radar astronomy Category of research using continuous-wave or pulsed transmissions, from Earth or from space satellites, to study Solar System bodies. In 1944, when Germany began the bombardment of London by V2 rockets, Stanley HEY was asked to modify an anti-aircraft gun-laying radar in order to give early warning of the approach of a V2. By re-directing the aerial beam to an elevation of 60 radar echoes were obtained from the rockets, but many other short-lived radar echoes were observed that had no connection with the rockets. Hey concluded that these transient echoes were associated with the entry of METEORS into the upper atmosphere. The radar waves are reflected from the long, thin columns of ionization formed at altitudes of about 100 km (60 mi) when a meteoroid evaporates as it plunges into the Earth's atmosphere. The meteor trails allow the measurement of the wind systems in the 100 km region of the atmosphere. The radar technique allowed meteors to be observed during daylight and through clouds, revealing that major METEOR SHOWERS were active in summer daytime. Methods of measuring the velocity of the entry of meteors into the atmosphere were developed and a long-standing controversy about the origin of the SPORADIC METEORS was settled.

radar astronomy This Magellan orbiter radar image shows Maat Mons on Venus. Radar observations have allowed planetary astronomers to penetrate Venus dense atmosphere and obtain an idea of the planets surface topography.

In 1947 November Frank Kerr (1918-2001) and Charles Shain (d.1960) of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia investigated the erratic changes in the strength of the radar echoes scattered from the Moon. They made use of the Australian Government's short-wave transmitter, normally used for broadcasting to North America, and were able to distinguish two types of fading of the lunar echoes: a rapid fading with a period of seconds was superimposed on a longer-period fading of about 30 minutes. They suggested that the rapid fading was associated with the libra-tion of the Moon and that the long-period fading might have an ionospheric origin. This was confirmed by JODRELL BANK, and it was discovered that the long-period fading was caused by the rotation of the plane of polarization of the radar waves as they passed through the Earth's IONOSPHERE (Faraday rotation). Further research showed that the radar waves were not reflected uniformly from the whole disk of the Moon, but from a region at the centre of the visible disk that had a radius of only one-third of the lunar radius. Another significant result of lunar radar work has been the systematic measurement of the total electron content between the Earth and the Moon derived from an analysis of the longer-period fading of the lunar echoes.

A radar measurement of the distance of Venus settled the ambiguities in the value of the SOLAR PARALLAX (hitherto known only to 0.1%) and the problem of the rate of rotation of the planet. Success was first achieved by NASA equipment at Goldstone on 1961 March 10 using a continuous-wave system. Pulsed radar contact was achieved at Jodrell Bank on April 8, soon followed by other successes in the United States and the Soviet Union. From these distance measurements a definitive value of the solar parallax was agreed. The spin of the planet affects the spectrum of the scattered radar waves, and in 1962 and 1963 measurements in the Soviet Union and the United States first established that the planet was in retrograde rotation with a period of 243 days. In 1962 the Soviet Union achieved radar contact with Mercury, followed by the Jet Propulsion Laboratory in the United States in 1963 May. In 1963 February radar contact was established with Mars by both the United States and the Soviet Union. Procedures were developed for radar mapping of the planetary surfaces. In the case of Mercury and Mars, optical imaging from space probes soon provided superior data, but the radar mapping of the perpetually cloud-covered surface of Venus has been of outstanding importance. Radar maps have been obtained of the planet's surface with a resolution of a few kilometres using APERTURE SYNTHESIS. In 1979 a computer analysis of several months of the data from the radar altimeter carried in the American PIONEER VENUS Orbiter produced a topographical map of more than 90% of the surface of Venus. The Soviet VENERA 15 and 16 spacecraft placed in orbit around Venus in 1983 carried an aperture synthesis radar system that mapped the whole planetary surface above 30 north latitude with a resolution of 1 to 2 km (c.1 mi). NASA's MAGELLAN satellite, launched in 1989, mapped 98% of Venus' surface with a resolution of around 100 m. Other objects in the Solar System detected by radar include a number of asteroids, the four Galilean satellites of Jupiter, and Saturn's rings. Radar echoes were obtained from Comet IRAS-ARAKI-ALCOCK by the Gold-stone and Arecibo antennae when it made a close approach to Earth (0.031 AU) on 1983 May 11.

radial velocity Component of velocity directed along the line of sight. Radial velocities are found from the Doppler effect, in which the spectrum lines from an approaching body are shifted to shorter wavelengths ('to the blue') and those from a receding body to longer wavelengths ('to the red'). At low velocities compared with the speed of light (such that relativity theory is not required), the velocity relative to the speed of light is directly proportional to the relative shift, and is considered positive for a receding body, negative for an approaching one.

radiant Circular area of sky, usually taken as a matter of observational convenience to have a diameter of 8, from which a METEOR SHOWER appears to emanate. Radiant positions are normally expressed in terms of right ascension and declination. Meteor showers usually take their name from the constellation in which the radiant lies - the PERSEIDS from Perseus, GEMINIDS from Gemini, and so on. The radiant effect, with meteors appearing anywhere in the sky with divergent paths whose backwards projection meets in a single area, is a result of perspective: in reality, shower members have essentially parallel trajectories in the upper atmosphere. Meteors close to the radiant appear foreshortened, whilst those 90 away have the greatest apparent path lengths. Meteor shower radiants move eastwards relative to the sky background by about one degree each day thanks to Earth's orbital motion.

radiant Meteors from a common source, occurring during a shower such as the Perseids, enter the atmosphere along parallel trajectories, becoming luminous at altitudes around 80100 km (5060 mi). If an observer on the ground plots the apparent positions of meteors on a chart of the background stars, they will appear to diverge from a single area of sky, the radiant. The radiant effect is a result of perspective.

A meteor shower radiant position may be determined from the backwards projection of plotted or photographed trails. The radiant can also be found from parallactic observations of a single meteor recorded at geographical locations separated by a few tens of kilometres.

radiation Energy transmitted in the form of electromagnetic waves or photons, or by subatomic particles. Radiation provides us with almost all the information we have about the Universe. In particular, the study of radiant energy is fundamental to our understanding of stellar structure, because it enables us to deduce the temperatures and luminosities of stars as well as their chemical constitutions.

Radiation behaves in two different ways depending upon the interaction with which it may be involved. During INTERFERENCE, DIFFRACTION and POLARIZATION it behaves like a wave, while during the photoelectric effect and the COMPTON EFFECT it behaves as though it were formed of a stream of particles. This wave-particle duality is not unique to radiation; entities such as ELECTRONS and PROTONS that are normally regarded as particles also exhibit a wave nature, as for example in the operation of the electron microscope.

As a wave, radiation has a wavelength, X, and a frequency, f or v, which are related by the equation Xf = c, where c is the velocity of LIGHT (see ELECTROMAGNETIC SPECTRUM). As a particle, called a PHOTON, or quantum, each particle has an energy given by E = hf, where h is the PLANCK CONSTANT.

Of particular interest to astronomy is the relationship between the temperature of the surface of a hot body and the way that surface emits radiation, which is affected by the nature of the surface itself. It is well known that not all surfaces raised to the same temperature radiate energy in the same way. Some reflect well, but others may be highly absorbing. By KIRCHHOFF'SLAW the efficiency of the emission by a heated object at a particular wavelength is proportional to the efficiency of its absorption at the same wavelength. Thus an object that absorbs with 100% efficiency over the whole SPECTRUM, known as a black body, will also be the most efficient when it comes to emitting radiation. The emission from a black body, known as BLACK BODY RADIATION, is a good approximation to the emission from many astronomical objects, including stars (SPECTRUM lines are mostly minor deviations from the overall emission). Black body radiation follows a bell-shaped curve (see PLANCK DISTRIBUTION). The peak of the distribution shifts to shorter wavelengths as the temperature increases (see WIEN'SLAW), which leads to the common experience that at moderate temperatures objects glow a dull red, then change colour successively through bright red, yellow, white to blue-white as the temperature is increased. The total emitted energy increases rapidly with temperature, leading to the STEFAN-BOLTZ-MANN LAW. At wavelengths significantly longer than that of the peak emission, the simpler Rayleigh-Jeans approximation may often be used, while at wavelengths shorter than the peak, the Wien approximation can be used.

