and the Crab Nebula is a notable example.

Chiron An outer Solar System body, c.180 km (c.110 mi) in diameter, given dual ASTEROID and COMET designations as (2060) Chiron and 95P/Chiron respectively. Its orbit, with a period of just over 50 years, has perihelion near 8.4 AU and aphelion at 18.8 AU. Chiron, therefore, crosses the path of Saturn and also approaches Uranus; consequently it is dynamically unstable. Over a time scale of order a million years (that is, much less than the age of the Solar System) it is to be expected that Chiron's orbit will change radically. It was the first object to be discovered in a class of bodies known as the CENTAURS, these being characterized as large objects inhabiting the outer planetary region and having rapidly evolving orbits.

When found, in 1977 by Charles Kowal (1940- ), Chiron appeared asteroidal in nature, but as its heliocentric distance decreased, passing perihelion in 1996 February, it began to develop characteristics associated with comets. In 1988 Chiron rapidly brightened, indicating the formation of some type of cometary coma; in the following years it developed a surrounding cloud of dust, and then the emission of cyanogen gas and other volatiles was detected. It is generally regarded as an escaped member of the EDGEWORTH-KUIPER BELT that has migrated inwards towards the Sun. See also ELST-PIZARRO; WILSON-HARRINGTON

Chladni, Ernst Florens Friedrich (1756-1827) German physicist known as the founder of acoustics. He was also a pioneer of meteoritics, the study of meteorites, which he was convinced were of extraterrestrial origin -a novel idea at the time. He assembled the first great collection of meteorites.

chondrite Most primitive and oldest of the METEORITES. Chondrites are STONY METEORITES that have not melted since their aggregation early in the history of the Solar System. They are mostly characterized by the presence of CHONDRULES, which are millimetre-sized spherules of rapidly cooled silicate melt droplets. On the basis of chemistry, chondrites are subdivided into three main groups, the CARBONACEOUS CHONDRITES (C), ORDINARY CHONDRITES (O) and ENSTATITE CHONDRITES (E). There are also two smaller classes, the Rumurutiites (R) and the Kakangari (K), each represented by only a single example. The groups have different oxygen isotope compositions, matrix, metal and chondrule contents and chondrule properties (such as size, type, and so on). The differences between the classes are primary, that is, they were established as the parent bodies accreted in different regions of the SOLAR NEBULA. In addition to these chemical classes, the chondrites are classified according to the processing that they have experienced, either thermal metamorphism or aqueous alteration. These secondary characteristics were established on the meteorites' parent bodies. A petrologic type from 3 to 6 indicates increasing thermal metamorphism. A petrolog-ic type from 3 to 1 indicates increasing aqueous alteration. Type 3 chondrites are the least altered; they are further subdivided into 3.0 to 3.9, on the basis of silicate heterogeneity and thermoluminescence.

chondrule Submillimetre to millimetre-sized spherules of rapidly cooled silicate melt droplets found in METECIRCINUS ( GEN. CIRCINI, ABBR. CIR) orites. Chondrules normally consist of olivine and/or pyroxene, with a variety of textures, depending on the starting materials and the cooling regimen. Pyroxene-rich chondrules are often composed of crystallites radiating from a point offset from the centre of the sphere; olivine-rich chondrules frequently have a blocky or barred appearance. The term 'chondrule' comes from the Greek chondros, meaning grain or seed. The origin of chondrules is still not known with certainty. At one time, they were thought to be fused drops of 'fiery rain' from the Sun. Other theories include that they were formed by the cooling of droplets produced by collisions between asteroids, or by direct condensation from a gas. Alternatively, chondrules might have formed by melting and subsequent quenching of small aggregates of dust grains in the pre-solar nebula; the heat source for such melting might have been shock waves in the nebula or energetic outflow from the Sun.

Christie, William Henry Mahoney (1845-1922) English astronomer, the eighth astronomer royal (1881-1910), who substantially improved the equipment at Greenwich Observatory, especially by acquiring the 28-inch (0.71-m) refractor. Christie was responsible for several important star catalogues produced by the Observatory, but he extended the observatory's work to include physical as well as positional astronomy, supervising programmes of photographic and spectroscopic stellar astronomy. With E. Walter maunder he initiated daily sunspot observations which led to discoveries about solar activity.

chromatic aberration Introduction of false colour into images formed by a lens. When light passes through a lens, it is bent or refracted. The degree of bending depends on the colour or wavelength of the light, so different colours follow different paths. The consequence is that the different colours in any image formed by the lens come to a focus at different points. This is chromatic aberration. It was a serious drawback in the first refracting telescopes. In a telescope, chromatic aberration appears as coloured fringes around the edges of objects. Chromatic aberration can be reduced or eliminated by using an achromat or an apochromat.

chromosphere Layer or region of the solar atmosphere lying above the photosphere and beneath the corona. The name chromosphere comes from the Latin meaning 'sphere of colour'. The term is also used for the layer above the photosphere of a star. The Sun's temperature rises to about 10,000-20,000 K in the chromosphere. The chromosphere is normally invisible because of the glare of the photosphere, but it can often be seen near the beginning and end of a total solar eclipse when it is visible as a spiky pink or red rim around the Moon's disk at the limb or edge of the Sun. Today, the chromosphere can be observed at any time across the full solar disk with a spectrograph or spectroheliograph that isolates a single colour of the Sun's light - for example, the red light of the hydrogen-alpha line (H-alpha) at a wavelength of 656.3 nm, or the violet light of ionized calcium, Ca II, at wavelengths of 396.8 and 393.4 nm, known as the calcium H and K lines. The monochromatic image made with a spectrograph is known as a spectrogram or spectroheliogram. Spectrograms show features such as fibrils, filaments, flocculi, plages and prominences. A large cellular pattern, known as the chromospheric network, is also revealed in spectrohe-liograms. It appears at the boundaries of the photosphere supergranulation, and contains magnetic fields that have been swept to the edges of these cells by the flow of material in the cell. A thin transition region separates the chromosphere and the corona. spicules containing chromospheric material penetrate well into the corona (to heights of 10,000 km/6000 mi above the photosphere) at the edges of the cells.

chromospheric network See chromosphere

Fourth-smallest constellation, lying in the southern sky between Centaurus and Triangulum Australe. Representing a drawing compass, it was one of the constellations introduced by Lacaille. Its brightest star is a Cir, mag. 3.18, spectral type A7p, distance 53 l.y.; there is a companion of mag. 8.5.

chronometer See marine chronometer

Chryse Planitia Extensive plains region on mars; it was the site of the viking 1 Lander probe. Centred near 27.0N 40.0W, it was shown by the Lander to consist of loose reddish material upon which were distributed large numbers of blocks of basaltic lava. Chryse occupies a large basin-like embayment into the cratered terrain of Mars, probably the infilled site of an ancient impact basin. A large number of prominent channels converge on the region. These channels have their origin in the eastern end of valles marineris and are of presumed fluvial origin. Recent altimetry and imaging data from mars global surveyor strongly suggest that the whole region from Chryse to the eastern end of Valles Mariner-is was once flooded.

Chryse Planitia Located near the east of Valles Marineris, Chryse Planitia was the landing site for the Viking 1 surface probe in 1976. The region is dominated by outflow channels created at a past epoch when liquid water flowed on Mars surface.

Circinus See feature article

circular velocity Velocity of a body in a circular orbit around a massive primary. Its value is given by GM/R, where M is the mass of the primary, R is the radius of the orbit and G is the gravitational constant. For the Earth the circular velocity ranges from about 7.8 km/s (4.8 mi/s) for the lowest artificial satellites, to 3.1 km/s (1.9 mi/s) for satellites in geosynchronous orbit, and to 1.0 km/s (0.6 mi/s) for the Moon. See also escape velocity

circumpolar star Star that never sets below the observer's horizon. For a star to be circumpolar at a given latitude its declination must be greater than 90 minus that latitude. For example, if the observer's latitude is 52, by subtracting 52 from 90 we get 38. Any star with a declination greater then 38 will therefore be circumpolar for that observer. At the equator, no stars are circumpolar whereas at the poles, all visible stars are circumpolar.

circumpolar star Provided it lies sufficiently close to the celestial pole, a star may describe a complete circle once per sidereal day without disappearing below the observers horizon. In the example shown, Alkaid, the end star on the Ploughs handle, is such a circumpolar star, while Arcturus is not.

circumstellar matter Material in the form of gas, dust or larger solid particles in close proximity to a star. Such material can form through several different processes and at several different stages of a star's life. The angular momentum of the material normally causes it to form a disk or ring centred on and orbiting the star. However the material may still be falling in towards the star (see fu orionis star) or be being ejected at velocities ranging from a few to many thousands of kilometres per second.

Almost all stars may be expected to be surrounded by circumstellar matter during and immediately after their formation. The collapse to a protostar will leave material behind in the form of a flattened envelope surrounding the star and a more amorphous gas cloud further out. Condensation of the more refractory elements and compounds within the envelope may lead to the formation of dust particles (see beta pictoris). These particles may then collide and stick together, building up to larger and larger sizes, and eventually perhaps forming planets. The young star usually starts to expel material at velocities of a few hundred kilometres per second (see stellar wind). Although the material is probably initially emitted isotropically, the surrounding equatorially concentrated envelope restricts the outflow to directions around the rotational poles of the protostar, resulting in bipolar flows and herbigharo objects. The stellar wind eventually evaporates the circumstellar envelope and brings a halt to planetary formation, if it is occurring. This process may explain why Uranus and Neptune are smaller than Jupiter and Saturn, since the protosun may have evaporated the remaining circumsolar material just as they were being formed. The edgeworthkuiper belt and the oort cloud are the last remnants of this material that once surrounded the Sun. With the evaporation of the dense circumstellar material close to the star, the ultraviolet radiation from the more massive stars can then penetrate into the surrounding lower density nebula producing hii regions (see also cosmogony).

Towards the end of the lives of solar-mass stars the outer layers of the star become unstable and are expelled at velocities of a few kilometres per second. The expelled material forms a cloud around the star up to a light-year across; it is heated by the ultraviolet radiation from the star. The resulting glowing material may then be seen as a planetary nebula. Subsequent higher velocity winds from the central star may sweep the centre of the nebula clear of material to produce a spherical shell that can be seen projected on to the sky as a ring (see ring nebula). The rotation of the star, its magnetic fields or a binary central star may lead to bipolar and many other shapes for the planetary nebulae.

Circumstellar material also arises within close binary stars, when one star is losing mass to form an accretion disk around the other. It also arises as the stellar winds from mira variables, wolf-rayet and other hot stars, as dust particles around carbon stars and red giants, as the regions producing emission lines within the spectra of p cyg, Be (see gamma cassiopeiae star), ttauri stars and so on. It also arises as the remnants of nova and supernova explosions.

cislunar Term used to describe an object or an event that lies or occurs in the space between the Earth and the Moon or between the Earth and the Moon's orbit.

civil twilight See twilight

CK Vulpeculae Slow nova of 1670. Its magnitude varied from 2.7 to less than 17.0. There is a suspicion that it may be a recurrent nova.

Clairaut, Alexis-Claude (171365) French mathematician and physicist who applied his expertise to celestial mechanics, winning fame for predicting the 1759 return of Halley's Comet to within 1 month. His analysis of the comet's orbit, which took into account perturbations he had discovered during work on the three-body problem, suggested that the comet would arrive at perihelion on or near April 15; the actual date was March 13. His earlier work on lunar theory led to a refined orbit (1752) and tables of lunar motion (1754) that remained unsurpassed for over a century. This work also helped to confirm Newton's theory of gravitation.

Clark, Alvan See alvan clark & sons

Clavius One of the largest lunar craters (58S 14W), diameter 225 km (140 mi); it is located in the southern lunar highlands. Clavius is an ancient impact site, the features of which have been largely obliterated by aeons of meteoric bombardment. It is often referred to as a walled depression because most of its rim is flush with the terrain outside its perimeter. Steep and rugged cliffs form the crater's inner walls, towering 3500 m (12,000 ft) above its convex floor.

Clementine Joint project between the US Strategic Defense Initiative and the national aeronautics and space administration (NASA). The objective of the mission was to test sensors and spacecraft components under extended exposure to the space environment and to make scientific observations of the moon and the near-Earth asteroid 1620 geographos. The observations included imaging at various wavelengths, including ultraviolet and infrared, laser ranging altimetry and charged-particle measurements.

Clementine The Moons south pole as imaged by the orbiting Clementine spacecraft in 1994. This picture has been compiled from some 1500 individual images. Results from Clementine support the possibility that ice deposits may be located in regions of permanent shadow at the lunar poles.

Clementine was launched on 1994 January 25. After two Earth flybys, the spacecraft entered orbit around the Moon four weeks later. Lunar mapping, which took place over approximately two months, was divided into two phases. During the first month, Clementine was in a 5-hour elliptical polar orbit with a perilune (point at which it was closest to the Moon) of about 400 km (250 mi) at 28S latitude. The orbit was then rotated to a perilune at 29N, where it remained for one more month. This allowed the entire surface of the Moon to be mapped for the first time, as well as altimetry coverage from 60S to 60N. Near-infrared and ultraviolet measurements provided the best maps of surface composition and geology yet obtained. Clementine data also indicated the presence of water ice in deep craters close to the lunar poles.

After leaving lunar orbit, a malfunction in one of the on-board computers occurred on May 7, causing a thruster to fire until it had used up all of its fuel. This left the spacecraft spinning out of control and meant that the planned flyby of Geographos had to be abandoned. The spacecraft remained in geocentric orbit until July 20, when it made its last lunar flyby before going into orbit around the Sun.

Cleomedes Lunar walled plain (27N 55E), diameter 126 km (78 mi); it is located north of the Mare CRISIUM. Cleomedes is encircled by extremely broad mountains, which rise more than 2500 m (8500 ft) above its floor. Its walls are very uneven, with many shallow impact craters. A group of mountainous peaks arises just north-west of Cleomedes' centre, south of which is the crater Cleomedes B. A forked rille first appears north-east of these peaks, then runs south-east. A sizeable crater caps the south rim.

Clerke, Agnes Mary (1842-1907) Irish astronomy writer whose works won her widespread acclaim and election to the Royal Astronomical Society as an honorary fellow (1903), an honour previously accorded to only two other women. Her most famous book was the authoritative A Popular History of Astronomy in the Nineteenth Century, published in 1885 and in several revised editions until 1902.


close binary BINARY STAR that has an orbital period of less than 30 years, implying that the two components are less than about 10 AU apart. Because of this proximity, most close binaries are SPECTROSCOPIC BINARIES and/or ECLIPSING BINARIES. MASS TRANSFER occurs at some stage in most close binaries, profoundly affecting the evolution of the component stars. If the two components in a close binary do not fill their ROCHE LOBES, the system is a detached binary. In a semidetached binary one star fills its Roche lobe and mass transfer occurs. In a CONTACT BINARY both stars fill their Roche lobes.

close binary In close binary systems, the two stars may be completely detached (a). Semidetached systems have one star whose atmosphere fills its Roche lobe, leading to mass transfer to the other component (b). Where both stars fill their Roche lobes, the pair share a common atmosphere and the system becomes a contact binary (c).

The evolution of close binaries depends on the initial masses of the two stars and their separation. The more massive star will evolve into a RED GIANT and fill its Roche lobe; material will spill through the inner LAGRANGIAN POINT on to its companion, thereby affecting its companion's evolution. The mass transfer can also alter the separation and orbital period of the binary star.

In binaries that are initially widely separated, material escaping from the Roche lobe of the evolved red giant immerses the system in material, creating a common envelope binary that contains the core of the red giant (a WHITE DWARF) and the companion star. Frictional forces cause the two components to approach, and thus the orbital period shortens. The common envelope is ejected and a CATACLYSMIC VARIABLE is left, in which the mass transfer from the companion on to the white dwarf causes the periodic outbursts seen in NOVAE, RECURRENT NOVAE, DWARF NOVAE and SYMBIOTIC STARS.

If one component of a close binary is massive enough, it may become a NEUTRON STAR or BLACK HOLE. Such binary systems are observed (see X-RAY BINARIES), but often a supernova explosion will blow the system apart into separate single stars.

closed universe Solution of Einstein's equations of GENERAL RELATIVITY in which the mass density of the Universe exceeds the CRITICAL DENSITY. This critical density is related to the HUBBLE CONSTANT and is estimated to be of the order of 9.2 X 10~27 kg/m3. This implies that the Universe will eventually collapse back to a 'Big Crunch'. See also BIG BANG THEORY

Clown Face Nebula See ESKIMO NEBULA

Cluster Part of the European Space Agency's INTERNATIONAL SOLAR TERRESTRIAL PHYSICS MISSIONS. Cluster and the SOLAR HELIOSPHERIC OBSERVATORY (SOHO) are contributing to an international effort involving many spacecraft from Europe, the USA, Japan and other countries. The four Cluster satellites complement observations by SOHO, which was launched in 1995. The first four spacecraft were lost in the failure of the first Ariane 5 launch vehicle in 1996 June, but another four were put into orbit in 2000 on Starsem Soyuz boosters. The objective of these spacecraft is to study the three-dimensional extent and dynamic behaviour of Earth's plasma environment, observing how solar particles interact with Earth's magnetic field. The identical cylindrically shaped spacecraft are in high Earth orbits, flying in a tetrahedral formation passing in and out of the Earth's MAGNETOSPHERE, crossing related features such as the magnetopause, bow shock and magnetotail. The Cluster satellites are equipped with 11 instruments provided by France, Sweden, the USA, the UK and Germany.

Cluster An artists impression of the four Cluster spacecraft orbiting in formation in Earths magnetosphere. Launched in 2000, the Cluster mission allows scientists to obtain three-dimensional measurements of particle densities and motions in near- Earth space.

cluster, star See STAR CLUSTER

cluster of galaxies See GALAXY CLUSTER


COLUMBA (gen. columbae, abbr. col)

Southern constellation originated by Petrus Plancius in 1592 from stars between Canis Major and Eridanus not previously incorporated in any figure. It supposedly represents the Biblical dove that followed Noah's Ark. The name of its brightest star, Phact, comes from the Arabic meaning 'ring dove'. j Col is a runaway star.

CNO cycle Abbreviation of carbon-nitrogen-oxygen cycle

Coalsack Dark, obscuring cloud, some 5 across, in the southern constellation crux (RA 12h 50m dec. -63). The Coalsack, like all such clouds of dust-laden gas, is seen only in silhouette, because behind it lies the bright background of the Milky Way. On a dark night the Coal-sack is very prominent: it appears to be the darkest spot in the entire sky, though that is purely an optical illusion. See also dark nebula; globule

Coathanger (Cr 399) Asterism in the constellation Vulpecula, a few degrees north-west of the tail of Sagitta (RA 19h 25m.4 dec. +2011'). Visible to the naked eye as an almost-resolved patch, the Coathanger is shown by any small optical instrument to be a group of about 15 stars. It is dominated by six 5th-magnitude stars lying in an east-west line, with a 'hook' comprising a further four stars looping south from its middle. The Coathanger has an overall magnitude +3.6 and a diameter of 60'. It is not a true cluster: the stars that make up the pattern lie at greatly differing distances, ranging from 218 to 1140 l.y.

coating Thin layers of material applied to optical components to improve their reflectivity or transmission; also the process of applying these coatings. Thin metal coatings are applied to mirror surfaces to increase their reflectivity. Aluminium is the most common but gold, silver and other metals are used for special applications such as infrared astronomy.

Refractive-index-matching materials such as magnesium fluoride are often applied to lenses to reduce the light reflected from their surfaces, thus increasing the amount transmitted. This is sometimes called blooming because the slow accumulation of dirt on early, uncoated optics resembled the bloom on a plum or grape, and it was noticed that the bloom improved the transmission of the optics. Multi-layer coatings are often used to produce more effective anti-reflection coatings.

COBE See cosmic background explorer

Coblentz, William Weber (1873-1962) American physicist and astronomer who worked for the US National Bureau of Standards and Lowell Observatory. Coblentz studied the infrared spectra of stars, nebulae and planets, pioneering the science of infrared spectroscopy. He was the first to verify planck'slaw, through his studies of black-body radiation.

Cocoon Nebula (IC 5146) Faint emission nebula surrounding a cluster of about 20 stars in the constellation cygnus (RA 21h 53m.4 dec. +4716'). The cluster has overall magnitude +7.2 and is at a distance of 3000 l.y. Star formation is probably still in progress in this rich Milky Way region. The Cocoon Nebula has an apparent diameter of 10'. It is crossed by obscuring lanes of dark material.

coelostat Two-mirror system that tracks motion of a celestial object, most usually the Sun, across the sky and allows light from it to be reflected into a fixed instrument.

A coelostat consists of a pair of plane mirrors, one of which is rotated by a motor east to west about a polar axis at half the Earth's rotation rate, thus counteracting the diurnal movement of the sky. Light from this mirror is reflected to a second, fixed mirror, which in turn directs the beam in a fixed direction, resulting in an image that is both stationary and non-rotating. Apparatus too heavy to be attached to a telescope, for example a spectrohelio-graph, may then be positioned to receive these reflected rays. The primary characteristic of the coelostat, which distinguishes it from the similar heliostat, is that the image it produces is non-rotating.

Coelostats are often used in solar observatories, mounted at the top of a tower with instruments such as spectro-graphs placed at the bottom, or even underground. This arrangement allows long focal lengths to be achieved, enabling high-dispersion spectra of the Sun to be produced. See also coude focus; siderostat

Coggia, Comet (C/1874 H1) Bright long-period comet discovered on 1874 April 17 by Jerome Eugene Coggia (1849-1919), Marseilles, France. The comet brightened rapidly during June as it approached Earth. Perihelion, 0.68 AU from the Sun, was reached on July 9. Around closest approach to Earth (0.25 AU) on July 18, the comet was of magnitude 0 and had a tail 60 long. Later in July, Comet Coggia faded rapidly as it headed southwards. The orbit is elliptical, with a period of 13,700 years.

cold dark matter Proposed as the missing mass component of galaxies. Flattened rotation curves of galaxies, and the velocities of stars at different distances from the centre of the galaxy, led astronomers to believe there was more mass present in the haloes of galaxies than was being seen. Mass in the form of baryons, or axions, was proposed as this cold dark component of galaxies. This matter might also provide enough mass to close the Universe (see closed universe). Recent measurements of the cosmic microwave background and brown dwarfs in galactic haloes have reduced the need to invoke dark matter to account for missing mass. See also missing mass problem

collapsar Obsolete name for a black hole

collimation Process of aligning the optical elements of a telescope. Certain sealed instruments, such as refractors, are generally collimated at the factory and need never be adjusted, but most telescopes with mirrors do require occasional re-alignment, especially amateur Newtonian telescopes built with Serrurier trusses where the secondary and focuser assembly are routinely separated from the primary for transport. The procedure for collimating a Newtonian telescope is usually carried out in two steps, first aligning the primary to direct light to the centre of the secondary, then adjusting the secondary to direct the light cone down the centre of the focuser.

The term is also used to describe an optical arrangement of lenses or mirrors used to bring incoming light rays into a parallel beam before they enter an instrument such as a spectroscope or x-ray telescope.

COMA BERENICES (gen. comae Berenices, abbr. com) collisionless process Process occurring within a plasma on a timescale that is shorter than the average time for collisions between particles in that plasma. An example of a collisionless process is the formation of the bow shock between the solar wind and interstellar matter.

colour index Difference in brightness of a star as measured at different wavelengths; it is used as a measure of the colour of the star. The different wavelengths are isolated by optical filters of coloured glass (for example blue and red) and light passing through each is expressed in magnitudes (B and R). The colour index B-R is zero for white stars (spectral type A0). It ranges from about 0.5 for the bluest stars to more than +2.0 for the reddest.

Colour index correlates well with the naked-eye perception of the colour of the brighter stars (the colour of fainter stars is hard to perceive for reasons to do with the physiology of the eye and the psychology of perception).

Colour index is principally a measurement of the temperature of stars. The bluest stars are hotter than 30,000 K, white stars have surfaces at temperatures of about 10,000 K. The reddest stars are very cool, say 3000 K. Cooler stars exist but emit so little light that they may best be studied by infrared techniques.

The colour index of light from a star may be changed as the light passes through interstellar space. Dust in space interacts more easily with the blue light from the star and disturbs the blue light from its straight path (scattering). Red light passes relatively freely around the dust and carries straight on to the observer's telescope. Thus the starlight is reddened (it would be more accurate to say 'de-blued'). The consequent increase of the star's colour index caused by the interstellar dust is called a colour excess due to interstellar reddening.

The colour index of stars is also modified by the presence of atoms in their atmospheres. The light in different wavebands is affected to different degrees. It is possible to isolate the various effects by measuring the colour index between different pairs of wavelengths. A colour index formed with ultraviolet and with blue light (U-B) may be compared with (B-R) in a colour-colour plot. The position of a star in the plot gives clues about its chemical composition, temperature and interstellar reddening.

The colour index is also used for Solar System bodies and can give clues about their mineral compositions.

colour-magnitude diagram Plot of the magnitude of a collection of stars versus their colour index. It is used as a diagnostic tool to study star clusters. See also hertzsprung-russell diagram

Columba See feature article

Columbia Name of the first space shuttle orbiter. It flew in 1981.

Columbus Orbital Facility European module to be attached to the international space station in 2004.

colure Great circle on the celestial sphere that passes through the two celestial poles. The equinoctial colure passes through the celestial poles and the vernal and autumnal equinoxes. The solstitial colure passes through the celestial poles and the winter and summer solstices.

coma aberration that makes off-axis star images grow small tails, giving them a comet-like appearance. Simple reflecting telescopes such as the newtonian telescope sometimes use a parabolic mirror to collect light and form an image. A perfect parabola will produce a perfect image in the centre of the field of view, that is, on its optical axis, and images close to the centre will also be very sharp. However, as the distance from the optical axis increases, the images start to grow tails pointing away from the centre. Eliminating these tails requires at least one extra optical component. Coma is not confined to mirrors but can appear in many optical systems.

Faint northern constellation, given permanent status in 1551 by Gerardus Mercator from stars previously regarded as belonging to Leo, though it had been referred to in ancient times. It supposedly represents the hair of Queen Berenice of Egypt, which she cut off in gratitude to the gods for the safe return of her husband from battle. Its brightest star is p Com, mag. 4.23. FS Com is a red giant that varies between 5th and 6th magnitudes every 2 months or so. 24 Com is an easy double for small telescopes, mags. 5.0 and 6.6, showing contrasting colours of orange and blue. Extending southwards for several degrees from 7 Com is the Coma Star Cluster, a V-shaped scattering of faint stars, traceable in binoculars. M64 is the black eye galaxy, and NGC 4565 is an edge-on spiral galaxy about 20 million l.y. away. Several members of the virgo cluster of galaxies can be found in Coma, most notably the face-on spiral M100; it also contains the more distant coma cluster.

coma, cometary Teardrop-shaped cloud of gas and dust surrounding a comet's nucleus. It is produced as a result of increased exposure to solar radiation once the comet is sufficiently close to the Sun (usually within 31 AU).

coma, cometary Release of volatile materials due to solar radiation leads to rapid development of an extensive temporary atmosphere, the coma, around a cometary nucleus. These Hubble Space Telescope images show the evolution of the coma surrounding the nucleus of C/1995 O1 HaleBopp.

Coma Berenices See feature article

Coma Cluster (Abell 1656) Nearest rich galaxy cluster, in the direction of coma berenices. It lies at a distance of about 330 million l.y., and has over 450 member galaxies with known redshifts (and galaxy counts suggest several thousand members in all). The structure of the X-ray gas shows prominent clumps, suggesting that smaller groups have recently fallen into the cluster. These clumps also contain an excess of galaxies with recent (but not ongoing) star formation, indicating interaction between the cluster environment and the individual galaxies. The cluster mass as estimated from galaxy velocities is 2 thousand million million solar masses. The Coma cluster is part of the extensive Perseus-Pisces supercluster of galaxies, which stretches almost halfway around the sky from our vantage point.

Coma Cluster This rich galaxy cluster, probably containing more than 1000 members, lies 350 million l.y. away. As in other such clusters, the most prominent members are giant elliptical galaxies.

comes Obsolete term for the fainter component of a binary star; it is now referred to as the companion.

comet Small Solar System body, consisting of frozen volatiles and dust. Comets are believed to be icy planetesi-mals remaining from the time of the Solar System's formation 4.6 billion years ago. The word 'comet' derives from the Greek kometes, a long-haired star, which aptly describes brighter examples.

comet Radiation pressure and solar wind effects result in a comets tail always pointing away from the Sun. An interesting consequence of this is that a comet therefore departs the inner Solar System tail-first.

The main, central body of a comet is the nucleus, typically only a few kilometres in diameter. The nucleus of Comet 1P/halley has dimensions of about 15 X 8 km (9 X 5 mi), but most comet nuclei are smaller. At large distances from the Sun, a cometary nucleus is inactive, and indistinguishable from an asteroid. Comet nuclei are thought to have a dark outer crust, which, when heated by solar radiation, particularly at distances substantially less than that of Jupiter (5 AU) from the Sun, can crack to expose fresh volatile material below. Sublimation of frozen gases leads to development of a temporary atmosphere, or COMA, surrounding the nucleus. New comets, or known PERIODIC COMETS returning to PERIHELION, are usually found at this stage, as fluorescence in the coma causes them to brighten and appear as a teardrop-shaped fuzzy spot. As a comet approaches the Sun more closely, a TAIL or tails may form. Brighter comets often show a straight ion tail (type I) and a curved dust tail (type II). Enveloped in the coma, the nucleus is not directly visible to Earth-based telescopes, but a bright spot may mark its position, and jets of material can be seen emerging from it in the brightest and most active comets. The occurrence of such features indicates that much of the gas emission from a comet comes from persistent active regions, which may cover 15-20% of the nucleus. As the nucleus rotates, active regions are alternately 'turned on' by solar heating or abruptly shut down as they are carried into shadow. Such behaviour led to the distinctive spiral structure in the coma of C/1995 O1 HALE-BOPP. Intermittent gas and dust emission can produce other features, including hoods and shells in the coma.

The whole comet may be enveloped in a vast tenuous cloud of hydrogen, detectable at ultraviolet wavelengths from spacecraft. Gas jets emerging from the nucleus carry away dust particles, which depart on parabolic trajectories to form a curved dust tail. Dust tails appear yellowish, the colour of reflected sunlight as confirmed by spectroscopy. The dust particles appear to be silicate grains, some 10 micrometres in size. Bigger flakes of dusty material, perhaps a few millimetres in size, are also carried away; in large numbers, these METEOROIDS can end up pursuing a common orbit around the Sun as a METEOR STREAM. Comets' tails point away from the Sun. Under certain circumstances, however, thin sheets of ejected dusty material may appear to point towards the Sun from the coma, forming an ANTITAIL, as a result of perspective.

Gas emerging from the nucleus is rapidly ionized by solar ultraviolet radiation. Positively charged ions are picked up by the interplanetary magnetic field in the SOLAR WIND and dragged away from the coma to form an ion tail (also commonly described as a plasma tail). In contrast with the dust tail, a comet's ion tail appears relatively straight and may show a marked bluish colour, which results from emissions at 420 nm wavelength, characteristic of carbon monoxide (CO) excitation. The ion tail can exhibit knots and twists resulting from changes in the interplanetary magnetic field in the comet's vicinity; reversals of the field's polarity can result in complete shearing of the ion tail - a DISCONNECTION EVENT - after which a new, differently oriented ion tail may develop.