The operation of these laws leads to a simple relationship between the colour of a star and its surface temperature, T. For example: 8540

radiation belts Regions of a planetary MAGNETOSPHERE that contain energetic ions and electrons trapped by the planet's magnetic field. These belts are usually doughnut-shaped regions. The particles trapped in the belts spiral along the magnetic field lines and bounce backwards and forwards between reflection points encountered as they approach the magnetic poles. The electron motion produces SYNCHROTRON RADIATION, a characteristic emission from the individual planetary systems. The particles are captured from the SOLAR WIND or are formed by collisions between COSMIC RAYS and ions in the planet's outer atmosphere. The Earth, Jupiter, Saturn, Uranus and Neptune are all known to possess radiation belts, while the magnetosphere of Mercury is too small compared to the planetary radius to support this phenomenon.

The radiation belts of Jupiter are vast in spatial scale and are additionally supplied by accelerated particles originating from the volcanic moon IO. The resulting particle flux is about 1000 times more intense than in the VAN ALLEN BELTS that surround the Earth. They present a formidable obstacle for spacecraft, such as GALILEO, since in the region of the most intense radiation the probe would receive an integrated dose of about 200,000 rads from the electrons and 50,000 rads from the protons. This may be compared with a dose of 500 rads, which is sufficient to kill a man. The intense bombardment of a spacecraft can saturate the sensitive instruments, upset the on-board computer systems and interfere with its communications with the Earth (see also SPACE WEATHER).

The radiation belts of Saturn were discovered by the PIONEER 11 spacecraft in 1979. The extensive system of rings and the satellites residing inside the magnetosphere of Saturn sweep away the charged particles, such that the fluxes maximize just outside the rings. The radiation belt particles around Uranus and Neptune also interact with the ring and satellite systems, but the fluxes are relatively low, probably due to the weak interaction between the solar wind and the planetary magnetospheres at these distances from the Sun.

radiation era Time before the universe became transparent to photons about 300,000 years after the BIG BANG. The period before this transition is called the radiation era since radiation dominated the thermal evolution of the universe up until that time. After the universe became transparent, matter began to dominate and atoms, stars and galaxies could form.

radiation pressure Pressure exerted on a surface by incident ELECTROMAGNETIC RADIATION. This pressure arises because each incident photon transfers a tiny quantity of momentum to the surface. For BLACK BODY RADIATION, the pressure is given by:

p = 4vT4 3c

where is the STEFAN-BOLTZMANN constant, T the temperature and c the velocity of light. One well-known observed effect concerns the dust tails of COMETS, which are affected by the Sun's radiation pressure and therefore always point more or less away from the Sun, so that when moving outwards, after perihelion, the dust tail of a comet precedes the nucleus. Radiation pressure also limits the maximum mass of a star to about 100 solar masses. At that size, the temperature of the star will be sufficiently high that the radiation pressure pushing outwards balances the inward forces of gravity. Since both gravity and radiation follow INVERSE SQUARE LAWS, once equality is reached at one point within the star, it will be equal throughout the star. Any material moving outwards will therefore continue to do so, and the star will become unstable, shedding material until its temperature is lowered sufficiently for it to become stable again.

radiation temperature Temperature of an object found by assuming that its emission at a particular wavelength, or over a small wavelength region, is that of a BLACK BODY. The temperature is obtained using the Planck equation (see PLANCK DISTRIBUTION). It is similar to the EFFECTIVE TEMPERATURE, except that the effective temperature uses the total emission at all wavelengths from the object and hence the STEFAN-BOLTZMANN LAW.

radiative transfer Most important process by which heat energy is transported from a star's hot interior to its surface. In radiative transfer, high-energy photons lose energy as they travel outwards through the hot plasma. This loss of energy occurs as the photons are scattered, mainly by free electrons. The photons can also lose energy if they are absorbed by an ion, and photo-ionization occurs. These processes take place in the radiative zones of the stars.

radioactive age dating Method of determining age by measuring the RADIOACTIVITY of a nuclide with a known half-life. The principle behind radioactive age dating is the fixed rate with which an unstable radioactive isotope (the parent) decays to a stable isotope (its daughter). The time taken for a radionuclide to decay to half its initial abundance is the half-life (T1/2) of the system. Several isotope systems with different half-lives are used to measure different events within Solar System history. Long-lived radionuclides, such as uranium or rubidium, are used to date the age of formation of meteorites. Short-lived radionuclides, such as plutonium, aluminium or manganese, date the formation interval between stellar processing of material and its incorporation into meteoritic components.

radioactivity Spontaneous emission of radiation from an atomic nucleus. An atom of a given chemical element is characterized by the number of protons in its nucleus, which is known as its atomic number. Nuclei of the same element may have different numbers of neutrons and thus different masses; these are called ISOTOPES of the element. Each nucleus with a particular number of protons and neutrons is called a nuclide. The sum of the numbers of protons and neutrons is the atomic weight. Some nuclides are unstable, generally because they contain too few or too many neutrons relative to the number of protons. These nuclides decay spontaneously into more stable nuclides. This process is called radioactivity or radioactive decay.

A heavy nucleus with too many protons may emit an alpha particle, consisting of two protons and two neutrons (a helium nucleus). A lighter nucleus may reduce the number of protons by capturing an electron or emitting a positron, thereby converting a proton to a neutron. If there are too many neutrons, one of them may be converted into a proton by emitting an electron (beta particle). Some of these transformations also produce gamma rays, which are highly energetic photons (X-rays). These processes transmute an unstable nucleus of one element into another that is stable; they are known as parent and daughter nuclides respectively. Each kind of unstable nuclide has a characteristic rate of decay. The time taken for half of the atoms of an isotope to decay is called its half-life; various nuclides have half-lives ranging from fractions of a second to billions of years.

The decay rate is unaffected by the chemical state of the atoms or by physical factors such as temperature and pressure; this property makes radioactivity an excellent chronometer for measuring ages of geological materials. For example, three different isotopes of uranium decay to three isotopes of lead with different half-lives, the longest of which is about 4.5 billion years. Precise measurements of abundances of these isotopes in minerals in the most primitive meteorites show that they formed (coincidental-ly) about 4.56 billion years ago, which defines the age of our Solar System. Other useful parent-daughter systems with long decay times include 40K-40Ar (the superscript is the atomic weight of the nuclide), 87Rb-87Sr and three nuclides of Sm-Nd. Shorter-lived nuclides produced by a nearby SUPERNOVA were present in the early Solar System. These nuclides are now extinct, but they have left evidence in the form of their stable daughter nuclides, which provide relative chronometers. The most important of these is 26Al, which decays to 26Mg with half-life 700,000 years. An excess 26Mg in aluminium-bearing minerals indicates that they formed before all of the 26Al had decayed; the amounts give relative formation times over a span of the first few million years of the Solar System's history. Other extinct parent-stable daughter pairs include 53Mn-53Cr and 129I-129Xe.

Energy released by radioactive decay is an important source of internal heat for planetary bodies, driving convection in Earth's mantle and core. In the early Solar System radioactive nuclides were more abundant, and species that are now extinct were then significant sources of energy. The abundance of 26Al was sufficient to melt planetes-imals a few hundred kilometres in size, and radioactive heating was the cause of thermal processing of meteorites.

radio astronomy Study of a wide range of objects from the coolest, most quiescent astronomical objects (namely the neutral hydrogen gas in the Galaxy) to some of the most energetic (such as PULSARS and QUASARS) in the radio region of the spectrum. Every type of astronomical object has been studied at radio wavelengths, including the Sun, planets, stars, nebulae, galaxies and the cosmic microwave background. The radio wavelength region covers the range from around 10 to 20 m (30 to 15 MHz), where the radio waves are absorbed and scattered by the ionosphere, and 350 urn (850 GHz), where atmospheric water vapour absorbs the radiation. The short-wavelength region from 350 um to 10 mm is now considered the domain of SUBMILLIMETRE-WAVE ASTRONOMY and MILLIMETRE-WAVE ASTRONOMY, and the region between 1 mm and 30 mm (300 GHz and 1 GHz) is also known as the MICROWAVE region. Radio astronomers use frequency as often as wavelength to characterize the signal because of the historical roots of the area.