Comas and ion tails show emission spectra characteristic of a number of molecular species comprising combinations of hydrogen, carbon, nitrogen, oxygen and sulphur, such as water (H2O), carbon monoxide (CO), carbon dioxide (CO2) and radicals such as cyanogen (CN) and hydroxyl (OH). Methane (CH4) and ammonia (NH3) are certainly present but are difficult to detect. When a comet is very close to the Sun, metallic emissions (particularly from sodium) occur; observations of C/1995 O1 Hale-Bopp in 1997 revealed the presence of a third, distinct sodium tail.

Although comets can be ejected from the Solar System, never to return, on hyperbolic trajectories following planetary encounters, none has yet been shown to enter from interplanetary space: the comets that we observe are grav-itationally bound to the Sun. A vast reservoir of cometary nuclei, the OORT CLOUD, surrounds the Solar System to a distance of 100,000 AU. Perturbations by passing stars or giant molecular clouds in the course of the Sun's orbit around the Galaxy can cause Oort cloud nuclei to fall inwards. There is strong evidence that such nuclei accumulate in a flattened disk - the EDGEWORTH-KUIPER BELT - at a distance of up to 1000 AU. From this region, further perturbations may lead nuclei to plunge inwards to perihelion as new long-period comets. Close encounter with one of the planets, particularly Jupiter, can dramatically alter the comet's orbit, changing some bodies from long-to short-period comets.

The division between short- and long-period comets is set purely arbitrarily at an orbital duration (perihelion to perihelion) of 200 years. Most comets have narrow elliptical orbits, which may be either direct or retrograde; lP/Halley, for example, has a 76-year retrograde orbit. Comet 2P/ENCKE has the shortest period, of 3.3 years. In total, some 150 short-period comets are known.

Comets lose material permanently at each perihelion passage and must eventually become defunct; the lifetimes of short-period comets are probably of the order of 10,000 years. The ultimate fate of a short-period comet appears to vary. Some disperse entirely into a diffuse cloud of dust and gas. Comet nuclei are fragile and may break into smaller fragments close to perihelion. Others, depleted of volatile material, may simply become inert, leaving a dark asteroid-like core with no tail activity.

The brightness of a comet is expressed as the equivalent stellar magnitude, as in the case of nebulae. Most comets show fadings and outbursts caused mainly by varying nuclear jet activity and solar effects. Predicting the apparent brightness of comets is notoriously difficult. It does appear that proximity to the Sun is a more significant factor than closeness to Earth. Comets are usually at their most active and, therefore, brightest just after perihelion.

Comets are normally named after those who discover them, up to a maximum of three names. In some cases -increasingly common early in the 21st century - comet discovery by automated telescopes or spacecraft is reflected in their names, examples including the many named after LINEAR or SOHO. Some, notably 1P/Halley, 2P/Encke and 27P/CROMMELIN, are named after the analyst who first determined their orbit. Short-periodic comets are identified by the prefix P/ and a number indicating the

comet Radiation pressure and solar wind effects result in a comet's tail always pointing away from the Sun. An interesting consequence of this is that a comet therefore departs the inner Solar System tail-first.

In most years, perhaps 25 comets become sufficiently bright to be observed with amateur telescopes. Spectacular naked-eye comets are rare and unpredictable: the brightest are usually new discoveries, making one of their first visits to the inner Solar System. The more predictable, short-period comets tend to be fainter, having already lost some of their volatile material. Among the brightest comets have been the kreutz sungrazers.

It has been speculated that the frequency of truly bright 'great' comets has been remarkably low in recent times compared with, say, the late 19th century. Statistically, however, the 20-year gap between C/1975 V1 west and C/1996 B2 hyakutake is not atypical. The next spectacular comet may appear at any time.

cometary globule Fan-shaped reflection nebula that is usually closely associated with a pre-main-sequence star, such as a ttauri star. The globule's appearance can superficially resemble that of a comet, but the two types of object are quite unrelated. There may be a bright rim to the 'head', and the 'tail' can be several light-years in length. Other recognized shapes for cometary globules include: an arc; a ring, sometimes with a star at the centre or on the rim; a biconical (hourglass) nebula with star at the 'waist'; and a linear wisp protruding from a star. Most cometaries shine by reflecting the light of their allied star, though some are ionized by the ultraviolet radiation of hot central stars (see hii region).

The heads of globules are denser regions within a larger nebula. The ultraviolet radiation and stellar wind from the associated star ionizes the gases on the outer surface of the globule causing it to glow, so producing the bright rim around the head. The gas and radiation pressure also drives away surrounding material to leave the dark tail formed of nebula material sheltered by the head.

Recently, the biconical type of nebula has been generalized to include any bipolar system that consists of two separate nebulae with a star lying between. Enlarging the class to incorporate the bipolar nebulae makes cometary globules evolutionarily less homogeneous. The subclass of bipolar nebulae are not all indications of stellar youth: some red giants lose mass by a bipolar flow, thereby generating bipolar nebulae.

The stars associated with cometary globules are often embedded within dusty equatorial disks, which are sometimes thick enough to render the stars optically invisible. Bright infrared sources are observed instead, which are the result of emission from the dust. The various cometary globule shapes may all be explained as viewing from different directions the basic model of a star plus a dust disk, with the disk constraining the star's light to shine into a bicone. See also hubble'svariable nebula

A similar effect occurs at the outer part of the asteroid belt. The Hilda group of asteroids at the 3:2 com-mensurability with Jupiter, and the asteroid Thule at 4:3, can only remain in orbits so close to Jupiter because their commensurabilities protect them from close approaches. The trojan asteroids are an extreme example of this effect, as they move in the same orbit as Jupiter, but their 1:1 commensurabilities with Jupiter ensure that they remain far from Jupiter - close to 60 ahead or behind.

There are many more commensurabilities in the satellite systems of the major planets than would be expected by chance, and there has been much recent effort to explain their origin. In the Jupiter system there is a complex commensurability involving the three satellites Io, Europa and Ganymede, which is a combination of two 2:1 commensurabilities. It is likely that this was formed by tidal evolution of the orbits due to the tides raised in Jupiter by the satellites, combined with the effect of energy dissipation of the tide raised in Io by Jupiter. In the Saturn system, the 2:1 commensurability between Mimas and Tethys, and that between Enceladus and Dione, were also probably caused by tidal evolution. The likely explanation of the 4:3 commensurability between Titan and Hyperion is that Hyperion is the sole survivor from many objects originally in the region, and its resonance has protected it from close approaches to Titan. There are also many effects of commensurabilities in the ring systems of the planets.

Common, Andrew Ainslie (1841-1903) English engineer and amateur astronomer who designed and built large reflecting telescopes and took some of the finest astronomical photographs of his time. In the late 1870s he obtained a 36-inch (0.9-m) mirror from the English optician George Calver (1834-1927) and designed and supervised the construction of a telescope and a rotating observatory to house the instrument. In 1880 Common took his first photographs with this telescope, and for the next five years he continued to improve his photographic technology. His photographs of the Orion Nebula (M42), which showed stars that were invisible to visual observers, demonstrated the value of astrophotography.

Commonwealth Scientific and Industrial Research Organisation (CSIRO) One of the world's largest and most diverse scientific research organizations, with a history dating back to 1916. CSIRO is the umbrella organization for the australia telescope national facility.

T cometary globule A strong outflowing stellar wind from a recently formed star draws this diffuse nebula into a comet-like shape. The young star's intense ultraviolet radiation ionizes gas in the nebula.

commensurability There are many cases of pairs of planets or satellites whose orbital periods are close to the ratio of two small integers, and these are called 'commen-surabilities'. They cause greatly increased perturbations between the two bodies by a resonance effect, and it is a major challenge of celestial mechanics to calculate these perturbations and the resulting effects on the orbits. The principal examples among the planets are the 5:2 commensurability between Jupiter and Saturn, 3:1 between Saturn and Uranus, 2:1 between Uranus and Neptune, and 3:2 between Neptune and Pluto. For Neptune and Pluto the commensurability is very close, and a special type of motion called a libration occurs. This prevents the two planets ever coming near to each other, even though their orbits cross, because they are never at that part of their orbits at the same time.

Cone Nebula A striking region of bright and dark nebulosity associated with star formation in the Milky Way in Monoceros. The Cone Nebula is named for the tapering dark intrusion silhouetted against the bright emission nebulosity in the vicinity of the young Christmas Tree Cluster.

compact object Object, such as a WHITE DWARF or NEUTRON STAR, that is of high mass contained within a volume of space, indicating that it is formed of DEGENERATE MATTER. See also BLACK HOLE

companion Fainter of the two components of a BINARY STAR. Often this is the less massive component, lying farther from the centre of mass. It is sometimes called the secondary. See also PRIMARY

comparator Instrument that enables two photographs of the same area of sky, taken at different times, to be rapidly alternated in order to reveal objects that have changed position or brightness. The most common example is the BLINK COMPARATOR, in which discordant images will appear to blink on and off or pulsate. The STEREO COMPARATOR uses binocular vision to make them appear to stand out of the plane of the picture, while in another type of comparator, they appear a different colour from the unchanging stars. The instruments are particularly used by hunters of novae and asteroids.

Compton effect Loss of energy by, and consequent increase in wavelength of, a PHOTON that collides with an ELECTRON and imparts some of its energy to it. In the inverse Compton effect, the photon gains energy from the electron.

Compton Gamma Ray Observatory Second of NASA's GREAT OBSERVATORIES, launched from the Space Shuttle Atlantis on 1991 April 5. The observatory was named in honour of Arthur Holly Compton (1892-1962), who was awarded the Nobel Prize for physics for work on gamma-ray detection techniques. Its four instruments provided unprecedented coverage of the electromagnetic spectrum, from 30 keV to 30 GeV. The Burst and Transient Source Experiment (BATSE) was designed to detect short outbursts. The Oriented Scintillation Spectrometer Experiment (OSSE) studied the spectrum of gamma-ray sources. The Imaging Compton Telescope (COMPTEL) mapped the gamma-ray sky at medium energies, and the Energetic Gamma Ray Experiment Telescope (EGRET) made an all-sky map of high-energy sources.

The observatory re-entered the Earth's atmosphere on 2000 June 4. During its lifetime, the telescope detected more than 400 gamma-ray sources, ten times more than were previously known. The origins of many of these sources remain unknown. Prior to Compton, 300 gamma-ray bursts had been detected; observations from the satellite recorded a further 2500.

One of BATSE's most important contributions was an all-sky map of gamma-ray burst positions, which confirmed that they originate well beyond our Galaxy. One of the major discoveries made by EGRET was a new class of quasars known as BLAZARS. The observatory also discovered a number of gamma-ray PULSARS, while observations of the galactic centre by OSSE revealed gamma radiation from the annihilation of positrons and electrons in the interstellar medium. See also GAMMA-RAY ASTRONOMY

Compton wavelength Length scale at which the wave-nature of a particle becomes significant. It is given by A mc
where h is the PLANCK CONSTANT, m is the rest mass of the particle and c is the velocity of light. For an electron its value is thus 2.4 X 10~12 m. The Compton wavelength is of significance for HAWKING RADIATION from BLACK HOLES and in COSMOLOGY, where it determines the earliest moment that can be understood using the laws of physics (the PLANCK TIME).

Cone Nebula (NGC 2264) Tapered region of dark nebulosity in the constellation MONOCEROS (RA 06h 41m.1 dec. +0953'); it obscures some of the extensive EMISSION NEBULA in the region of the Christmas Tree Cluster. The bright nebulosity into which the Cone Nebula intrudes covers an area about 20' across and lies at a distance of 2400 l.y. The Cone Nebula is difficult to observe visually: it is best revealed in long-exposure images.

conic section Curve that is obtained by taking a cross-section across a circular cone. The curve will be a circle, ellipse, parabola or hyperbola depending on the angle at which the cross-section is taken. The significance for astronomy is that these curves are also the possible paths of a body moving under the gravitational attraction of a primary body.

conjunction Alignment of two Solar System bodies with the Earth so that they appear in almost the same position in the sky as viewed from Earth. The INFERIOR PLANETS, Mercury and Venus, can align in this way either between the Sun and the Earth, when they said to be at INFERIOR CONJUNCTION, or when they lie on the opposite side of the Sun to the Earth, and are at SUPERIOR CONJUNCTION. The superior planets can only come to superior conjunction. When a planet is at conjunction its ELONGATION is 0.

conjunction Superior planets come to conjunction with the Sun when on the far side of it, as seen from Earth (and are therefore lost from view). The inferior planets, Mercury and Venus, can undergo conjunction at two stages in their orbit. At superior conjunction, they lie on the far side of the Sun from Earth, while at inferior conjunction they are between the Sun and the Earth. Under certain circumstance, Mercury and Venus can transit across the Suns disk at inferior conjunction.

The strict definition of conjunction is when two bodies have the same celestial longitude as seen from Earth and because of the inclination of the various planetary orbits to the ecliptic, exact coincidence of position is rare. The term is also used to describe the apparent close approach in the sky of two or more planets, or of the Moon and one or more planets.

constellation Arbitrary grouping of stars, 88 of which are recognized by modern astronomers. Various constellation systems have been developed by civilizations over the ages; the one we follow is based on that of the ancient

Greeks, although it actually originated around 4000 years ago with the Sumerian people of the Middle East. The Chinese and Egyptian systems were completely different. In particular, the Chinese of the 3rd century ad had 283 constellations incorporating nearly 1500 stars. Many of the Chinese figures were small and faint, and some consisted merely of single stars.

A constellation pattern has no real significance: the stars are at very different distances from us and appear close together only because of a line-of-sight effect. This is well demonstrated by a and p Centauri, which lie side-by-side in the sky although in fact a is 4 l.y. away while p is over 100 times as distant.

ptolemy gave a list of 48 constellations in his Almagest. He did not include the far southern sky, which was below the horizon from his observing site in Alexandria, and there were gaps between his constellations; but all the figures he listed are retained by modern astronomers, although with somewhat different boundaries. Many of those he named are drawn from ancient mythology; for example the legend of Perseus and Andromeda is well represented. The gaps left by Ptolemy were filled in by others, notably Johannes hevelius.

Twelve new southern constellations, mostly representing exotic animals, including a bird of paradise and a flying fish, were formed in around 1600 by two Dutch navigators, Pieter Dirkszoon Keyser and Frederick de Houtman. A far more detailed survey of the southern sky was made in 1751-52 by the Frenchman lacaille, who catalogued nearly 10,000 stars from the Cape of Good Hope, modern South Africa. He introduced 14 new constellations to fill the gaps between Keyser and de Hout-man's figures, mostly named after instruments of science and the arts, such as the Microscope, the Telescope and the Painter's Easel. Lacaille also divided up the large Greek constellation Argo Navis into three more manageable sections - Carina, Puppis and Vela.

After Lacaille there was a period when almost every astronomer who produced a star map felt obliged to introduce new constellations, often with cumbersome names such as Globus Aerostaticus, the hot-air balloon, Machina Electrica, the electrical machine, and Officina Typograph-ica, the printing shop, all of which have been rejected. Another of these rejected constellations - Quadrans Muralis, the Mural Quadrant, invented by the Frenchman J.J. Lalande - is remembered because of the January meteor shower called the quadrantids, which radiates from the area it once occupied.

Finally, in 1922, the international astronomical union put matters on a more systematic footing. They fixed the accepted number of constellations at 88, and in 1928 adopted rigorous boundaries to the constellations based on circles of right ascension and declination, a system originally introduced for just the southern constellations in 1879 by Benjamin gould. The accompanying table gives the official list and their abbreviations.

It is clear that the constellations are very unequal in size and importance; they range from the vast hydra to the tiny but brilliant crux. It is also true that there are some constellations that seem to have no justification for a separate existence, horologium and leo minor being good examples. Only three constellations have more than one first-magnitude star (taking the first-magnitude limit as the conventional 1.49) - centaurus, crux and orion. Note also that although Ophiuchus is not reckoned as a zodiacal constellation, it does cross the zodiac between scorpius and sagittarius. See also individual constellations; table, pages 94-95

contact binary Binary star in which both components fill their roche lobes. Common envelope binaries are systems in which material has escaped from the Roche lobes to surround both stars. The exchange of mass and energy within contact binaries is not well understood.

The best-known stable contact binaries are the w ursae majoris stars, which consist of stars of spectral type G and K, with a typical mass ratio of 2:1. Material from the stars has escaped the Roche lobes and surrounds the components in a common envelope. Other observed stable contact binaries have massive early-type components or consist of cool supergiants.

If a star in a contact binary is much more massive and larger than its companion, then it may undergo a catastrophic tidal instability, with the less massive star being pulled into its envelope. Resulting shock waves dissipate energy, expelling the outer envelope of the massive component and slowing the orbit of the binary system. planetary nebulae with double cores may be the result of such a process.

continuous creation Idea that matter is constantly created in the gaps between galaxies so that the perfect cosmological principle applies in an expanding universe. This was a necessary feature in the steady-state theory of cosmology.

continuous spectrum (continuum) Spectrum containing photons with a smooth distribution of wavelengths; it has no breaks or gaps and no absorption lines or emission lines, though these can be superimposed.

continuum Property that is seamless or has no 'smallest value'. A continuum can be subdivided into infinitely small pieces and is still the same thing, whereas a quantized medium has a 'smallest' quantity. Space in Isaac Newton's theory of mechanics is a 'continuum', and spacetime in relativity is a continuum.

Contour NASA discovery programme mission, to be launched in 2002 July, to fly past three comets, taking images, making comparative spectral maps of their nuclei and analysing the dust flowing from them. The spacecraft will fly past Comet 2P/encke in 2003 November, Comet 73P/schwassmann-wachmann-3 in 2006 June and Comet 6P/d'arrest in 2008 August.

convection Process of energy transfer in a gas or liquid as a result of the movement of matter from a hotter to a colder region and back again. For astronomy, convection is mostly of significance in stellar interiors and in planetary atmospheres. In planetary atmospheres, convective circulation combined with Coriolis forces leads to the formation of HADLEY CELLS, where warm gas rises and moves away from the equator, eventually cooling and returning at lower levels. Convection in stars is still poorly understood, although numerical models are improving. The models are based upon mixing length theory. This assumes that the convective elements move a characteristic length, known as the mixing length, releasing all their excess energy only at the top of their movement, or absorbing their entire energy deficit only at the bottom. The transfer of energy thus occurs as a result of both the upward and downward phases of convective motion. Between the top and bottom of the mixing length the material changes ADIABATICALLY.

conjunction Superior planets come to conjunction with the Sun when on the far side of it, as seen from Earth (and are therefore lost from view). The inferior planets, Mercury and Venus, can undergo conjunction at two stages in their orbit. At superior conjunction, they lie on the far side of the Sun from Earth, while at inferior conjunction they are between the Sun and the Earth. Under certain circumstance, Mercury and Venus can transit across the Sun's disk at inferior conjunction.

Cooke, Troughton & Simms English firm of telescope and scientific instrument manufacturers. John Troughton, Sr (c.1716-88) made high-precision sextants, quadrants and other scientific instruments from the mid-1750s. Around 1780, his nephew John Troughton, Jr (c.1739-1807) perfected a means of precisely dividing the circular scales for surveying and astronomical devices. John, Jr's brother, Edward Troughton (1756-1835), made the first mural circle, an innovative transit telescope, designed in 1806 and completed in 1812 for GREENWICH OBSERVATORY.

William Simms (1793-1860) improved methods for dividing transit circles. In 1824 he joined forces with the Troughtons, forming the partnership of Troughton & Simms. He was eventually succeeded by his son William Simms, Jr (1817-1907) and the latter's cousin, James Simms (1828-1915); James' two sons, William Simms III (1860-1938) and James Simms, Jr (1862-1939), also joined the family's optical business.

The Englishman Thomas Cooke (1807-68), a maker of high-quality refracting telescope lenses, was joined by his sons Charles Frederick Cooke (1836-98) and Thomas Cooke, Jr (1839-1919). At their Buckingham Works in York, England, Thomas Cooke & Sons made many fine large refracting telescopes for observatories round the world. The telescope built in 1881 for Liege University, with a 10-inch (250-mm) lens, was widely regarded as one of their finest. In 1871 the firm completed what was then the world's largest refractor, a 25-inch (0.63-m) instrument for Robert Sterling Newall (1812-89). Cooke's master optician was Harold Dennis Taylor (1862-1943), who invented the 'Cooke photographic lens' in the 1880s. In 1892 he made the first three-element apochromatic lens, designed to virtually eliminate chromatic aberration and to have sufficient colour correction for use in visual studies and photography.

In 1922 Thomas Cooke & Sons bought out Troughton & Simms, forming by merger the firm of Cooke, Troughton & Simms, Ltd. The company continued to make telescopes and other scientific instruments until the late 1930s, when it sold its telescope-making operation to GRUBB, PARSONS & CO.

Coordinated Universal Time (UTC) Tmescale derived from atomic clocks and based on civil time as kept at the Greenwich meridian. Popularly known as GREENWICH MEAN TIME (GMT), UTC is used as the basis for generating radio time signals. However, variations in the Earth's rotation mean that time as kept by atomic clocks (UTC) and that derived from observations of the stars (UT1, see UNIVERSAL TIME) gradually diverge. If this were allowed to go unchecked, the long term effect would be for time kept by clocks and that as shown by the Sun to become increasingly out of step. In order to avoid this, the two are kept within 0.9 second of one another through the periodic introduction of a LEAP SECOND into the UTC timescale, thus causing it to differ from INTERNATIONAL ATOMIC TIME (TAI) by an integral number of seconds.

co-orbital satellite Natural satellite or moon of one of the major planets occupying the same orbital distance as a similar object. An example of such a pair is the Saturnian satellites EPIMETHEUS and JANUS. In some cases small satellites may occupy the LAGRANGIAN POINTS associated with one of the larger moons of the giant planets, for example, CALYPSO and TELESTO, the satellites of Saturn linked with TETHYS. Ring systems might also be regarded as being sets of tiny co-orbiting satellites. See also SHEPHERD MOON; TROJAN ASTEROID

Copenhagen Observatory One of the world's oldest astronomical institutions, established in 1642 to provide accurate stellar positions for maritime navigation. Two hundred years later, it moved from its city centre site to the suburbs of Copenhagen. Today the observatory is operated by the University of Copenhagen as part of the Niels Bohr Institute for Astronomy, Physics and Geophysics. Its researchers use the telescopes of the european southern observatory as well as the Danish 1.54-m (60-in.) telescope, completed in 1979 at la silla observatory.

Copernican system heliocentric theory of the Solar System advanced by Nicholas copernicus, in which the Earth and the other planets revolve around the Sun, and only the Moon revolves around the Earth; the apparent daily motion of the sky is a consequence of the Earth's axial rotation, and the stars are otherwise motionless because they are extremely distant.

Copernican system Copernicus model of the Solar System put the Sun at the centre, with Earth and the other planets in orbit about it. Replacing the earlier geocentric system, this removed Earth from any special position in the Solar System.

Shortly after 1500, Copernicus became interested in the problems inherent in the established ptolemaic system of an Earth-centred cosmology. Why, for instance, if the planets revolved around the Earth, did the retrograde, or reverse, motions of Mars, Jupiter and Saturn synchronize with the terrestrial year? The ancient geocentric cosmology required the operation of a host of eccentric circles and epicycles to explain such motions, whereas a heliocentric system could account for them simply as a consequence of the Earth's orbital motion.

By 1513 Copernicus had developed a workable heliocentric model that addressed various problems in planetary dynamics. It appears to incorporate elements drawn from the work of the Arab astronomers al-tusI and Ibn al-Shatir (1304-75/6). The Copernican system seriously threatened the universally accepted physics of aristotle, and Copernicus feared ridicule from other scholars. However, he was finally prevailed upon to publish his de revolutionibus orbium coelestium just before his death in 1543. The Copernican system was still based on circular rather than elliptical orbits (and was as a consequence worse at predicting planetary positions than the Ptolemaic system it sought to replace), and it placed the Sun at the centre of the cosmos, in a privileged position; but it did offer a logical sequence of planetary revolution periods that increased with distance from the Sun, an important step forward. More importantly, it marked the beginning of the end for geocentricism.

Copernicus, Nicholas (1473-1543) Polish churchman and astronomer who proposed that the planets revolve around a fixed Sun. ('Copernicus' is the Latinized form of his name, by which he is almost always known; the Polish form is Mikolaj Kopernik.) Through his study of planetary motions, he developed a heliocentric theory of the Universe in which the planets' motions in the sky were explained by having them orbit the Sun, as opposed to the geocentric (Earth-centred) model that had been favoured since the days of aristotle and ptolemy. The motion of the sky was now simply a result of the Earth's axial rotation, and, relative to the celestial sphere, the stars remained fixed as the Earth orbited the Sun because they were so distant. An account of his work, de revolutionibus orbium coelestium, was published in 1543.

Copernicus studied canonical law and medicine at the universities of Cracow, Bologna and Padua. At Bologna (1496-1500), he learned astronomy and astrology from Domenico de Novara (1454-1504), using his observations of the 1497 March 9 lunar occultation of Aldebaran to calculate the Moon's diameter. Copernicus returned to Poland in 1503, as a canon in the cathedral chapter of Warmia; he also practised medicine but maintained a lively interest in astronomy, being invited (1514) to reform the julian calendar.

In Copernicus' day it was universally accepted that the Earth was solidly fixed at the centre of the cosmos. Ptolemy had explained planetary motions by having epicycles turn on larger circles around each planet's orbit (see ptolemaic system). Copernicus explored the consequences of fixing the Sun in the centre of the planetary system, with the Earth and the other planets orbiting it. Being a classical scholar rather than a self-conscious innovator, he began to examine the ancient Greek writers to see whether precedents for a heliocentric system existed, and found several, most notably in the writings of Heraclides of Pontus (388-315 bc) and aristarchus of Samos. Precisely what motivated Copernicus' radical departure from traditional astronomy is not known, but it is likely that he was also influenced by works that were critical of the Ptolemaic system, including the Epitome of Ptolemy's Almagest by regiomontanus and Disputations Against Divinatory Astrology by Giovanni della Mirandola (1463-94). While in Cracow, he may have encountered the writings of al-tuIsiI.

In the copernican system, the epicycles of the superior planets (Mars, Jupiter and Saturn) could be discarded because their motions were explained by the effects of the Earth's orbit around a centralized Sun; for the inferior planets (Mercury and Venus) Copernicus centred their epicycles on the Sun instead of on separate carrying circles. Mercury, the swiftest planet, was closest to the Sun, and Saturn, the slowest, orbited at the outer bound of the Solar System, the other planets falling into place according to their periods of revolution.

In 1514 Copernicus first described his new model of the Solar System in a small tract, the commentariolus ('Little Commentary'), which he distributed to only a few colleagues. The heliocentric theory was set forth in greater detail by his student rhaeticus in the work Narratio prima, published in 1540/41. Copernicus' famous book De revolutionibus orbium coelestium, considered to be the definitive statement of his system of planetary motions, did not appear until the year of his death. It was Rhaeticus who had finally persuaded him to publish. Copernicus' reticence had had nothing to do with fear of persecution by the Catholic Church: as an ecclesiastical lawyer, he knew that the Church had no dogmatic rulings on scientific matters. What he had feared was academic ridicule in Europe's universities for seeming to contradict common sense.

In De revolutionibus, Copernicus refuted the ancient arguments for the immobility of the Earth, citing the advantages of the new Sun-centred model, which correctly ordered the planets by the rate at which they appeared to move through the heavens, and explaining the phenomenon of retrograde motion. He correctly explained that the motions of the stars that would be produced by a moving Earth were not observable simply because the stars were so far away - 'so vast, without any question, is the divine handiwork of the Almighty Creator'.

Following the publication of De revolutionibus, most astronomers considered the Copernican system as merely a hypothetical scheme - a means of predicting planetary positions, lacking any basis in physical reality and impossible to confirm by astronomical observations. Most astronomers continued to follow Aristotle's physics, in which the Earth was viewed as a perfect, immovable body rather than a transient, moving entity, while the Ptolemaic system actually gave more accurate planetary positions than Copernicus' original scheme. Not until the discoveries of Johannes KEPLER and GALILEO did the Copernican system begin to make physical as well as geometrical sense. As a consequence of Galileo's writings, De revolutionibus was in 1616 placed on the Index of prohibited books by the Catholic Church. The book was not actually banned, however, but small alterations were introduced to make the Copernican system appear entirely hypothetical. But by then Copernicus' book was already being superseded by Kepler's Astronomia nova (1609) and the Rudolphine Tables (1627), which provided the basis for explaining the irregularities in the motions of the planets that had not been satisfactorily accounted for in Copernican system.

Copernicus Large lunar crater (10N 20W), known for its complex system of EJECTA and bright RAYS. Situated on the north shore of Mare Nubium, Copernicus dominates the Moon's north-west quadrant. The crater's lava-flooded floor, 92 km (57 mi) wide with multiple central peaks, lies nearly 4 km (2.5 mi) below its highly detailed walls. Dominating the inner ramparts of Copernicus are massive arc-shaped landslides, which formed by collapse and subsidence of the debris left over from the violent impact that created the main crater. A bright, broad (30 km/20 mi) blanket of ejecta surrounds the polygonal walls. Beyond this ring of bright material, Copernicus' majestic rays, best seen at full moon, radiate for hundreds of kilometres. These features testify to the relative 'youth' of Copernicus, which is 800 million years old. Numerous chains of craterlets curve outwards from the main crater in every direction.

Twin mountain ridges, running roughly east-west and separated by a spacious valley, divide the floor of Copernicus in half. The north group of mountains is composed of three major peaks of modest altitudes, the highest peak attaining 750 m (2400 ft). The south ridge, with a huge pyramidal mountain at its centre, is longer than the north ridge.

Copernicus (OAO-3) US astronomy satellite launched in 1972 August to study stellar ultraviolet and X-rays. It observed CYGNUS X-1, a BLACK HOLE candidate. It ceased operations in 1981.

Cor Caroli The star a Canum Venaticorum, visual mag. 2.89 (but slightly variable), distance 110 l.y. It is of spectral type A0p, with unusually prominent lines of silicon and europium. Cor Caroli is the prototype of the class of ALPHA2 CANUM VENATICORUM STARS; like all stars in this class, it varies slightly as it rotates, due to surface patches of different composition brought about by strong localized magnetic fields. Small telescopes show it to be a double star, with an unrelated companion of mag. 5.61. Cor Caroli is Latin for 'Charles' heart', a name bestowed by British royalists to commemorate the beheaded King Charles I of England.

Cordelia One of the small inner satellites of URANUS, discovered in 1986 by the VOYAGER 2 imaging team. Cordelia is c.26 km (c.16 mi) in size. It takes 0.335 days to circuit the planet, at a distance of 49,800 km (30,900 mi) from its centre, in a near-circular, near-equatorial orbit. With OPHELIA it acts as a SHEPHERD MOON to Uranus' Epsilon Ring, Cordelia being just inside the orbit of that ring.

Cordoba Durchmusterung (CD) Catalogue of 614,000 southern stars to 10th magnitude, initiated by John Macon Thome (1843-1908) at Cordoba Observatory, Argentina, and covering the sky from dec. 22 to 90. The four main volumes were completed by 1914, and a fifth was added in 1932 by Charles PERRINE. Together with the bonner durchmusterung and cape photographic durchmusterung, which it complements, the CD represents the last major survey made by visual observations, before photographic surveys became the norm.