The initial discovery that radio waves from outside the Solar System were detectable on Earth was made in 1931 by Karl JANSKY, whose name is used for the unit of measurement of radio signal. The words 'radio astronomy' were first used in the late 1940s. The discovery was not thought to be of great significance to the development of astronomy until after World War II, when the new techniques that had been developed during the War were used to study these radio emissions and it was appreciated that non-thermal processes must be operating in the Sun and in more distant regions of the Universe. Originally (reflecting the nature of the equipment), radio emission was measured in decibels since it was 'signal strength'; now FLUX DENSITY as weak as micro-janskys can be detected. The 250-ft (76.2-m) Mark I RADIO TELESCOPE at JODRELL BANK is an early example of a large steerable telescope; it began work in 1957. The ARECIBO telescope in Puerto Rico (with a diameter of 305 m) is suspended across a sinkhole; it started operations in the early 1960s. However, the power of the radio telescope is seen to best advantage when several radio telescopes are used together, either through APERTURE SYNTHESIS, to improve the detail in maps, or through INTERFEROMETRY, to improve positional accuracy (see also MULTI-ELEMENT RADIO-LINKED INTERFEROMETER NETWORK; RADIO INTERFEROMETER; VERY LONG BASELINE INTERFEROMETRY).

Two early discoveries (in the 1950s) confirmed the importance of radio astronomy. One was the development by Soviet physicists of the theory of SYNCHROTRON RADIATION in the Milky Way. It was shown that the motion of relativistic electrons in the galactic magnetic field would produce radio emission such that the difficulty in the observed spectrum was removed. The other discovery concerned the nature of the localized (point-like) sources of emission. Although localized sources existed in the Milky Way (particularly SUPERNOVA REMNANTS such as the Crab Nebula), collaboration with the astronomers using the 200-inch (5-m) optical telescope on Palomar led to the surprising conclusion that the majority of the sources were distant extragalactic objects. These became known as RADIO GALAXIES. An important stage was reached in 1960 when, using the 200-inch telescope, Rudolph Minkowski identified one of these radio galaxies with a galaxy having a REDSHIFT of 0.46 - the most distant object then known, assessed to be 4.5 X 109 l.y. away, with a recessional velocity of 40% of the speed of light. Radio astronomy is able to detect radio galaxies to much larger distances than can be observed optically. The number of radio sources increases with distance, showing that the Universe is evolving.

Another discovery from radio astronomy, by Arno PENZIAS and Robert Wilson in 1965, namely that of the COSMIC MICROWAVE BACKGROUND, confirmed the theory that the Universe evolves with time. Recent work in this area has been undertaken with the COSMIC BACKGROUND EXPLORER (COBE) satellite, and a new satellite (PLANCK) will continue the study is greater detail.

Yet another important discovery by radio astronomers was that of pulsars, by the group at Cambridge (including Anthony HEWISH and Jocelyn BELL) in 1967. The initial discovery was of a source emitting a pulse of radio waves every 1.337 seconds with a precision better than one part in 107. Over 700 pulsars have been discovered, and although there are very few optical identifications it is widely accepted that they are rapidly rotating NEUTRON STARS - the collapsed very high-density remnants of stars that have undergone a SUPERNOVA explosion. The beam of radio emission, from the pole of the neutron star, swings rapidly across our line of sight, like the beam of light from a lighthouse. Pulsars have been detected with millisecond periods, in binary systems, and even with planets orbiting around them. The final stage of stellar evolution following the supernova explosion is the creation of a supernova remnant, which can be observed at radio wavelengths as the material from the supernova interacts with the interstellar medium.

Neutral hydrogen was known to be the principal component of the Galaxy; Hendrik van de HULST predicted that it should be possible to observe a spectral line from it in 1945. The line at 21 cm (TWENTY-ONE CENTIMETRE LINE) was detected in 1951. The motion of the neutral hydrogen gas can be studied using the 21-cm line, which gave the first unambiguous determination of the spiral structure of the Galaxy. Many other galaxies are close enough to be resolved and mapped in the radio part of the spectrum, and it has been found that the neutral gas extends farther from the galaxy centres than the visible light and stars indicate. The motions of the neutral hydrogen gas can be used to track the distribution of the mass in the Galaxy, showing that in most normal galaxies only about 10% of the mass of the galaxy is observed (the rest is DARK MATTER).

Several important molecules are observed in the radio region: OH (the hydroxyl radical) at 18 cm was the first, in 1963. Over 100 molecules have been detected from interstellar space in the Galaxy, from other galaxies, and from the envelopes around late-type stars in the Galaxy. The emission from molecules can be enhanced by the MASER process, and the masers in the outer envelopes of late-type stars can be accurately tracked showing the motions, and expansion, of the stellar envelope. These changes can be seen on short time-scales (months and years). Until molecular hydrogen could be directly detected, CO (carbon monoxide) was a very important molecule because it was co-located with molecular hydrogen in cold clouds and could be used to trace it. See also INTERSTELLAR MATTER CYGNUS A was the first radio galaxy to be detected, the strongest source of radio emission outside the Galaxy. The lobes are only around 3 million years old, compared to perhaps 10 billion years old for the stars in the central galaxy. Both Cygnus A and CENTAURUS A are thought to be galaxies in collision. In Centaurus A the small spiral galaxy, engulfed by the giant elliptical galaxy, can be seen in the INFRARED, and the optical image shows a large dark dust lane across the galaxy. Centaurus A is also an X-ray source. The optical galaxy M87 coincides with the radio source Virgo A, a powerful radio galaxy. A jet of gas 8000 l.y. long can be seen, and the light is polarized, showing that the synchrotron mechanism is at work, and there is a strong magnetic field present.

radio galaxy Galaxy that emits in the radio region of the ELECTROMAGNETIC SPECTRUM at rates around a million times stronger than the Milky Way. Many radio galaxies have optical counterparts, which are almost invariably ELLIPTICAL GALAXIES. The emission from a typical radio galaxy is concentrated in two huge radio LOBES lying well outside the galaxy and frequently located symmetrically about it. In appearance these lobes look as though they have been explosively ejected from the central galaxy. The lobe sizes for the biggest radio galaxies are around 16 million l.y., comparable with the size of a typical cluster of galaxies (our own Galaxy is around 65,000 l.y. across). These giant galaxies are the largest known objects in the Universe.

radio galaxy Jets emerging from the active nucleus of Cygnus A produce two lobes of strong radio emission to either side of the galaxy, as seen in this image from the Very Large Array. Many radio galaxies show similar patterns of emission as material they eject impacts on the surrounding intergalactic medium.

The positions of the radio galaxies must be determined with very high precision, using very long baseline interferometry. This enables telescopes to image the faint optical counterparts. The structure of the galaxies, the lobes and jets can be mapped using aperture synthesis, so that details can be investigated using telescopes such as the very large array. Radio astronomers have found that the extended radio lobes are almost invariably connected to the central galaxy by jets, which seem to originate from the galactic nucleus. High-frequency VLBI observations have revealed strong, point-like, active radio sources in the nuclei of many objects. It seems clear that the basic 'engine' that drives the explosion lies in the galaxy's nucleus, although it is unclear what the engine might be. Whilst the collision of two galaxies may contribute to the engine, interacting galaxies are usually weak radio sources. Radio galaxies may have more in common with quasars and bl lacertae objects, which are as powerful radio emitters as radio galaxies (or slightly more powerful). Radio galaxies have more emission at lower frequencies: in the famous third Cambridge catalogue (3C) about 70% of sources are found to be radio galaxies, but in surveys made at higher frequency, such as the Parkes 2700 MHz survey, less than 50% are believed to be radio galaxies. Radio galaxies may be old quasars or failed quasars. There is still much to learn.

radio interferometer Two (or more) radio telescopes separated by a distance greater than (or comparable to) their diameters that are looking at the same astronomical object; their signals are combined electronically. Radio interferometers were developed in order to permit radio telescopes to attain much higher resolution than was possible with a single telescope (usually more than 60"). Radio telescopes are used as interferometers to get more accurate positions for radio sources, allowing telescopes at other wavelengths (most commonly optical telescopes) to identify the source of the radio emission, and to map sources in greater detail to identify the places of most intense radio emission.

When the two signals from the two separate telescopes are combined correctly (see intensity interferometer) they will either reinforce or cancel each other, depending on the amount of delay between the two signals. If the two telescopes of the interferometer are arranged along an east-west line on the Earth, a radio source moving across the sky will produce a series of maxima and minima as the delays between the signals received by the two telescopes change. It can be difficult to decide whether the delay is zero or a whole number of wavelengths, but the ambiguities can often be removed by observing with the radio telescopes at different separations from each other (changing the amount of the delay). For this reason many radio interferometer telescopes are mounted on railway tracks, which permit the spacings to be changed easily.

In a normal radio interferometer with two telescopes, the voltage levels from the signals received at each telescope are not added together but multiplied. In this case the power output of the combined signal is proportional to the product of the individual voltages and the individual telescope dish diameters, so a small dish can be very effective as an interferometer when working with a second larger dish of large collecting area. Interferometers can reach resolving powers of better than a thousandth of an arcsecond.

The first radio interferometers were built in the early 1950s and were used to measure the sizes and positions of the strong sources then known. Objects such as the peculiar galaxy cygnus a were shown to be emitting vast quantities of radio energy from small regions. The identification of the strong radio source Taurus A with the crab nebula supernova remnant was only possible after an accurate position had been measured with the Dover Heights Sea Interferometer near Sydney, Australia. This instrument used only one aerial perched on the top of a cliff overlooking the sea; the second aerial was formed by the reflection of the first in the water.

radio scintillation Irregular rapid changes in the apparent intensity of radio sources, caused by variations in the electron density in the ionosphere (mainly the F2 layer) and in the interplanetary gas. It occurs only with sources of less than 1" diameter. See also scintar

radio telescope Instrument used to collect and measure electromagnetic radiation emitted by astronomical bodies in the radio region of the spectrum. The radio region extends from around 10 mm to around 10-20 m. The short-wavelength region from 10 mm to 350 urn is now defined as the millimeter-wave astronomy and submillimetre-wave astronomy range, but much of the equipment and many of the techniques are the same as those in the radio region. Almost all the types of object studied with optical telescopes have also been observed with radio telescopes. These include the Sun, planets, stars, gaseous nebulae and galaxies. Furthermore, radio astronomy has been responsible for the discovery of several new and unsuspected types of astronomical phenomena, such as quasars, pulsars and the cosmic microwave background.

radio telescope Just like an optical reflector, a radio telescope depends on collection of electromagnetic radiation by a large aperture, parabolic reflector, often in the form of a dish. Radio waves are brought to a focus on the detector. Most radio telescopes can be steered in altitude and azimuth.

The first radio telescope was built by Karl jansky in the early 1930s, and he recognized that the 'noise' he was detecting came from the Milky Way. Following World War II, there was a glut of cheap (or free) radar and communications equipment. Using this equipment, experimental telescopes were built, mainly in Australia and England, but there was interest in many countries including USA, Soviet Union, France and Japan. Since radio wavelengths are large, the paraboloid receiving dish does not have to be solid (as in an optical telescope): if the radio telescope is operating at wavelengths longer than around 20 cm, it can use mesh instead. There are several types of radio telescope in addition to the standard steerable telescope. Occasionally, radio dishes have been built on equatorial mountings, similar to those used for most optical telescopes, but these have not proved popular. Radio telescopes are normally built with altazimuth mountings. That is, their basic movements are up-down and around parallel to the horizon. The alt-az mount, as it is known, presents many engineering advantages, particularly with the advent of powerful computer control systems. In fact, many new optical telescopes are being built with alt-az mounts. The largest steerable telescopes are the 100-m Effelsberg Radio Telescope (near Bonn) and the 100-m Green Bank Telescope (in Virginia), followed closely by one of the oldest, the 250-ft (76.2-m) jodrell bank telescope. The third largest steerable telescope is the 64-m (210-ft) parkes radio telescope in Australia. The 300-m (1000-ft) telescope in arecibo, Puerto Rico, is suspended over a sinkhole and does not move, although tracking capability is provided by the secondary mirror. Even with these very large telescopes the resolving power is poor by optical standards (perhaps around 60"-100" at typical wavelengths); the problem has been overcome by combining several radio telescopes using aperture synthesis to mimic a bigger telescope (up to the size of the Earth). Another approach is to use two (or more) telescopes as a radio interferometer to improve the resolution. very long baseline interferometry using a few telescopes, very widely separated, can produce a resolving power of a few thousandths of an arcsecond.

A single radio telescope does not produce a picture directly (unlike an optical telescope); the dish must be scanned forwards and backwards to build it up, and the data then fed through a computer. The computer is also vital for controlling the telescope (which may weigh 1000 tonnes), pointing it accurately at the required position in the sky, moving continually in two coordinates (altitude and azimuth), and correcting for the inevitable deformation of a large dish. The accuracy required is often better than 10", which becomes millimetres when translated into dish movements. Most radio telescopes are not located inside buildings, so the computers have to correct for weather too, such as high winds, cold and heat. Radio telescopes can also be adversely influenced by ice, rain and lightning.

radio waves electromagnetic radiation with wavelengths ranging from millimetres to hundreds of kilometres. Radio waves were first generated artificially by Heinrich Hertz (1857-94) in 1888. Radio astronomers measure radio waves in frequency units (cycles per second, called hertz, symbol Hz) as often as in wavelength units, for example the twenty-one centimetre line of neutral hydrogen and the four lines of oh at 1612, 1665, 1667 and 1720 MHz. To convert from one to the other is quite straightforward: frequency X wavelength = velocity of light (jk = c).

radio window Range of wavelengths, from around a few millimetres to about 20 m, to which the terrestrial atmosphere is transparent. The window has several subdivisions, which have become separate areas of astronomy because of the techniques used for observing them: the submillimetre, the millimetre and the microwave. At short wavelengths, the radio waves are absorbed by water vapour in Earth's atmosphere, and at radius vector Imaginary straight line between an orbiting celestial body and its primary, such as the line from the Sun to a planet. See also kepler'slaws

Ramsden eyepiece Basic telescope eyepiece consisting of two simple elements. While it has a wide field of view, the Ramsden eyepiece suffers from chromatic aberration and poor eye relief, so the kellner eyepiece is usually preferred. It is named after the English optician Jesse Ramsden (1735-1800).

Ranger First NASA space probes to investigate another Solar System body, in this case the Moon. Of the nine probes launched in 1961-65, only the last three were successful. The first success came with Ranger 7 in 1964 July, when 4316 television images of the surface at ever-increasing resolutions were returned before the spacecraft hit the region now known as Mare Cognitum. A mare area was selected because of its relatively level and uncratered surface, two essential criteria for the future apollo programme manned landings.

The final wide-angle frame covered an area about 1.6 km (1 mi) square, showing craters down to about 9m (30 ft) in diameter. The best-resolution images showed craters as small as 1 m (3 ft) in an area about 30 by 49 m (100 by 160 ft).

Ranger 8 was targeted to a point in southwest Mare Tranquillitatis, while Ranger 9 headed for a more scientific target, the crater Alphonsus. The images showed that the lunar mare areas were free from small, deep craters and widespread rock fields, and that they would support the weight of a spacecraft. The Rangers also provided accurate values of the Moon's radius and mass ratio with respect to the Earth.