Cordoba Observatory National observatory of Argentina, founded in 1871, some 650 km (400 mi) north-west of Buenos Aires; its most celebrated director was Benjamin GOULD, towards the end of the 19th century. The observatory is best known for its determinations of stellar positions published in the cordoba durchmusterung and the Astrographic Catalogue (see carte du ciel). It has a 1.5-m (60-in.) reflector, and its astronomers also use major multinational facilities such as the GEMINI OBSERVATORY.

core (planetary) Dense central region of a planet, having a composition distinct from the outer layers (MANTLE and CRUST). A planetary core forms when heavier components sink to the planet's centre during DIFFERENTIATION. Earth's core, which makes up nearly a third of the planet's mass, is composed mostly of iron alloyed with nickel and lighter elements, of which sulphur is probably the most abundant. The core is mostly liquid, but with a small solid inner core. Convective motions in the liquid iron generate Earth's magnetic field. Most of the core's properties are inferred by SEISMOLOGY. Mercury, Venus and Mars have iron cores as deduced from their densities and moments of inertia; the Moon is depleted in iron but may have a small core. The larger icy satellites of the outer planets probably possess rocky cores beneath icy mantles. The densities of the GAS GIANT planets imply that they possess cores of rock and icy material with masses of order 10 times Earth's mass; these cores probably formed by ACCRETION before the planets acquired their massive gaseous envelopes. see also COSMOGONY

core (stellar) Innermost region of a star; it is the region in which HYDROGEN BURNING takes place when the star is on the MAIN SEQUENCE. The Sun's core is believed to extend out to a quarter of the solar radius. Main-sequence stars of about 1.5 solar masses have radiative cores, in which the most important method of energy transport is by RADIATIVE DIFFUSION. Stars above this mass limit have convec-tive cores, in which energy transport is mainly by CONVECTION. see also STELLAR INTERIOR

Coriolis effect To an Earthbound observer, anything that moves freely across the globe, such as an artillery shell or the wind, appears to curve slightly - to its right in the northern hemisphere and to its left in the southern hemisphere. This is the Coriolis effect. In 1835 Gaspard de Copernicus One of the most prominent rayed craters on the Moon, Copernicus has a diameter of 92 km (57 mi) and shows terraced walls and central peaks. The rays and ejecta blanket surrounding the crater are well shown in this Hubble Space Telescope image.

corona (1) Loops in the Sun's inner corona imaged by the TRACE spacecraft in 1998. Like many other features in the solar corona, these are shaped by magnetic fields.

corona (1) Loops in the Suns inner corona imaged by the TRACE spacecraft in 1998. Like many other features in the solar corona, these are shaped by magnetic fields.

corona (2) A radar image of Artemis Corona obtained by the Magellan orbiter. Artemis Corona is the largest such feature on Venus.

corona (2) A radar image of Artemis Corona obtained by the Magellan orbiter. Artemis Corona is the largest such feature on Venus.

Coriolis (1792-1843) first explained that this apparent curvature was not caused by some mysterious force. It simply shows that the observer is on a rotating frame of reference, namely the spinning Earth. The Coriolis effect accounts for the direction of circulation of air around cyclones.

corona (1) Outermost region of the solar atmosphere, above the CHROMOSPHERE and TRANSITION REGION. The corona (from Latin 'crown') becomes visible as a white halo surrounding the Sun at a TOTAL SOLAR ECLIPSE, and can be observed at other times using a special instrument called a CORONAGRAPH. Coronal material is heated to temperatures of millions of Kelvin, and consequently emits energy at extreme-ultraviolet and X-ray wavelengths. Observations at these wavelengths show the corona over the whole of the Sun's face, with the cooler, underlying PHOTOSPHERE appearing dark.

The visible, white light corona may be divided into the inner K CORONA, within two or three solar radii of the photosphere, and the outer F CORONA. The K corona shines by visible sunlight that is weakly scattered by free electrons. It is about one-millionth as intense as the photosphere. Observations of the K corona indicate there are up to ten million billion (1012) electrons per cubic metre at the base of the corona. The F corona is caused by sunlight scattered from solid dust particles. The shape of the inner K corona reflects constraint of electrons by MAGNETIC FIELDS. At the base of coronal streamers, the electrons are concentrated within magnetized loops rooted in the photosphere. Farther out, the streamers narrow into long, radial stalks that stretch tens of millions of kilometres into space; some may reach as far as halfway to Earth.

At times of reduced activity, near the minimum of the SOLAR CYCLE, coronal streamers are located mainly near the solar equator. The coronal streamers are the source of the low-speed SOLAR WIND. Near maximum in the activity cycle, the corona becomes crowded with streamers both near the equator and the Sun's poles.

The corona's high temperatures are capable of stripping iron atoms of 13 of their electrons, producing an ionized form whose emission in green light at wavelength 530.3 nm was first detected at the total eclipse of 1869 August 7. Initially taken - mistakenly - to be the signature of a new element ('coronium'), FeXIV emission provides a useful tracer for coronal structure. Further evidence for the corona's high temperatures comes from observation of its radio emission. Gas in the corona is completely ionized, and is therefore in the form of a PLASMA.

Recent observations of the corona from spacecraft suggest that at least some of the heat of the corona is related to the Sun's ever-changing magnetic fields. These observations have been made using a soft X-ray telescope on the YOHKOH spacecraft, and with ultraviolet and extreme-ultraviolet telescopes aboard SOHO and TRACE.

Images of the Sun at extreme-ultraviolet and X-ray wavelengths reveal dark regions, called coronal holes, and bright regions, known as coronal loops. Coronal holes are characterized by open magnetic fields that allow hot material to escape. At least some of the high-speed component of the solar wind flows out of the coronal holes nearly always present at the Sun's poles. Coronal holes extending towards the solar equator become common close to the minimum of the solar cycle.

The hottest and densest material in the low corona is located in thin, bright magnetized loops that shape, mould and constrain the million-degree plasma. These coronal loops have strong magnetic fields that link regions of opposite polarity in the underlying photosphere. Coronal heating is usually greatest where the magnetic fields are strongest. High-resolution TRACE images have demonstrated that the coronal loops are heated in their legs, suggesting the injection of hot material from somewhere near the loop footpoints in the photosphere below.

Coronal loops can rise from inside the Sun, sink back down into it, or expand out into space, constantly changing and causing the corona to vary in brightness and structure. These changing magnetic loops can heat the corona by coming together and releasing stored magnetic energy when they make contact. This method of coronal heating is termed MAGNETIC RECONNECTION. It can occur when newly emerging magnetic fields rise through the photosphere to encounter pre-existing ones in the corona, or when internal motions force existing coronal loops together.

There are about 50,000 small magnetic loops in the low corona; these come and go every 40 hours or so, forming an ever-changing magnetic carpet. SOHO observations indicate that energy is frequently released when these loops interact and reconnect, providing another possible source of coronal heating. Bursts of powerful energy released during magnetic reconnection can explain sudden, brief intense explosions on the Sun called FLARES. Numerous low-level flares, called microflares or nanoflares, might contribute to coronal heating. See also E CORONA; T CORONA

corona (2) Ovoid-shaped feature, of which several hundred are known on VENUS. Venusian coronae, typically 200 to 600 km (120-370 mi) across, are the foci of concentric and radial TECTONIC deformation and associated, probably basaltic, volcanism. They result from the uplift, followed by relaxation, of the LrTHOSPHERE by thermally buoyant PLUMES. Uranus' satellite MIRANDA has three coronae, which are polygonal (rather than ovoid), concentrically banded features 200-300 km (120-190 mi) in diameter; this is comparable with Miranda's 235 km (146 mi) radius. Miranda's coronae are believed to be produced by a combination of tectonic disruption and CRYOVOLCANISM.

Corona Australis See feature article

Corona Borealis See feature article

coronagraph Instrument, used in conjunction with a telescope, that has an occulting disk to block out the brightness of the Sun's PHOTOSPHERE, providing an artificial eclipse, with additional precautions for removing all traces of stray light. It was invented by Bernard LYOT. A coronagraph is used to observe the faint solar CORONA in white light, or in all the colours combined, at any time. The best images, with the finest detail, are obtained from coronagraphs placed on satellites above the Earth's atmosphere, whose scattered light otherwise degrades the image. There are two coronagraph instruments aboard the SOHO spacecraft, for example.

coronal hole See CORONA

coronal mass ejection (CME) Transient ejection into interplanetary space of plasma and magnetic fields from the solar CORONA, seen in sequential images taken with a CORONAGRAPH. A coronal mass ejection expands away from the Sun at supersonic speeds up to 1200 km/s (750 mi/s), becoming larger than the Sun in a few hours and removing up to 50 million tonnes of material. CMEs are often associated with eruptive PROMINENCES in the chromosphere, and sometimes with solar FLARES in the lower corona.

coronal mass ejection SOHO images of a huge coronal mass ejection on 2000 February 27. The event was recorded using the LASCO C2 (left) and C3 (right) coronagraphs.

Coronal mass ejections are more frequent at the maximum of the SOLAR CYCLE, when they occur at a rate of about 3.5 events per day. They can, however, occur at any time in the solar cycle, and unlike coronal streamers, they are not confined to equatorial latitudes at solar minimum.

CMEs are most readily seen when directed perpendicular to the line of sight, expanding outwards from the solar limb; Earth-directed events are seen as diffuse, expanding rings described as halo coronal mass ejections. Earth-directed CMEs can cause intense MAGNETIC STORMS, and trigger enhanced auroral activity. Coronal mass ejections produce intense shock waves in the SOLAR WIND, and accelerate vast quantities of energetic particles. A large CME may release as much as 1025 Joule of energy, comparable to that in a solar flare. Like flares, CMEs are believed to result from release of stored magnetic stress. In view of their influence on the near-Earth space environment, CMEs are monitored by national centres and defence agencies.

CORONA AUSTRALIS (gen. coronae australis, abbr. cra)

Small southern constellation consisting of an arc of stars under the feet of Sagittarius, known since ancient Greek times when it was visualized as a crown or wreath. Its brightest stars, a and p CrA, are only of mag. 4.1. y CrA is a tight 5th-magnitude binary requiring apertures over 100 mm (4 in.) to split.

CORONA BOREALIS (gen. coronae borealis, abbr. crb) Distinctive northern constellation between Bootes and Hercules, representing the crown worn by Ariadne when she married Dionysus, who later threw it into the sky in celebration. Its brightest star is alphekka, mag. 2.22. v CrB is an easy binocular double, mags. 5.2 and 5.4, while CrB, mags. 5.0 and 6.0, requires a small telescope to divide. In the bowl of the crown is R CrB, a yellow supergiant (spectral type F8 or G0) that can suddenly fade by up to 9 magnitudes. when carbon accumulates in its atmosphere (see r coronae borealis star). T CrB is a recurrent nova known as the blaze star.

coronal streamer See corona

coronium Chemical element initially hypothesized to explain an unidentified green emission line observed in the solar corona (as well as others found later). The coronium lines were eventually found to be forbidden lines of highly ionized iron (Fe XIV) and other common elements.

Corvus See feature article, page 100

Cos B European gamma-ray astronomy satellite; it was launched in 1975 August. It remained in operation until 1982 April, detecting 25 gamma-ray sources during its lifetime.

cosmic abundance Relative amounts of elements produced during the first three minutes of the big bang. The Big Bang model predicted how the amounts of various elements were created under the conditions present in the early universe. Primarily hydrogen, helium and correlation function Mathematical way of statistically quantifying the distribution of galaxies in cosmology. Two-point and three-point correlation functions are normally calculated using the observed galaxy distributions, and then compared with theoretical distributions derived from cosmological models. Since correlation functions are statistical in nature, if the models give the same correlation function as the actual data, then the model is successful, even though the exact locations of the individual galaxies and/or clusters are not precisely what we see. See also big bang theory.

SOHO images of a huge coronal mass ejection on 2000 February 27. The event was recorded using the LASCO C2 (left) and C3 (right) coronagraphs.

CORVUS (gen. corvi, abbr. crv) Modest constellation south of Virgo whose most distinctive feature is a quadrilateral of stars similar to the Keystone of Hercules. It represents a crow, which in Greek legend was sent by Apollo to carry a cup (the neighbouring constellation Crater) to fetch water. 8 Crv (Algorab, abbreviated from the Arabic meaning 'raven's wing') is a wide and unequal double for small telescopes, mags. 2.9 and 9.2. Corvus contains the antennae, a famous pair of colliding galaxies small amounts of lithium and beryllium were produced by nucleosynthesis reactions under high temperatures and densities in the early universe. After about 15 minutes, the temperature of the universe had dropped and no further nucleosynthesis could occur until stars formed. Thus all of the heavier elements must have been synthesized in stars and during supernova explosions. Calculations indicate that only one 3He or 4He atom is formed for every 10,000 hydrogen atoms, and one lithium or beryllium atom for every hundred million hydrogen atoms. All of the remaining elements in the Universe were thus created in the centres of stars or during supernova explosions.

Cosmic Background Explorer (COBE) This all-sky temperature map derived from COBE data shows radiation in the direction towards which Earth is moving to be blue-shifted and therefore hotter. Conversely, in the opposite direction, the radiation appears cooler due to redshift. The data suggest that our Local Group of galaxies moves at 600 km/s (375 mi/s) relative to the background radiation.

Cosmic Background Explorer (COBE) This all-sky temperature map derived from COBE data shows radiation in the direction towards which Earth is moving to be blueshifted and therefore hotter. Conversely, in the opposite direction, the radiation appears cooler due to redshift. The data suggest that our Local Group of galaxies moves at 600 km/s (375 mi/s) relative to the background radiation.

Cosmic Background Explorer (COBE) NASA satellite launched in 1989 November to study cosmic microwave background radiation - the leftover radiation from the big bang. COBE made precise measurements of the diffuse radiation between 1 jm and 1 cm over the whole celestial sphere. As well as measuring the spectrum of this radiation, COBE also studied fluctuations in its temperature distribution, and the spectrum and angular distribution of diffuse infrared background radiation.

The spacecraft's instruments were cooled by liquid helium and protected from solar heating by a conical sun shade. The satellite rotated at 1 rpm and performed a complete scan of the celestial sphere every six months.

COBE first determined that the cosmic microwave background has a black body temperature of 2.73 K. Its overall view of the sky showed that one-half of the sky is slightly warmer than the other half as the result of the Solar System's motion through space. Then, in 1992, scientists announced that COBE had found 'ripples' in the background radiation, regions that were no more than 30-millionths of a degree warmer or cooler than the average temperature. These minute variations were attributed to gaseous structures several hundred million light-years across that existed in the Universe around the time the first galaxies were forming.

COBE's scientific operations were terminated at the end of 1993. Five years later, astronomers released the first maps to show a background infrared glow across the sky. Based on data from the Diffuse Infrared Background Experiment aboard COBE, they revealed the 'fossil radiation' from dust that was heated by hidden stars in the very early Universe.

cosmic dust See micrometeorite

cosmic microwave background (CMB) Remnant radiation from the creation of the Universe. Early cos-mologists predicted theoretically that the Universe originally began as a singularity that expanded into a small, hot 'soup' of radiation and elementary particles. As it expanded, the temperature dropped and the nuclei of atoms became stable. Finally, after the first 300,000 years, the Universe was cold enough for the radiation to decouple from the matter. As the Universe continued to expand and cool, the radiation and matter evolved independently. The radiation cooled (redshifted) as the Universe expanded but remained homogeneous and isotropic on a large scale; the matter clumped owing to gravitational interactions on smaller scales. Predictions were made that at the present stage of the Universe, the temperature of this background radiation field should have cooled to around 5 K.

cosmic microwave background This all-sky temperature map from COBE, from which the effects due to the motion of the Earth have been subtracted, shows ripples in the cosmic microwave background. Although the extent of these variations is tiny 30 millionths of a degree they are believed to have been sufficient to trigger formation of the first galaxies after the Big Bang.