RAS Abbreviation of royal astronomical society

Rasalgethi The star a Herculis, an irregularly variable red giant with an estimated diameter about 400 times the Sun's, distance about 400 l.y. It is of spectral type M5 Ib or II and is so large that it pulsates in size, ranging between about mags. 2.7 and 4.0 with no set period. It is also a binary, with a wide bluish companion of mag. 5.4, easily visible in small telescopes, which orbits it in 3600 years or so. The name comes from the Arabic ra's al-jathf, meaning 'the kneeler's head'; the figure of Hercules is traditionally depicted in a kneeling position.

Rasalhague The star a Ophiuchi, visual mag. 2.08, distance 47 l.y., spectral type A5 V. Its name comes from the Arabic ra's al-hawwa', meaning 'head of the serpent-bearer', since Ophiuchus was traditionally depicted holding Serpens.

RASC Abbreviation of royal astronomical society of canada

RASNZ Abbreviation of royal astronomical society of New Zealand

RATAN-600 See special astrophysical observatory Rayet, Georges Antoine Pons See wolf, charles Joseph Etienne

Rayleigh scattering Scattering of light by particles smaller than the wavelength of the light. It is named after the British physicist Lord Rayleigh (1842-1919). The efficiency of the scattering increases rapidly as the wavelength decreases, thus leading to blue skies on Earth, since the blue light from the Sun is scattered much more than the red by molecules in the atmosphere.

rays Radial ALBEDO features that diverge from young craters on planetary bodies that lack atmospheres; these systems of straight or curved streaks look like splash marks. Rays are prominent on the Moon and Mercury; they are also seen on some satellites of the outer planets. Rays are believed to be surface deposits of finely comminuted EJECTA excavated from craters. Rays diverge from well-formed, young craters for typical distances of 10 diameters. They traverse relief without deviation or interruption, indicating ballistic emplacement. Most rays are brighter than the underlying surface, probably due to light scattering from small grains. Rays often contain small secondary craters, which are caused by the impact of large chunks of excavated material. It is not known why this material tends to go in specific directions rather than being uniformly distributed. Probably most craters have rays when first formed, but they are gradually erased by micrometeorite bombardment and by darkening from exposure to the SOLAR WIND. TYCHO, the most prominent lunar rayed crater, 87 km (54 mi) in diameter, has rays extending to 23 diameters; they are easily visible in a small telescope, particularly close to full moon. Tycho's age is estimated at about 100 million years, testifying to the slowness of erasure and thus the much greater ages of most other lunar craters.

rays This Galileo image of the Moons northeast quadrant, was taken as the spacecraft sped past Earth en route to Jupiter. A bright ray is seen crossing the oval Mare Serenitatis at left.

R Coronae Borealis star (RCB) Star that exhibits sudden, unpredictable fades. R Coronae Borealis stars are high-luminosity stars of spectral types Bpe-R, carbon- and helium-rich but hydrogen deficient. The fades may be anywhere between 1 and 9 magnitudes; the resultant deep minima may last for a few weeks, months or for more than a year. These fades occur completely at random, without the least sign of any periodicity, and once a decline has commenced it is usually rapid, especially if the fading is of several magnitudes. The subsequent rise back to maximum is very much slower, and there are many fluctuations in brightness as the star rises. R Coronae Borealis stars spend most of their time at maximum, some then pulsating about 0.5 magnitude in a semi-periodic manner. Several show periods of around 35 to 50 days for these slow pulsations.

R Coronae Borealis star The light-curve of a typical R Coronae Borealis star shows long intervals spent at maximum brightness, punctuated by irregular episodes of abrupt dimming caused by condensation of carbon in the stellar atmosphere. Some fades are deeper than others, and the time taken to recover to maximum varies from one maximum to the next.

R Coronae Borealis stars are a small class of SUPERGIANTS, numbering about three dozen. The type star, R Coronae Borealis, was first found to be a variable in 1795. It is normally about 6th magnitude, and may remain at this maximum brightness for as long as ten years before suddenly beginning to fade. The minimum may be anywhere between 7th and 15th magnitude: there is no way of determining just how much it will fade. It remains at minimum for a few weeks or several years: again, there is no way of knowing for how long. Other stars in this group behave in a similar manner, although they are not as bright at maximum as R Coronae Borealis. RY Sagittarii when at maximum is about half a magnitude fainter than R Coronae Borealis, and the remaining members of the group are very much fainter.

In those stars for which adequate spectra are available, chemical abundances are estimated at 67% carbon, 27% hydrogen and 6% light metals.The greatest attention spec-troscopically has been given to the two brightest examples, R Coronae Borealis and RY Sagittarii. Investigators agree that both stars are much more abundant in carbon than hydrogen and also that lithium is overabundant in both, with the C2 absorption bands only weakly present. Other RCB stars have strong C2 absorption bands and are presumably cooler.

During the initial decline the normal ABSORPTION spectrum is rapidly replaced by a rich EMISSION SPECTRUM. All RCB variables have an infrared excess. Variations in the infrared do not occur at the same time as changes in the visual, and changes in either may be unrelated to changes in the other.

A number of models have been proposed to account for the behaviour of RCB variables. In one, a gas cloud forms above the photosphere and moves outwards. As it does so it cools and condenses, forming small particles of graphite. These are very efficient absorbers of light and so would account for the sudden declines in brightness. The cloud would disperse, becoming at first patchy and transparent in places. Later, the temperature and pressure would become so low that the carbon particles would condense into soot, revealing the star once more.

Alternatively, particles form in the upper photosphere, cutting off the chromosphere from its source of excitation so that it gradually decays. A third theory suggests that a cloud is ejected from the star. The chromos-pheric spectrum then results from an eclipse by the dense cloud in the observer's line of sight. This spectrum is in many ways similar to that of the solar chromosphere during a total eclipse of the Sun. At first the cloud might be much smaller than the star, but as it moved away it would grow in size and, if it were centred over the star, cause an eclipse that would result in the deep declines that are observed.

The RCB variables provide a very good example of why professional astronomers welcome the assistance of amateurs. The professionals are interested in certain parts of the light-curves of these stars, especially the commencement of deep declines. The cost of modern instruments and the demands on them mean that they cannot be used night after night, often over periods of years, in the hope that a decline of an RCB star will be observed. Professionals therefore rely on amateurs to monitor these stars continuously and to advise them whenever a fade is detected. In this way the amateurs also produce the data needed to determine how RCB stars pulsate at maxima and the period in which they do so.

Reber, Grote (1911- ) American pioneer of radio astronomy. In 1932, while working as a designer of radio sets, he heard of Karl JANSKY's discovery of cosmic radio emission and set about designing and building equipment to investigate the phenomenon for himself. Six years later, he completed a 9.57-m (31.4-ft) paraboloidal dish erected behind his house, and the ancillary instrumentation. Using a receiver working at a frequency of 160 MHz (wavelength 1.87 m), he succeeded in recording radio emission from the Milky Way.

From then until after World War II, when Stanley hey and others began to follow up their wartime research, Reber was the world's only radio astronomer. He detected radio emission from many sources (then called 'radio stars') that did not correspond to visible stars, such as cassiopeia a and cygnus a. He also found that the Sun and the Andromeda Galaxy emitted at radio wavelengths. In 1948, with the potential of radio astronomy rapidly coming to be appreciated, he tried unsuccessfully to raise funds for a 67-m (220-ft) US radio telescope; instead the world's first large dish was constructed at jodrell bank, UK, under the direction of Bernard lovell. Reber continued to map the radio sky, particularly at 1-2 MHz (150-300 m) with modest equipment of his own construction, moving to Tasmania in 1954.

reciprocity failure In astrophotography, pronounced diminution in the effective sensitivity or speed of a photographic emulsion with longer than optimum exposures. In bright conditions, image brightness is inversely proportional to exposure time (hence reciprocity). Thus, doubling the exposure time exactly compensates for a halving of image brightness. But the reciprocity fails with the very low image brightnesses encountered in astropho-tography, and much longer exposures are necessary. One way of countering this effect is to subject the film to hypersensitization.

recombination Capture of an electron by a positive ion. In a planetary nebula or diffuse nebula, hydrogen atoms (as well as other atoms and ions) are ionized (or further ionized) by ultraviolet radiation from hot stars. Protons and heavier ions then capture the free electrons, which takes the ions to their next lower ionization states (hydrogen becoming neutral). Recombination to an atom or ion can take place on any energy level. As electrons move from the levels on to which they are captured to lower levels, they radiate emission lines.

Recta Rupes (Straight Wall) Lunar fault running from 20S 8W to 23S 9W. This fault is radial to the Mare imbrium; it was probably formed by the shock wave from that impact and then covered by its ejecta. Later, basalts flooded the region, placing stress on the fracture until it activated as a fault. The fault has a slope angle of approximately 40, though in telescopes it appears much steeper. The mare ridges surrounding Recta Rupes are from the submerged walls of an ancient crater (see ghost crater).

recurrent nova (NR) binary star system in which the secondary is a late type (G, K or M) giant filling its roche lobe and transferring material to a less massive white dwarf primary. At intervals of years to tens of years this accreted material on the surface of the white dwarf explodes in a thermonuclear reaction, causing the brightness of the star to increase by 7 to 10 magnitudes for about 100 days in the visible and for double that at other wavelengths. These stars are brighter at minimum than an ordinary nova, but not because of the cycle length; the stars with longer cycles tend to show faster development and brighter maximum luminosity.

Recurrent novae may be regarded as ordinary novae that have had more than one outburst (of lesser amplitude than normal novae) or as dwarf novae with very long intervals between outbursts. Typical recurrent novae are: T Coronae Borealis, which had outbursts in 1866 and 1946 (with two smaller outbursts in 1963 and 1975); RS Ophiuchi, with outbursts in 1898, 1933, 1958 and 1985; and T Pyxidis, which flared up in 1890, 1902, 1920, 1944 and 1966. At present only six confirmed recurrent novae are known, with a few additional suspected systems. Apart from RS Ophiuchi and T Pyxidis, which closely resemble one another, the systems show considerable variations in their characteristics.

reddening Phenomenon exhibited by radiation as it passes through the interstellar medium and is absorbed by gas or dust and re-radiated. Most of this re-radiation is at a longer wavelength than the original incident radiation. As such, visible light is re-radiated at a 'redder' wavelength than it originally had. Generically, radiation is described as reddening as it travels, and this description is even applied in the infrared.

red dwarf Star at the lower (cool) end of the main sequence with spectral type K or M. Red dwarfs' surface temperatures are between 2500 and 5000 K and their masses are in the range 0.08 to 0.8 times that of the Sun. Red dwarfs are the most common type of star in our Galaxy, comprising at least 80% of the stellar population. Examples are barnard'sstar and proxima centauri.

Red dwarfs have a similar composition to the Sun, with a thick outer convection zone compared to the radius of the star. They exhibit many solar-type phenomena, such as spots, a chromosphere, a corona and flares. The rotation rate of many red dwarfs has been determined from fluctuations in their light-curves due to the presence of large, cool starspots analogous to sunspots. The slower the rotation, the less active the star. Red dwarfs with rotation rates above about 3 to 5 km/s (2-3 mi/s) often show flare activity and are known as flare stars.

All red dwarfs close enough to be studied in detail show a spectrum consistent with the existence of a chromosphere. X-ray emission suggests the existence of coro-nae; the more active the red dwarf, the higher the X-ray emission, suggesting hotter coronae. See also dwarf star

red giant Star that has finished burning hydrogen in its core and that is experiencing hydrogen shell burning. As a consequence, its atmosphere expands and its effective temperature falls to between 2000 and 4000 K, making it appear red in colour. Red giants have spectral type K or M and lie in the upper right hand part of the hertzsprung-russell diagram. They have diameters 10 to 1000 times that of the Sun. arcturus and aldebaran are red giants.

Red giants are often variable stars, with slowly pulsating surface layers. Because of the red giants' large radii, the gravitational effect falls considerably at their outer layers; they often lose substantial amounts of material from stellar winds, producing a planetary nebula.

The Infrared Astronomy Satellite (IRAS) discovered many red giant stars embedded within extensive dust shells, which, radiating at a temperature of a few hundred Kelvin, are detectable only in the infrared.

Red Planet Common name for the planet mars, owing to its striking red colour.

redshift (z) Lengthening of the wavelengths of electromagnetic radiation caused by the expansion of the Universe and curved spacetime. Wavelength shifts are commonly ascribed to the doppler effect, first described by Christiaan Doppler (1803-53) in 1842. Though the 'redshifts' that pertain to the Universe are not caused by the Doppler effect, it must be examined first.

The Doppler effect is encountered in all manner of wave motion, from water waves through sound waves to light. Its most familiar aspect is the drop in the pitch of sound from a passing automobile or aircraft. As a body approaches, the waves seem to be shifted to shorter wavelengths; if the body is receding, the wavelengths appear longer. The Doppler shift is readily detected in astronomical bodies by the shifts in the wavelengths of spectrum lines. An approaching body has its spectrum lines shifted towards longer wavelengths, in the vernacular of astronomy 'towards the blue' (a 'blue shift', no matter what the lines' actual colours). The lines of a receding object are shifted towards longer wavelengths, similarly 'towards the red' (a 'red shift'). The greater the speed along the line of sight (the RADIAL VELOCITY), the greater the wavelength shift. Under ordinary low-velocity circumstances (such that relativity is not required), the relative shift (change in wavelength divided by wavelength) equals the radial velocity (v) divided by the speed of light (c), or where Xobserved and X0 are respectively the observed and rest (laboratory) wavelengths.

Half the stars show blue shifts in their spectra, the other half red shifts. The single-word term 'redshift', however, is reserved for galaxies. The 'redshift' (z) is simply the observed relative wavelength change: which in Doppler notation is v/c, or v = zc, which works as long as the velocities and distances are - by the standards of cosmology - small.

In 1914 Vesto M. SLIPHER, at the Lowell Observatory in Arizona, began studying the spectra of objects then called 'spiral nebulae', but today known to be galaxies. He found that 11 of the 15 he examined displayed redshifts, thereby indicating a strong preference for recessional motion. Years later, Edwin HUBBLE at the Mount Wilson Observatory in California proved these objects to be (by present standards) relatively nearby galaxies, each rather like our own Galaxy. By 1929 Hubble had succeeded in proving that the redshift of a galaxy is directly proportional to its distance from Earth. This discovery, today called the HUBBLE LAW, can be written as v = H0d = zc, where d is the distance to a galaxy and H0 is a term of proportionality called the HUBBLE CONSTANT. The value of H0, which has been hotly contended, is currently set at around 70 km/s/Mpc; that is, the recession velocity of a galaxy increases by 70 km/s for every megaparsec away from the Earth. The Hubble law tells us that the Universe is expanding - its galaxies (really the clusters of galaxies) are getting farther apart.

If megaparsecs and kilometres are scaled to each other, H0 has units of inverse seconds. The inverse of H0 therefore has units of seconds, and thus gives an age for the Universe (discounting gravitational drag or Einstein's accelerative cosmological force) of around 13 billion years. The Hubble law is also reversed to find distances to remote galaxies simply by measuring their redshifts. If v = zc = H0d, then d = zc/H0. 'Redshift distances' are being used to map great portions of the Universe.

Redshifts of the most distant galaxies are so great that they can be determined through colour alone. At maximum, z exceeds 5. The simple Doppler formula would suggest that such bodies are moving away at over five times the speed of light, which is impossible. The expansion of the Universe is properly described by the theory of GENERAL RELATIVITY, formulated by Albert EINSTEIN. According to this description, space expands with time. Galaxies are not moving through space, but are riding along with space as it grows larger. Redshifts of galaxies caused by the expansion of space are therefore not the result of the Doppler effect. When a galaxy radiates energy, its photons become trapped in the web of space along with everything else. As the photons approach us, space expands, and therefore the photons stretch and their wavelengths lengthen and shift to the red. The farther away the galaxy, the longer the flight time and the greater the stretch. Distant galaxies therefore have greater red-shifts than nearby ones. Superimposed on those redshifts related to the expansion of the Universe are genuine Doppler shifts caused by the local motions of galaxies through space. Such Doppler effects can mask the effects of expansion and have historically made the determination of H0 quite difficult.