In 1965 Arno penzias and Robert Wilson attempted to rid the large Bell Telephone Laboratory microwave antenna of background noise. This noise appeared to be constant and, if it originated outside the antenna, was extraterrestrial and isotropic and represented a background temperature of 3 K. They realized that they were indeed observing the cosmic microwave background radiation that is a remnant of the early Universe.

Astronomers realized that this cosmic microwave background held clues to the physical conditions early in the evolution of the Universe. NASA launched a satellite, the cosmic background explorer (COBE), to investigate this radiation in detail. COBE mapped the radiation in frequency as well as spatially with unprecedented accuracy and confirmed the background temperature at 2.73 K. It also mapped the temperature variations in all directions. This mapping provided evidence for the inflationary scenario of the early Universe.

cosmic rays Charged particles (protons and the nuclei of heavier atoms) that arrive at the top of the atmosphere and come from cosmic space. Their energies extend up to 1020 electron volts (eV) per particle and thus represent the highest individual particle energies known. The particles that come from space and hit the top of the atmosphere are called primary cosmic rays, and the particles that hit the ground are called secondary cosmic rays. Initially it was thought that cosmic rays comprised some form of ultra-penetrating gamma radiation, hence the term cosmic rays, but later work showed that particles, mainly protons, were responsible. Telescopes have been operated in many different environments, from satellites to the bottom of deep mines; directional telescopes in mines have shown upward-moving cosmic rays caused by neutrinos that have penetrated the whole Earth before interacting to cause detectable secondary rays. Special ground-based telescopes can detect the extensive air showers that occur when a primary particle hits an oxygen or nitrogen nucleus in the upper levels of the atmosphere. The initial secondary particles are pions, but these decay to the more stable muons. At ground level the showers contain mainly electrons with perhaps 5% muons.

The discovery of cosmic rays is credited to the Austrian physicist Victor hess, who, in 1912, carried electrometers aloft in a balloon in an attempt to discover why it had proved impossible to eliminate completely a small residual background reading in the electrometers at ground level. Hess found that, after first falling, the reading started to increase as the balloon ascended and he made the remarkable claim that there was a need 'to have recourse to a new hypothesis; either invoking the assumption of the presence at great altitudes of unknown matter or the assumption of an extraterrestrial source of penetrating radiation'. After arguments lasting some years the extraterrestrial origin idea finally won; Hess was awarded the Nobel Prize in 1936.

Since cosmic rays are charged particles, they are deflected by magnetic fields in the Galaxy and beyond, so it is sometimes difficult to be certain about their origins.

However, cosmic rays and gamma rays (see GAMMA-RAY ASTRONOMY) are both high-energy signatures, so gamma rays can help locate the origins of cosmic ray sources. It has been suggested that CYGNUS X-3 provides a significant fraction of the cosmic rays in the Galaxy, but the galactic centre also produces cosmic rays, as may other objects such as supernovae. Some of the highest-energy cosmic rays come from the VIRGO CLUSTER; it contains the galaxy M87, which has a JET showing that there may be a super-massive BLACK HOLE at its centre. Low-energy cosmic rays are emitted by the Sun. One of the unusual features of these is the presence of comparatively large fluxes of the elements lithium, beryllium and boron, which are identified as fragments of heavier cosmic ray nuclei that have struck nuclei of the interstellar medium. The presence of a very small fraction of radioactive beryllium-l0 (10Be), taken with other data, leads to an estimated lifetime of low-energy cosmic rays of at least 20 X 106 years. Low-energy cosmic rays cause the production of radioactive carbon (14C) and beryllium (10Be) in the atmosphere, which leads to the possibility of studying intensity variations over 103-107 year periods from studies of 14C in tree rings and 10Be in deep-sea sediments.

cosmic year Time taken for the Sun to complete one revolution about the centre of the Milky Way galaxy -about 220 million Earth years. The entire Galaxy is rotating, gradually changing shape as it does so, and the Sun is located some 28,000 l.y. from the galactic centre.

cosmochemistry Study of the chemical reactions occurring in space and of the products of those reactions. The atoms forming most elements are synthesized inside stars (see ASTROCHEMISTRY) and then a small proportion is returned to interstellar space during SUPERNOVA explosions. The exceptions to this are principally hydrogen and helium-4, and to a much lesser extent deuterium (hydro-gen-2), helium-3 and lithium-7, which were created in the first few seconds of the Big Bang. The roughly 25% (by mass) of the matter in the Universe that is helium-4, for example, resulted from reactions such as n + e+ p + ve where n is neutron, e+ is a positron (positive electron), p is a proton and ve is an electron neutrino. Such reactions were in equilibrium during the early stages of the Big Bang and left a balance of 1 neutron to 6 protons when they ceased. The surviving neutrons (for the neutron is an unstable particle) eventually combined with protons to produce the helium nuclei. Hydrogen and helium form about 98% by mass of all the material in the interstellar medium, with carbon, nitrogen, oxygen, neon, magnesium, silicon, sulphur, argon and iron making up most of the balance. Many other elements have been detected, and we may expect all those found on Earth, plus some of those that do not occur naturally, such as technetium and plutonium, to occur in space. However, the molecules that can currently be observed are mostly combinations of hydrogen, carbon, nitrogen and/or oxygen (see INTERSTELLAR MOLECULES). One significant reason for this is that it is only molecules in the form of gases that can be identified with certainty (from their radio spectra). The composition of solid materials, such as interstellar dust particles (see INTERSTELLAR MATTER), can be only roughly characterized.

Once formed, the atoms in interstellar space can take part in chemical reactions, but the conditions differ so much from that of a terrestrial chemistry laboratory, that the reactions and their products are often very unusual by 'normal' chemistry's standards.

Some molecules, such as TiO, CN and C2, are robust enough to exist in the outer atmospheres of cool stars. However, most molecules will be dissociated by the ultraviolet radiation in starlight if they float freely in space. The interiors of GIANT MOLECULAR CLOUDS (GMCs) are thus the sites where the vast majority of interstellar molecules are to be found. There they are sheltered by dust from the stellar ultraviolet radiation.

Even though GMCs are much denser than the average for the interstellar medium, they are still very hard vacuums by terrestrial laboratory standards. Thus interactions usually take place between two atoms in isolation, and this can make it difficult for stable chemical reactions to occur. For example, if two neutral hydrogen atoms encounter each other and join to form a molecule, the molecule will be highly excited because of the kinetic energy possessed by the atoms. It will thus be unstable, and under terrestrial conditions would lose its excess energy to a third particle to form the stable hydrogen molecule. That third particle is unlikely to be nearby within a GMC, and so the molecule will rapidly dissociate back to the two original hydrogen atoms. A possible way out of this would be for the excited hydrogen molecule to radiate away its excess energy, but most of the lower energy transitions of the hydrogen molecule are forbidden, and so this is unlikely to occur.

Stable reactions can occur when one of the particles is ionized, perhaps as a result of a collision with a COSMIC RAY particle. Thus water can be formed from an ionized hydrogen molecule in combination with oxygen through the following set of reactions:
H2+ + H2 H3+ + H
H3+ + O OH+ + H2

It is thought that many molecules are formed only on the surfaces of the dust particles within the GMC. Individual atoms are adsorbed on to the dust particle and are then in such close proximity that relatively normal reactions can occur. The daughter molecule may be ejected back into space by the energy released during the reaction, or perhaps a cosmic ray particle passing through the dust particle may heat it sufficiently to evaporate all the molecules that have accumulated on its surface.

There is currently a 'standard model' that gives a general explanation of the formation of the Solar System. Many details remain to be worked out, and some parts of cosmic microwave background This all-sky temperature map from COBE, from which the effects due to the motion of the Earth have been subtracted, shows 'ripples' in the cosmic microwave background. Although the extent of these variations is tiny - 30 millionths of a degree - they are believed to have been sufficient to trigger formation of the first galaxies after the Big Bang.

It is generally accepted that planetary systems are not rare, but are a common product of star formation. Stars form from clouds of interstellar gas, mostly hydrogen and helium, containing a small fraction of heavier elements, mostly in the form of dust grains. If the gas reaches a critical density it begins to collapse under its self-gravity. This collapse may be initiated by a number of phenomena: cooling of the gas; loss of a supporting magnetic field; or an external perturbation that compresses the gas. Most star formation is observed to occur in clusters, as portions of a large cloud, with a total mass perhaps thousands of times that of the Sun, collapse over an interval of millions of years. The most massive stars have short lifetimes and explode as SUPERNOVAE within the cloud, mixing their material back into the medium and triggering further collapses by their shock waves. As a stellar-mass portion of the cloud collapses, it loses energy by infrared radiation from dust grains and molecules. ANGULAR MOMENTUM is conserved, and any initial rotation increases in speed as the cloud contracts. Clouds with larger amounts of angular momentum may produce a BINARY STAR. For lesser amounts, turbulence and magnetic coupling redistribute angular momentum, allowing most of the mass to fall towards the centre, while a portion remains in a rotating PROTOPLANETARY DISK orbiting the PROTOSTAR.

The protoplanetary disk is heated by the kinetic energy of its infall. Its inner part becomes hot enough to vaporize the dust, and the gas nearest the protostar is ionized. In the outer part, grains of interstellar dust may survive; as the disk cools, heavier elements recondense in the inner region. Dust grains collide and stick together to produce centimetre-sized aggregate particles, which settle towards the central plane of the disk, where they form a thin, dense particle layer. Particles in this layer are subject to drag exerted by the gas, which causes bodies of different sizes to move at different rates; they may also collide due to turbulence. They continue to grow, forming PLANETESIMALS of kilometre size. At this stage they are less affected by the drag of the nebular gas, and gravitational interactions become important. Their perturbations stir up eccentricities and inclinations, causing the orbits of the planetesi-mals to cross, which results in frequent collisions. These collisions may produce both growth and fragmentation, but the energy loss in inelastic collisions ensures that growth dominates. By this process of ACCRETION, a small number of large bodies, or PROTOPLANETS, forms. Collisions among them continue for millions of years until the remaining bodies are few and widely separated, yielding planets on stable orbits. The Moon is believed to have formed from debris after a Mars-sized protoplanet collided with the still-growing Earth. The final sweeping up of smaller bodies takes tens of millions of years.

Farther from the protosun, cooler temperatures allow ice to condense, providing more material and allowing growth of larger protoplanets. If these protoplanets reach a critical size, several times Earth's mass, they can capture gas from the protoplanetary disk, growing into giant planets like Jupiter and Saturn. After a few million years the gaseous component of the disk is driven off by activity of the newly formed star as it goes through its TTAURI phase. Still farther out, the growth times of the protoplanets are too long, and the disk may dissipate before they can capture large amounts of gas; planets like Uranus and Neptune result, their makeup being dominated by the accretion of icy cometary planetesimals.

This model for planetary formation provides a plausible qualitative explanation for the major features of the Solar System, although some parts are uncertain and many details remain to be worked out. The degree to which our planetary system is typical or unique is not known. The existence of extrasolar planets in systems quite unlike our own shows that other outcomes are possible. The final form of a planetary system probably depends on both systematic factors, such as the mass and angular momentum of a protostellar cloud, its temperature and composition, and so on, as well as on random events, such as the proximity of other stars and the outcomes of collisions between protoplanets.

cosmological constant Constant in Einstein's FIELD EQUATIONS of GENERAL RELATIVITY, originally added to stabilize the universe. If one writes down the general rela-tivistic equation for the universe, one finds that the universe cannot be static, but must either expand or contract. When Einstein derived his 'universe' equation, astronomers had no idea the Universe was actually in a state of expansion.

This cosmological constant can provide an attractive or repulsive force that operates throughout the entire universe and causes expansion, contraction or stability depending on its value. Shortly after Einstein published the addition to his theory, Edwin HUBBLE announced his observations of galaxies that showed the Universe was indeed in a state of constant expansion. Einstein quickly retracted his cosmological constant as being unnecessary and announced it as his biggest blunder. Even after astrophysicists realized that no cosmological constant was needed, some kept it and gave it a very small value, or a time-dependent value (in the case of inflationary theory). Lately, new observations of universal acceleration combined with the low mass density seen in the Universe indicate that it might be necessary after all. See also BIG BANG THEORY cosmological distance scale Methods and scales with which astronomers determine the distance between galaxies and clusters in the visible Universe. In order to understand our Universe, the distances to galaxies must be determined. In order to measure these tremendous distances, astronomers rely on a variety of methods.

Distances in the Solar System can be measured by radar, and nearby stellar distances can be measured by direct stellar PARALLAX. In order to determine the distances to more distant stars, model-dependent methods like main-sequence fitting and the period-luminosity relationship for CEPHEID VARIABLES must be used. Measuring the distances to nearby galaxies relies upon detection of Cepheid variables or other bright stellar phenomena such as supernovae in those galaxies. These methods are fairly accurate for nearby galaxies, but become more difficult and uncertain for more distant galaxies. Other methods such as the TULLY-FISHER RELATION can be used for distance estimates of spiral galaxies, while distances to elliptical galaxies require assumptions about the average size of giant ellipticals in a cluster. Farther and farther out into the Universe, distance measurement becomes more and more uncertain. Until recently this has prevented us from determining what type of universe we live in.

This series of methods is also called the cosmic distance 'ladder', where each rung represents a method to determine the distance to more distant objects. Each method also depends on the previous one for accurate calibration. The final rung of the cosmological distance scale is the HUBBLE LAW. This relationship is used for objects so far away that we cannot image individual stars, molecular clouds, or even individual supernovae, and it is essential to have a reliable value for the HUBBLE CONSTANT to use it.

cosmological model Any of several theories or ideas that attempt to describe and explain the form and evolution of our UNIVERSE. The original cosmological model held that the Earth was the centre of the Universe, with the Sun, Moon and planets orbiting around us and the stars affixed to the CELESTIAL SPHERE. This geocentric (Earth-centred) model was in favour for over 1000 years after PTOLEMY first wrote it down. This model failed to explain the elliptical

shapes of planetary orbits, the westward drift or retrograde motion of the planets, and the phases of Venus. The geocentric model was subsequently replaced by the more powerful heliocentric or 'Sun-centred' model. The heliocentric model was capable of explaining retrograde motion much more easily than its predecessor. Although copernicus was the first to show that the heliocentric model was far better in explaining the observations than the geocentric model, it was not until after galileo's time that it became politically acceptable to publish such an idea. The current accepted cosmological model, called the big bang, is based on Einstein's theory of general relativity and maintains that at one moment spacetime was collapsed into an infinitely small point, and subsequently the entire Universe expanded and continues to expand. Other cosmological models include the anisotropic models, the steady-state theory and brans-dicke theory.

cosmological principle Assumption that spacetime is

homogeneous and isotropic on the largest of scales and that we do not live in a preferred place in the Universe. This sweeping principle allows us to solve Einstein's field equations and to build cosmological models that can be tested with observations.

cosmological redshift Shift of spectral lines due to the expansion of the Universe. Galaxies and clusters are located in spacetime, which is uniformly expanding and carrying the galaxies along with it; thus the galaxies exhibit a velocity relative to a distant observer. The 'Doppler-type' effect is not a result of the sources' velocity in spacetime, but is caused by the expansion of spacetime itself. Edwin hubble showed in 1929 that the Universe is expanding, and further that the cosmological velocity is linearly related to the distance of the source. See also hubble law

cosmology Study of the structure of the Universe on the largest scale. Contained within it is cosmogony.

Our early view of the Universe was prejudiced by the belief that we occupied a special place within it - at the centre. Only in the 20th century have we realized that the Earth is but a small planet of a dim star, located in the outer suburbs of a typical galaxy. Perhaps the most important astronomical discovery of the early 20th century was hubble's realization that the dim nebulae he observed were in fact enormous systems of thousands of millions of stars lying far outside our Galaxy. Soon after this discovery, astronomers realized that these galaxies were all receding from the Earth. Hubble, along with other astronomers, obtained optical spectra of many galaxies and found that their spectral lines were always shifted towards the red (longer wavelengths). He interpreted these redshifts as being a universal doppler effect, caused by the expansion of the Universe. Furthermore, the speed he inferred was found to be proportional to the galaxy's distance, a relationship known as the hubble law.