The Doppler formula (v = zc) gives velocities when the redshifts are low, much less than 1. But when the redshift z approaches 1, zc no longer equals v, and a correct formula, given by the theory of general relativity, must be used. Unfortunately, there is no unique simple formula, because the relation depends on the structure of the Universe. Indeed, we can use redshift and distance measurements to learn about the structure of the Universe.

The redshift of a given body tells us how much the Universe has expanded since the light left that body. The fractional change in wavelength during the light time of a photon is XobservedAemitted, where ^emitted is the 'rest' wavelength X0. This ratio equals the fractional degree to which the Universe has expanded, given by ^observed/remitted, where i?emitted is the distance between two galaxies at the time of photon emission and -Robserved is the distance at the time it was received. The redshift z is therefore Thus -Robserved/-Remitted, which could be thought of as a local 'radius' of the Universe, equals z + 1. That is, if the red-shift z for a given galaxy is 0.5, the Universe has expanded by a factor of 1.5 since the light departed the galaxy on its way to us.

The most highly redshifted photons are those in the COSMIC MICROWAVE BACKGROUND radiation. These photons have been travelling towards us since matter and radiation decoupled from each other (that is, when the Universe went from being opaque to transparent) about 300,000 years after the Big Bang. They exhibit a redshift of z = 1000, meaning that their wavelengths have been stretched a thousand-fold during their 13-billion-year journey to Earth.

redshift survey Method of mapping the three-dimensional distribution of galaxies in space, with the aim of determining the LARGE-SCALE STRUCTURE of the Universe, by measuring the REDSHIFTS of a large, controlled sample of galaxies, either to a certain magnitude limit or in a specific region of the sky. A notable early achievement was the CfA Redshift Survey by Margaret GELLER and her team. Current redshift surveys, such as the 6dF (Six-degree Field) Galaxy Survey or the SLOAN DIGITAL SKY SURVEY, employ bundles of optical fibres, each precisely aimed at a different galaxy, to enable the spectra - and hence the redshifts - of several hundred galaxies to be obtained at one pointing of the telescope.

redshift survey Combining data from several surveys, this plot shows redshifts for nearly 14,000 galaxies. Among the large-scale structure revealed is the Virgo Cluster, near the centre of the plot.

Rees, Martin John (1942- ) English cosmologist and, from 1995, the fifteenth ASTRONOMER ROYAL, who has studied quasars, galaxy formation and clustering, and the cosmic microwave background radiation. His models of quasars were the first to show convincingly that they are supermassive black holes at the centres of distant galaxies. In the 1960s, his studies of quasar redshifts showed that their distribution increased with increasing distance, providing strong observational evidence for the Big Bang theory, and against the steady-state model.

reference stars Stars whose positions on the celestial sphere are known to a high degree of accuracy, allowing them to be used to measure the relative positions of other celestial objects. Those whose positions are most accurately known are classed as FUNDAMENTAL STARS, their positions recorded in a FUNDAMENTAL CATALOGUE.

reflectance spectroscopy Technique that involves shining light on to a geological sample so that the interaction of the light with the atoms and crystal structure of the material produces a characteristic pattern of absorption and reflectance bands. This technique is used by planetary geologists to determine the mineralogical properties of the surfaces of planets and satellites without having actually to sample the rocks or subject them to geochemical analysis in the laboratory. See also spectroscopy

reflecting telescope (reflector) Telescope that collects and focuses light using mirrors. After an unsuccessful attempt by James gregory in 1663 to make an all-mirror telescope, the first workable reflecting telescope was demonstrated to the Royal Society, London, by Isaac newton in 1668. The lenses of the time all suffered from chromatic aberration, and the idea was to replace refracting lenses with a curved mirror. With the reflecting surface on the front face of the mirror, there was no need for the material of which the mirror was made to be transparent, so materials other than glass could be used. The first reflectors experimented with metal mirrors, as these were easier to manufacture. An alloy made from 68 parts copper and 32 parts tin, called speculum metal, was routinely used until the late 19th century, and it was not until 200 years after Newton that Jean foucault made the first silver-on-glass mirror.

The newtonian telescope uses a parabolic mirror surface to reflect the light falling upon it back to a single point known as the prime focus. By introducing a small flat secondary mirror at 45 just before this focus, the cone of light is diverted through a right angle to an eyepiece mounted on the outside of the telescope tube.

The magnification of the image is the focal length of the mirror divided by the focal length of the eyepiece. With a mirror of 2000 mm focal length, for example, a 20-mm eyepiece will magnify 100 times. The focal ratio (focal length divided by aperture) of common types of amateur reflectors is usually around ^8 or ^6. This means, for example, that a typical ^8 200-mm (8-in.) reflector will have a tube about 1.6 m (5.2 ft) long.

In the 17th century, other mirror arrangements were devised. Some, like the gregorian telescope, had such long focal lengths that they proved unwieldy and soon fell out of favour. The development in the late 17th century of the cassegrain telescope, in which light is reflected back from the secondary through a hole in the primary mirror, allowed long focal lengths to be accommodated in shorter, less cumbersome tube assemblies. In order not to have to perforate the primary, a small flat mirror can be introduced into the optical system to direct the light path outside the tube (sometimes along the declination axis). This system is known as the Cassegrain/coude (see coude focus).

Very large mirrors suffer from distortion as they expand and contract with changes in temperature, posing problems for the constructors of large professional reflectors. This problem was reduced in the 1930s by making telescope mirrors from the newly developed Pyrex, which has a very low coefficient of linear expansion. The recent introduction of quartz and ceramics means that modern mirrors hardly expand or contract at all.

Bernhard schmidt and Dmitri maksutov designed and built the first corrector-plate telescopes. They were initially intended for photographic use, but can be adapted to provide portable instruments for visual observing. All the various systems incorporate a corrector plate in front of the main mirror. There are several optical designs for this type of telescope, each giving a wide field at the prime focus which enables photographs to be taken of large areas of the sky.

The most important and delicate parts of any reflecting telescope are the reflecting surfaces, and over the years many different materials have been tried in the quest for a durable coating that will reflect the greatest possible amount of light. For many years silver was used, as a newly applied burnished finish will reflect 95% of visible light. Silver's tendency to oxidize and tarnish renders it less durable than other coats, but it can readily be renewed. Aluminium was used from the 1930s. It reflects 87% of visible light, but its application requires the mirror to be placed in vacuum during deposition, so aluminiz-ing is a job for professionals.