It is now known that both the shift and the speed-distance proportionality follow naturally from an overall expansion of the scale size of the Universe. Galaxies are redshifted because the Universe has a different scale size now compared with the size it was when the light was emitted from the galaxies. Nevertheless, time has shown that Hubble was correct in his interpretation that the speed of recession is proportional to distance. Today, the constant of proportionality bears his name (see hubble constant).

Close to the Sun, the distance of galaxies can be determined from the properties of some variable stars they contain - such as the Cepheids - or from the size of HII regions. As we move farther out into the Universe, however, these methods become increasingly inaccurate. Eventually, distances can only be estimated by measuring the redshift and relying on the accuracy of the Hubble relation.

Unfortunately, for some of the most distant objects, such as quasars, we do not have adequate confirmation that this procedure for determining distances is valid. Some astronomers believe that at least part of the quasar redshifts may originate from unknown 'non-cosmological' causes.

Attempts to determine the nature of the reshifts and the expansion rate have occupied much of the available time on large telescopes. Today the question is still unresolved.

However, perhaps the most important cosmological problems that remain at the beginning of the 21st century are to determine the rate at which the universal expansion is taking place (determining the Hubble constant), how it has expanded in the past and how it will continue to behave in the future. To these must be added the question of whether the overall geometry of the Universe is 'closed' or 'open'. In an open universe, the total volume of space is infinite, the universe has no boundary and will expand for ever. closed universes contain a finite amount of space, may or may not have boundaries and will eventually collapse back on themselves.

Attempts to obtain a grand view of the Universe have led to the construction of cosmological models. A starting point for many cosmologists has been the finding that the Universe appears much the same in all directions (the so-called isotropy) and at all distances (homogeneity). However, the expansion of the Universe would at first seem to suggest that the overall density of the distribution of galaxies must decrease so that they become more sparsely distributed as time goes on.

Cosmological models have included the steady-state theory of Hermann bondi, Thomas gold and Fred hoyle in which the Universe is the same not only in all places but also at all times. It therefore had no beginning, will have no end and never changes at all when viewed on the large scale. This theory required matter to be created as the Universe expanded in order that the overall density of galaxies should not decrease. For this reason it is also referred to as the continuous creation model.

On the other hand, according to supporters of the big bang models originally proposed by George gamow, Ralph Alpher (1921- ) and Robert Herman (1914-97), the whole Universe was created in a single instant about 20 billion years ago and is presently expanding (the modern consensus value is about 15 billion years). In the future it may continue to expand or possibly collapse back on itself depending on the total amount of matter and energy in it, that is, whether or not the Universe is open or closed. An important cosmological question is the missing mass problem: the amount of matter we see in the Universe is far smaller (by a factor of about 100) than the amount we infer from the motions of the galaxies.

Definitive observations to discriminate between cosmo-logical models are hard to make. The most informative parts of the Universe are those farthest away. Unfortunately, the objects we observe in such regions are faint and their nature is unknown. It is extremely difficult to tell to what extent quasars, for example, are similar to the nearer - and more familiar - objects. And if we cannot make comparisons, we cannot use them as standards to test cosmologi-cal models. It is also not known if our Earth-derived physical laws are applicable in the Universe at large.

Cosmologists have made several important discoveries, including an attempt to determine the Hubble constant. The deceleration parameter, which determines whether the Universe will expand for ever or eventually collapse back on itself, has also been estimated. Recent observations of supernovae have shed some light on the values of these important cosmological parameters. They indicate that the Universe is probably accelerating and will never collapse back again.

Another important discovery in cosmology was the cosmic microwave background radiation, which provided strong evidence against the steady-state theory. Its discovery also brought with it problems of its own: we do not understand how this background radiation can be so uniform in all directions when it comes from different parts of the Universe that have never been in communication with each other. Attempts have been made to invoke a very rapid period of expansion in the Universe's history in order to remove this difficulty (the so-called inflationary universe), but these attempts appear to many cosmolo-gists to be less than convincing (see inflation).

cosmological constant A supernova (arrowed) in a high-redshift galaxy, imaged by the Hubble Space Telescope. Observations of objects such as this support the suggestion that a cosmological constant might, after all, be required to describe the expansion of the Universe.

Cosmos satellite At the present time, the best observational evidence favours an inflationary Big Bang model for the Universe with a Hubble constant of around 68 km/s/Mpc.

Cosmology is presently based on a considerable amount of speculation fuelled by relatively little observational material. Future generations of cosmologists will be presented with plenty of problems - and opportunities. Even if we were to obtain a good understanding of the present and future behaviour of the Universe, we would still be far from comprehending what happened before the Big Bang.

Crab Nebula The remnant of a supernova that exploded in the constellation of Taurus in 1054. The expanding gas cloud has filamentary structure and a pulsar at its heart.

Cosmos satellite Blanket name used by the former Soviet Union for most of its scientific and military satellites. By the end of 2001, 2386 had been launched.

coude focus Focal point of an equatorially mounted telescope in which the light path is directed along the polar axis to a fixed position that remains stationary, regardless of the orientation of the telescope. Instruments such as high-dispersion spectroscopes, which are too large or heavy to be mounted on a moving telescope, may be placed at the coude focus, which is often located in an adjacent room or even separate floor. A series of auxiliary mirrors is used to direct the converging beam of light from the secondary mirror, down the hollow polar axis of the telescope mount, to the slit of the spectrograph. The word coude is French for 'elbow', and describes the bending of the light path.

coude focus In this optical configuration, mirrors direct light from the telescope along the polar axis to a fixed observing position. This has several advantages for observations that require use of heavy or bulky detectors.

counterglow Alternative name for gegenschein

Cowling, Thomas George (1906-90) English mathematician and theoretical astrophysicist who pioneered the study of magnetic fields in stars. Cowling worked out mathematical models that demonstrated the importance of radiation and convection in making and transporting nuclear energy through a star's atmospheric layers.

CPD Abbreviation of cape photographic durchmusterung

Crab Nebula (Ml, NGC 1952) supernova remnant some 6500 l.y. away and located in Taurus (RA 05h 34m.5 dec. +2200') about a degree from Tau.

Crab Nebula The remnant of a supernova that exploded in the constellation of Taurus in 1054. The expanding gas cloud has filamentary structure and a pulsar at its heart.

On 1054 July 4 Chinese astronomers recorded a 'guest star' in what is now the constellation Taurus. The star shone about as brightly as the planet Venus, being visible even in daylight. Surprisingly few records of so prominent an object have been found elsewhere in the world, though diligent searches are turning up some, including two rock engravings in the southwestern United States that may depict the supernova near the crescent moon.

Today the remnant of that explosion can be seen as an emission nebula, a cloud of gas that originated in the star itself and now has expanded to a size of about 8 X 12 l.y. That nebula bears various names: Messier 1, NGC1952, Taurus A and Taurus X-1. The name by which it is best known, however, is the Crab Nebula. Lord rosse named it for its visual resemblance to a crab when observed through his 72-inch (1.8-m) reflector.

The Crab Nebula is a supernova remnant - the nebula, radio and/or X-ray source left over from a supernova. Supernova remnants usually take the form of an expanding, hollow shell. The Crab Nebula, however, emits from its centre outwards, and is a member of a very rare group of remnants known as plerions or filled supernova remnants.

A supernova's material is ejected at speeds of typically 10,000 km/s (6000 mi/s), so that it rams violently into the surrounding interstellar gas. The effect of this continuous collision is to keep the gas hot long after the supernova explosion. The gas is warmed to several million degrees, emitting light and X-rays. In so doing, the gas is slowed down, so that eventually the remnant ceases to glow. Depending on its environment, a supernova remnant may shine for tens of thousands of years. The Crab Nebula is, therefore, a relatively young specimen.

The nature of the explosion is such that the gas forms filamentary structures. The filaments give old supernova remnants a wispy appearance. In the Crab, the filaments are indeed present: on colour photographs they glow with the characteristic red of hot hydrogen, like strands of red cotton wrapped around a soft yellow glow. It is the yellow glow that is distinctive in the case of the Crab Nebula. Within the filaments is an ionized gas in which electrons, freed from their parent atoms, are spiraling in intense magnetic fields and emitting synchrotron radiation.

The Crab Nebula is exceptional amongst plerions because the synchrotron radiation extends from the radio domain to the visible. It is the only synchrotron nebula that can be seen in a small telescope. In order to emit at such short wavelengths, the electrons must be very energetic indeed. The very act of producing synchrotron radiation removes energy from the electrons, so in the Crab Nebula there has to be a continuing supply of energetic electrons now, 900 years after the star was seen to explode. The source of these energetic electrons is the neutron star or pulsar found at the centre of the nebula. The crab pulsar, spinning at the rate of 30 times per second, continuously sprays out both radiation and electrons to replenish the synchrotron radiation.

The Crab Nebula holds other surprises yet. Only recently a very faint extension was noted from its northern edge -a broad, parallel-sided jet of gas. The origin of this jet is still obscure. Various theories have been proposed, but none seems quite convincing at present. The Crab Nebula certainly will retain a vital place in the study of supernovae and supernova remnants for a considerable time.

coude focus In this optical configuration, mirrors direct light from the telescope along the polar axis to a fixed observing position. This has several advantages for observations that require use of heavy or bulky detectors.

Crab Pulsar Pulsar with a period of 0s.033 at the heart of the crab nebula (Ml). It is the remains of the supernova of 1054, and its discovery confirmed that pulsars are related to neutron stars. The Crab Pulsar is one of very few pulsars to be identified optically, and it has been seen to pulse in the optical, X-ray (with the einstein observatory) and gamma-ray (with the compton gamma ray observatory) regions, with the same period. In between the main pulse there is another weak pulse, called the inter-pulse. The hubble space telescope has taken a remarkable picture (at optical wavelengths) of the pulsar in which knots of gas can be seen in the pulsar jet, and wisps of gas are seen like ripples on a pond or a whirlpool. These are very variable.

Crabtree, William (161044) English cloth-merchant, apparently self-taught in astronomy. By 1636 he had become conversant with the researches of Galileo, Johannes Kepler, Pierre Gassendi and Rene Descartes (1596-1650), owned a telescope and several other astronomical instruments, and had become the centre of a group of astronomical correspondents in north-west England that included Jeremiah horrocks, William gas-coigne and Christopher Towneley (1604-74). Crabtree used his observations of eclipses, occultations and sunspots to advance the Copernican system. In 1639 he and Horrocks observed, from Salford, Lancashire, the first recorded transit of Venus across the Sun's disk.

Crater See feature article

crater Circular depression on the surface of a planet, satellite or other Solar System body. The term is typically applied to features of volcanic or impact origin. Volcanic craters are steep-walled depressions at the top or on the flanks of a volcano; they are often the major vents for eruption. They are the result of explosions or collapse at the top of a volcanic conduit. Impact craters are observed on all kinds of surface; they are formed by high velocity collisions of interplanetary bodies. A primary impact crater is typically surrounded by a concentric zone of blocky and hummocky terrain formed by ejecta. Larger ejecta fragments may produce secondary craters.

On bodies lacking an atmosphere, even micromete-orites may produce impact craters, known as micro-craters, less than a micrometre in diameter. Where an atmosphere is present, its thickness determines the minimum possible crater size. On Venus, with its thick, dense atmosphere, the smallest primary impact craters are a few kilometres in diameter. Mars' rarefied atmosphere, however, leads to a minimum size of between 10 and 15 metres.

High-velocity passage through an atmosphere often results in fragmentation of an impactor. If break-up happens close to the planet's solid surface, so that the impactor fragments' high velocity is retained, primary impact craters smaller than the above limits may form. Impact craters show changes in structure as a function of their size. Relatively small craters (usually less than 1 km in diameter) are typically bowl-shaped. Larger ones have an uplifted floor. Still larger craters have a central peak on their floor. The largest (tens, hundreds to a few thousand kilometres across) have concentric rings on their floor and are known as multi-ring basins. The diameter at which there is a transition from one type of crater structure to another one depends on the nature of the target material (for example rock or ice) and on the target body's gravitational acceleration. For example, the onset diameter of central-peaked craters is 8-15 km (5-9 mi) on Venus, 10-20 km (6-12 mi) on Mercury, 5-15 km (3-9 mi) on Mars, and 25-40 km (16-25 mi) on the Moon (see crater, lunar).

crater, lunar Roughly circular depression seen on all parts of the lunar surface. Craters range from micrometre-size features (microcraters), which are known from examination of lunar samples returned to Earth by apollo astronauts and luna robotic missions, to so-called multi-ring basins, the largest of which is about 2500 km (1600 mi) in diameter. On the whole Moon, numbers of craters larger than 1000, 100, 10 and 1 km in diameter are, respectively, two, more than 200, c.104 and c.106. This close to power function extends to smaller sizes.

The shapes and structures of most craters show systematic variations with crater size. Craters with diameters greater than 10 km (6 mi) generally have prominent rims raised above the surrounding terrain. Some large craters are quite pristine and show morphological changes as their sizes increase. Craters with diameters smaller than 15-20 km (9-12 mi) are bowl-shaped. Above this size, pristine craters have flattened floors with hummocky surfaces. Craters larger than 25-40 km (16-25 mi) have a mountain at the centre of the flat floor and are known as central peak craters. At larger diameters, greater than 100-120 km (62-75 mi), the central peak is surrounded by a fragmentary ring of smaller peaks; such craters are known as central peak basins. Craters with diameters greater than 200-300 km (120-190 mi) have one or more concentric rings of peaks but lack a central peak; they are known as peak-ring and multi-ring basins. Bowl-shaped craters are often called simple craters, and those with a flattened floor and central peaks are called complex craters. Inner slopes of complex craters are steep, often with characteristic terraces formed by slumping and sliding. Many large craters are filled with ejecta from other craters, which partly or completely bury their interior structures.

The outer flanks of the rims of large lunar craters are gently sloping; they consist of hummocky terrain, which grades back to the general (pre-crater) level. Beyond the hummocky terrain region are sometimes found large numbers of small, irregular secondary craters, which often occur in groups and lines elongated roughly radial or concentric to the main, or primary, crater. Even farther out from the primary crater rim, roughly radial patterns of brightening of the surface, known as rays, are visible when the Sun is high over the horizon. In plan view, the primary crater rims may range from nearly circular, through polygonal, to quite irregular.

Small craters, in the size range from 10 km (6 mi) down to a few centimetres, generally have simple bowl-like shapes. The inner walls are usually smooth, with occasional outcrops of bedrock; they show only minor modification

CRATER (gen.crateris, abbr. crt)

Faint constellation south of Leo representing a cup or chalice, linked with the legend of neighbouring Corvus. Its brightest star is 8 Crt, mag. 3.56, spectral type G9 or K0 III, distance 195 l.y. by slumping. Flat floors and central mounds are rare. In plan view, the majority of small craters are near circular; some are elongated in a manner that is typical for secondary craters and for craters associated with lunar SINUOUS RILLES. Small craters show obvious progression from fresh-looking, morphologically prominent and relatively deep forms to subdued shallow features that are obviously a result of crater degradation due to impact 'GARDENING' and slumping. Some of the smallest craters (microcraters) seen on fragments of rocks and minerals have smooth interiors and rounded rims, showing evidence of plastic flow.

The morphological features of most craters - raised rims and hummocky exteriors - are consistent with the idea that they were formed by some explosive process that excavated material from the depression and deposited it to form the crater rim and ejecta blanket. The depletion in volatiles of lunar rocks argues against a volcanic-type explosion, while the systematic presence of meteoritic material in the ejecta of small (REGOLITH) and large (highland BRECCIA) craters suggests that the crater-forming explosions resulted from HYPERVELOCITY IMPACTS of METEOROIDS. This idea agrees well with the close to random distribution of primary craters on the homogeneously aged surface of the Moon. Large fragments of the target material, thrown out at a relatively high speed, form secondary craters, while smaller particles, thrown out with even higher speed, produce the bright rays.

Volcanic eruptions almost certainly occurred on the Moon, producing the extensive areas of dark deposits that can be seen thinly covering the surface in some places.

C ring Saturn's C ring imaged by Voyager 2 during its flyby in 1981 August. Seen as single by Earth-based observers, the C ring was revealed by Voyager to be made up of hundreds of separate component ringlets.

C ring Saturns C ring imaged by Voyager 2 during its flyby in 1981 August. Seen as single by Earth-based observers, the C ring was revealed by Voyager to be made up of hundreds of separate component ringlets.

CRUX (gen. crucis, abbr. cru)

Smallest of all the 88 constellations, but one of the most famous and distinctive. Its four brightest members form a shape that has been likened more to a kite than a cross; its symmetry is disturbed by the off-centre of e Cru. Crux was formed from stars in the hind-legs of Centaurus during the 16th century. The long axis of the cross points to the south celestial pole. a Cru (ACRUX) is a sparkling double for small telescopes with a wider binocular companion of mag. 4.9. 7 Cru (Gacrux) is a wide binocular double; j Cru is an easy double for small telescopes of mags. 4.0 and 5.1. Objects of particular interest in Crux include NGC 4755, known as the jewel box or KAPPA CRUCIS CLUSTER, and the dark COALSACK nebula. See also BECRUX

These deposits are sometimes observed in association with kilometre-size shallow craters, which are known as dark-haloed craters. These craters are good candidates for volcanic landforms, especially those located along rilles or in straight lines, similar to those located on the floor of the crater ALPHONSUS, or those spatially associated with source areas of the youngest volcanism. Some kilometre-size and smaller rimless craters, especially those in association with fractures and cracks, may have formed by drainage of material into subsurface openings.

It is now considered certain that the great majority of lunar craters are of impact origin, although many large craters were subsequently modified by volcanic and TECTONIC processes. The most obvious example is the extensive flooding by lava flows of the large basins on the side facing the Earth. This process, which happened mainly between 4000 and 3000 million years ago, led to the formation of circular lunar maria, for example Mare IMBRI-UM and Mare CRISIUM.

The relative numbers of craters of a given size seen on different parts of the surface has been used to obtain relative ages of the terrains: the older a surface is, the greater the degree of cratering. Also, when ejecta from a crater is seen to lie on top of other features it provides a time marker for those features. By combining these relative ages with absolute ages measured on rocks returned to Earth, the calibration function of transformation of relative numbers of craters of a given size into millions and billions of years has been worked out. This calibration curve can be applied to other bodies and serves as the only technique now available to estimate absolute ages of different terrains throughout the Solar System; major events on different planets and satellites can be correlated in time.

Crepe ring Common name for Saturn's C RING.

crescent PHASE of the MOON between new and first quarter, or between last quarter and new, or of an inferior planet between inferior CONJUNCTION and greatest ELONGATION, when less than half its illuminated side is visible.

Cressida One of the small inner satellites of URANUS, discovered in 1986 by the VOYAGER 2 imaging team. Cressida is c.66 km (c.41 mi) in size. It takes 0.464 days to circuit the planet, at a distance of 61,800 km (38,400 mi) from its centre, in a near-circular, near-equatorial orbit.

Crimean Astrophysical Observatory (CrAO) Principal astronomical institution in the Ukraine, and one of the largest scientific centres in the republic. CrAO began before World War I as a station of the PULKOVO OBSERVATORY, but its modern history began in 1945 when the Soviet government decided to build a new facility at Nauchny, 12 km (8 mi) south-east of Bakhchisaraj; this continues as the observatory's main centre. Its main instruments are the 2.6-m (104-in.) Shajn Telescope (commissioned in 1961), two 1.25-m (50-in.) telescopes, and a large gamma-ray telescope having 48 mirrors and a total collecting area of 54 m2 (580 sq ft). The observatory also operates a 22-m (72 ft) radio telescope near Simeiz.

C ring Innermost of the rings of SATURN visible from Earth. It lies inside the B RING at a distance of between 74,500 km (46,300 mi) and 92,000 km (57,200 mi) from the planet's centre.

Crisium, Mare (Sea of Crises) Lunar lava plain, roughly 500 km (310 mi) in diameter, located in the north-east quadrant of the Moon. The region's geology reveals that an enormous impact struck the area, producing a multi-ring impact BASIN. At a later time, lavas flooded the inner areas of the basin, but were generally contained by an inner ring, which is the shape of the present mare. This feature appears oblong due to foreshortening, though its actual shape is more hexagonal. A shelf lies just inside the margin, marking an inner ring of Crisium.

cross-staff Simple 14th-century instrument consisting of two attached pieces of wood in the shape of a cross and used as a sighting device for determining the angular distance between two objects.

Cruithne earth-crossing asteroid with an orbital period very close to one year; number 3753. Discovered in 1986, the peculiar orbital properties of Cruithne were not identified until 1997. There are no known trojan asteroids of the Earth occupying the lagrangian points, but Cruithne has a regular dynamical relationship to our planet, delineating a horseshoe-shaped path relative to us as it orbits the Sun. The Saturnian moons janus and epimetheus also follow horseshoe orbits, but centred on that planet. Cruithne is about 5 km (3 mi) in size. See table at near-earth asteroid.

critical density Amount of mass needed to make the Universe adopt a flat geometry. The size of the critical mass depends on several cosmological parameters and is given.

Crommelin, Andrew Claude de la Cherois (1865-1939) English astronomer, born in Ireland, expert on the orbits of periodic comets and asteroids. With Philip Herbert Cowell (1870-1949), he used improved methods, based on a combination of mathematical theory and actual observations, to predict the perihelion date of Hal-ley's Comet at its 1910 apparition. The prediction was accurate to within 3 days, far exceeding the accuracy of predictions for prior returns of the comet. Crommelin was a skilled observer, and took part in four major expeditions to observe total eclipses of the Sun, helping to confirm Einstein's general theory of relativity.

Crommelin, Comet 27P/ Periodic comet, first discovered by Jean Louis pons in 1818 and again, independently, by Jerome Eugene Coggia (1849-1919) and Friedrich Winnecke (1835-97) at the later apparition of 1873. The identity of this as a single object seen at different returns was not established until 1928, when a comet found by G. Forbes was shown by Andrew crommelin to have the same orbital elements. Initially designated Pons-Cog-gia-Winnecke-Forbes, the comet was renamed Crom-melin in 1948. The comet was recovered in 1956 only 10 from the expected position by Ludmilla Pajdusakova (1916-79) and Michael Hendrie (1931- ). 27P/Crom-melin has an orbital period of 27.4 years, and its most recent return in 1983-84 was used to test observing programmes ahead of 1P/Halley's 1986 perihelion.

crossed lens Convex lens designed to minimize spherical aberration. A crossed lens has two convex spherical surfaces, the radii of which are in the ratio of six to one. This arrangement minimizes spherical aberration for a parallel beam of light.

crossing time Measure of the internal dynamics of a star cluster, galaxy or cluster of galaxies, given by the ratio of a relevant radius to the particle velocity. The times taken for a system to relax into a well-mixed configuration, for a galaxy merger to play itself out, or for the inicrust Outermost layer of a rocky planet; it is composed of the lightest silicate minerals that float to the surface during differentiation. Earth's crust is divided into continental (granitic) and oceanic (basaltic) provinces, with thicknesses of order 20-50 km (10-30 mi) and 5-10 km (3-6 mi), respectively. The crust is compositionally distinct from the denser material of the mantle. The Moon's crust is c.60 km (c.40 mi) thick.

Crux See feature article cryovolcanism Form of low-temperature volcanism on icy planetary bodies in which the eruptive fluid (magma) is water, water-ammonia mixture or brine rather than liquid silicate. The fluid originates in a warm layer at depth and escapes through fractures. It freezes on the surface, creating flow features and flood plains. Cryovolcanic features have been identified on larger icy satellites of the outer planets.

CSIRO Abbreviation of commonwealth scientific and industrial research organisation

C star Abbreviation of carbon star cubewano trans-neptunian object that does not have a resonant orbit and is, therefore, distinct from a plutino; cubewanos are members of the edgeworth-kuiper belt. The term cubewano is derived from the pronunciation of the preliminary designation of the first trans-Neptunian object discovered, 1992 QB1.

culmination Passage of a celestial body across the meridian due south or due north of the observer. circumpolar stars cross the meridian twice, the events being known as upper culmination (between pole and zenith) and lower culmination (between pole and horizon). Non-circumpo-lar objects obtain their maximum altitude above the horizon at culmination. See also transit

Cunitz, Maria (1610-64) German astronomer and mathematician who published a simplified version of Johannes Kepler's tables of planetary motion, making them much more widely available than before. For this work she earned the title 'Urania Propitia', meaning 'she who is closest to Urania', the muse of astronomy.

Curtis, Heber Doust (1872-1942) American astronomer who correctly described the 'spiral nebulae' as other galaxies lying far beyond the Milky Way. He joined the Lick Observatory in 1902, becoming expert at astrophotography and spectroscopy. With Lick's director Wallace campbell he undertook a project to measure the radial velocities of all northern-hemisphere stars brighter than visual magnitude 5.5, in the process discovering many spectroscopic binaries. Using Lick's Crossley Reflector, Curtis amassed the finest collection of photographs ever taken of nebulae. He found that 'spiral nebulae' were scarce along the galactic plane (the ZONE OF AVOIDANCE) and that many spirals showed dark dust lanes along their horizontal planes, similar to the Milky Way's. These observations and his spectroscopic work convinced him that the bright, diffuse nebulae resided inside our Galaxy, but that the 'spiral nebulae' were other galaxies lying at much greater distances. This was at variance with the 'metagalaxy' model proposed by Harlow SHAPLEY (see GREAT DEBATE).

Curtis became director of the Allegheny Observatory (1920-30), where he designed and built a wide variety of astronomical equipment, including 'measuring engines' to obtain precise positions of celestial objects from photographic plates, and specialized instruments for observing solar eclipses. During the 1925 total eclipse expedition to New Haven, Connecticut, he became the first to obtain an infrared spectrum of the solar corona and chromosphere. Curtis continued his solar astronomy at the University of Michigan's observatory, which he directed from 1930 to 1942, and at its McMath-Hulbert Observatory, which he helped to found.

cryovolcanism Water erupting from a possible ice volcano produced this flow-like deposit on Jupiter's satellite Ganymede. Imaged in 1997 by the Galileo orbiter, this feature is about 55 km (34 mi) long and up to 20 km (13 mi) wide.

curvature of spacetime Distortion of the spacetime CONTINUUM due primarily to the presence of mass. Einstein's theory of GENERAL RELATIVITY is a geometric theory in which the FIELD EQUATIONS describe how much effect mass-energy has on the shape of the four-dimensional SPACETIME continuum in which we live. Mass-energy curves or distorts spacetime, and particle paths are constrained to follow this curvature. If a particle is observed approaching the Sun, it appears to be attracted to the Sun by some force, or, equivalently, its path is seen to alter because of the curvature of spacetime around the Sun.

cusp Either of the two pointed extremities, or 'horns', of the Moon or an inferior planet when at its CRESCENT phase. The term literally means the point of meeting of two curves, in this case the curves being the limb of the Moon or planet and the TERMINATOR (the boundary between the illuminated and non-illuminated hemispheres).

cusp caps of Venus Bright hoods or caps at the north and south points of the planet, especially conspicuous at the latter. They were discovered 1813 December 29 by the German astronomer Franz Paula von Gruithuisen

CYGNUS (gen. cygni, abbr. cyg) Large and easily recognized constellation, also known as the Northern Cross, representing the swan into which the god Zeus turned himself before seducing Leda, the queen of Sparta. Its brightest star, DENEB, is an immensely luminous supergiant, a member of the so-called SUMMER TRIANGLE. p Cyg, better known as ALBIREO, is a beautiful coloured double for the smallest apertures, while 61 Cyg is an easy pair of orange dwarfs (see SIXTY-ONE CYGNI). CHI CYGNI is a red MIRA STAR, ranging between mags. 3.3 and 14 with a period of about 400 days; P CYGNI is a highly luminous variable. Cygnus lies in the Milky Way and contains rich starfields. Its largest nebula is the CYGNUS RIFT, a dark cloud that bifurcates the Milky Way as it heads south. Near Deneb is the NORTH AMERICA NEBULA, NGC 7000, while near Cyg is the VEIL NEBULA, a supernova remnant. M39 is a large open star cluster easily seen in binoculars and just on the naked-eye limit. NGC 6826 is an 8th-magnitude planetary nebula popularly known as the BLINKING PLANETARY. Two celebrated objects in Cygnus within reach only of professional instruments are CYGNUS X-1, a black hole X-ray source that is in orbit around a visible star, and CYGNUS A, a strong radio source thought to be two galaxies in collision.

CV Serpentis ECLIPSING BINARY star associated with the bright hydrogen nebula E41. It has a period of 29d.64, and its magnitude ranges from 9.86 to 10.81(mB). It has Wolf-Rayet and O8-O9 stellar components. CV Serpentis exhibits strange behaviour in that eclipses are sometimes observed when the Wolf-Rayet component eclipses the O-type star, and sometimes when the O-type star is the eclipsing body. Also, predicted eclipses do not always occur.

Cygnus See feature article

Cygnus A Strongest source of radio emission outside the Galaxy, and the first RADIO GALAXY to be detected. It emits a million times more energy in the radio part of the spectrum than the Milky Way. Cygnus A is thought to be two galaxies in collision; it has two radio LOBES centred on the parent galaxy. It is also an X-ray source.

Cygnus Loop See VEIL NEBULA

Cygnus Rift (Northern Coalsack) Northern part of a large DARK NEBULA that runs along the Milky Way from Cygnus through to Ophiuchus. The nebula as a whole is known as the Great Rift. The clouds forming the Great Rift are about 2000 l.y. away and have a mass of about one million solar masses. The Great Rift causes the Milky Way to appear to split just below Deneb (a Cyg), with the main branch running down through Altair (a Aql) and a spur forking towards p Oph.

Cygnus X-1 Binary star system around 6000 l.y. (2 kpc) away that is a strong source of X-rays. It is associated with a faint star (HDE 226868), a blue supergiant, with an orbital period of 5.6 days. The blue supergiant is around 30 times as massive as the Sun, and the invisible companion has a mass of around 10 solar masses, which is too big for a white dwarf or a neutron star, so it may be a black hole. The X-rays sometimes vary in intensity on a time-scale of milliseconds. Cygnus X-1 is also a radio source.

Cygnus X-3 Source of X-rays, gamma rays and cosmic rays (and small radio flares), emitting more energy than all but a very few other objects in the Galaxy. Cygnus X-3 consists of a NEUTRON STAR orbiting a larger companion, with an orbital period of 4.8 hours. It is situated on the edge of the Galaxy, 36,000 l.y. from Earth.

Cyrillus Polygonal lunar crater, 97 km (60 mi) in diameter. With CATHARINA, it is a member of the THEOPHILUS chain. Its north-east wall is overlapped by the younger impact crater Theophilus. Cyrillus' east rim is straightened, and its interior walls gradually slope to the floor, which is broken at two places by lengthy landslips parallel to the outer ramparts. The west rim of Cyrillus is cut by a long north-south cleft; a long, curvy ridge abuts the north-east walls. Cyrillus adjoins Catharina through a wide, shallow valley that cuts through its south wall. The floor of Cyrillus shows much detail, including a group of three mountains lying east of centre. Although quite large, these mountains are lower and more heavily eroded than Theophilus' central mountains, thus proving that Cyrillus is the older crater. A sinuous rille originates at a volcanic vent beyond Cyrillus' south wall; it tops that wall, continues down to the south half of the crater's floor and extends towards a craterlet just south-west of centre.