Other metals have been tried, but their hardness requires abrasives to be used to remove them before recoating. Rhodium is least affected by chemical and physical ageing but is very expensive and has less reflectivity than either silver or aluminium. Overcoating aluminium-coated mirrors has been experimented with to try to reduce oxidation and has proved successful in extending the life of the mirror surface. The length of time between aluminizations, however, will always depend on the care taken in looking after the telescope, the amount of use it gets and the air pollution at the telescope site. See also refracting telescope; tilted-component telescope

reflection grating diffraction grating in which light is dispersed into a spectrum by means of a series of closely spaced, equidistant parallel grooves ruled on to a metal surface. A typical spacing density for the grooves in an optical reflection grating for astronomy is about 1000 grooves per millimetre, and the size of the separation (about one micrometre) is not much bigger than the wavelength of light. Light of different wavelengths therefore reflects from the grating in different directions (diffraction) creating a high-quality spectrum. See also spectroscope; transmission grating

reflection nebula Interstellar gas and dust cloud that is seen because it scatters light from another source. That source is usually a nearby star. The efficiency of the scattering process increases rapidly as the wavelength of the light decreases, and so these nebulae often appear blue in colour. The nebulosity around the pleiades is a well-known example of a reflection nebula.

reflection nebula A small part of the nebulosity surrounding the Pleiades star cluster can be seen in this Hubble Space Telescope image. The star Merope is just out of frame at top right.

reflection variable (R) Recently designated type of extrinsic variable; it is a close binary system, in which light from the hot primary is reflected (re-radiated) from the surface of the cooler secondary. The light-curves of reflection variables have visual amplitudes of 0.5-1.0 magnitude and are essentially sinusoidal, with the maximum occurring when the hot star passes in front of the cool companion. Eclipses may occur in certain systems.

reflector See reflecting telescope

refracting telescope (refractor) Telescope that utilizes the refraction of light through lenses to form images of distant objects. In its simplest form, a refracting telescope consists of two lenses, an objective and an eyepiece. In practice, both objective and eyepiece are compound lenses, consisting of two or more components. The objective is a lens of large aperture and long focal length that forms an image of a remote object in its focal plane. The eyepiece is a smaller lens of short focal length that magnifies the image. The magnification equals the focal length of the objective divided by that of the eyepiece.

refracting telescope The optical layout of a refractor is shown in this schematic cutaway. Light is collected by the main, objective lens at the front end of the telescope and brought to a focus. The observer views a magnified image of the focused light through a smaller lens, or set of lenses, making up the eyepiece. The telescope shown here is on a German equatorial mount.

The invention of the refracting telescope is usually credited to the Dutch optician Hans lippershey, in 1608. The first astronomer to make serious regular telescopic observations was galileo in 1610, with telescopes of his own design and construction. The Galilean refractor had a convex ('positive') objective and a concave ('negative') eyepiece which was placed in front of the focal point of the objective. As a result it produced bright, erect images, but the field of view was small and the instruments were difficult to use.

Simple lenses and telescope systems suffer from a range of optical defects, or aberrations, notably chromatic aberration and spherical aberration. Seventeenth-century astronomers realized that spherical aberration and, to a lesser extent, chromatic aberration, could be reduced by making gently curved lenses of very long focal length. This approach resulted in some extraordinary instruments. For example, Johannes hevelius constructed several telescopes up to 46 m (150 ft) in length, with wooden lattice tubes which flexed badly. Christiaan huygens developed an alternative design - the aerial telescope - with the objective fitted in a short tube at the top of a mast, and connected only by a cord to an eyepiece at ground level.

In 1729 the English lawyer and amateur optician Chester Moor Hall (1703-71) realized that chromatic aberration could be substantially reduced by combining a positive lens made from ordinary crown glass with a negative lens made from flint glass - a harder material which contained lead and had a higher refractive index and a higher dispersion (a measure of the extent to which the different wavelengths are separated out by refraction). By careful selection of lens shapes, the chromatic aberration introduced by one lens could to a large extent be cancelled out by the other. A lens of this type is called achromatic, and the first one was built for Hall by the optician George Bass in 1733.

An achromat (two-lens objective) can bring only two wavelengths to a focus at exactly the same point, but the residual spread of focal points for the other wavelengths -the secondary spectrum - can be reduced by a factor of 30 or so compared with a single-lens objective. An apochromat is an objective - normally a triplet made from three types of glass - which brings three wavelengths to the same focus and reduces the secondary spectrum even further. In practice, very few astronomical refractors have anything other than two-element objectives. Eyepieces, too, must be designed to minimize optical aberrations.

With continuing improvements in materials, skills and optical technology, the 19th century became the heyday of the refractor. Although most refractors were conventional instruments, specialized designs were produced for specific purposes (see transit instrument; heliometer). Coude refractors used mirrors placed after the objective to reflect light to a fixed observing position, the coude focus. In another variant, light was reflected into a fixed refractor by a siderostat or coelostat.

The building of big refractors culminated in the 36-inch (0.9-m) instrument at lick observatory, and the 40-inch (1-m) refractor of the yerkes observatory, installed in 1888 and 1897, respectively. The Yerkes instrument is highly unlikely ever to be surpassed. Compared with modern reflectors, large refractors have many disadvantages. Chromatic aberration cannot wholly be eliminated. As lenses become larger and thicker a significant amount of light is absorbed in them. Since a lens can be supported only around its edge (whereas a mirror can be supported across the whole of its rear surface), a large lens tends to flex under its own weight; the long tube itself tends to flex and needs to be housed in a large, expensive building. An achromatic doublet has four surfaces to be accurately shaped whereas a mirror has only one. All these factors combine to make a refractor very much more expensive than a reflector of the same aperture.

Smaller refractors are widely used as guide instruments and still have a number of attractions for visual observers. Their optical components are less likely to go out of alignment than those of a reflector (see collimation). Refractors often seem to be less affected by temperature changes than reflectors, and the lack of obstructions in the light path together with the wholly enclosed tube (which cuts down air currents) is reckoned by devotees to contribute to better, steadier images on average, than those produced by reflectors of equal aperture. See also reflecting telescope

refraction, atmospheric See atmospheric refraction

refractor See refracting telescope

Regiomontanus (1436-76) Name by which the German astronomer and mathematician Johannes Muller was known. Regiomontanus was one of the first publishers of astronomy literature, completing the translation of Ptolemy's Almagest (which was published posthumously in 1496) begun by his mentor Georg von purbach. This work was not just a translation but also a critique of the Ptolemaic system, and it encouraged Nicholas Copernicus to devise his heliocentric model. After his death, a letter by Regiomontanus was discovered which stated in part that 'the motion of the stars must vary a tiny bit on account of the motion of the Earth'; this is possibly the earliest exposition of the idea of stellar parallax.

Regiomontanus Irregularly shaped lunar crater (30S 48E), 129 X 105 km (80 X 65 mi) in diameter. This crater is from the earliest period of the Moon, and so has experienced considerable degradation from impact erosion. Indeed, the walls and rims to the south and west are nearly gone, along with any evidence of this crater's ejecta. A prominent central peak is offset to the north because of loss of the crater's original wall as a result of the impact that created purbach.

regolith Loose incoherent material of any origin on the surface of a planet or satellite. The term was first suggested for Earth's materials at the end of the 19th century, but it was not widely used until the 1960s, when it was applied to lunar surface material formed by multiple meteorite impacts. Lunar regolith consists mostly of unsorted fragments (from micrometres to metres across) of local bedrock, with an admixture of material brought ballistically from distant areas. Among the other components of regolith are the products of impact melting, such as pieces of glass (often containing unmelted mineral grains), and so-called regolith breccia, in which small fragments of the bedrock and pieces of glass are cemented within a fine-grained matrix of the same composition. Lunar regolith is a few metres thick in lunar maria and thicker in lunar highlands. Regolith of Mercury and asteroids seems to be rather similar to lunar regolith. Regolith of Mars is thought to have been formed not only by meteorite bombardment but also by aeolian and, in the planet's early history, fluvial erosion and accumulation. Regolith of icy satellites is fragmented ice with an admixture of non-icy components, such as organic materials and possibly silicates.

regression of the nodes Slow retrograde (westwards) motion of the nodes of the Moon's orbit on the ecliptic, as a result of the gravitational attraction of the Sun. A full circuit takes 18.6 years, and it is the cause of the largest components of the nutation of the Earth's axis of rotation, which also has a period of 18.6 years. In fact, regression of the nodes is a common property of all planets and satellites. For the planets it is caused by perturbations of other planets, and for satellites it is caused mainly by the oblateness of the planet. The direction of motion is opposite to the direction of motion of the satellite, so for retrograde satellites (for example Triton, and some artificial Earth satellites), the nodes move around the equatorial plane of the planet in the direct sense.

Regulus The star a Leonis, visual mag. 1.36, distance 77 l.y., spectral type B7 V. It is the faintest first-magnitude star. Regulus has a wide companion of mag. 7.7 visible in binoculars and small telescopes. The name comes from the Latin meaning 'little king'.

Reinmuth, Karl (1892-1979) German astronomer who worked at the Konigstuhl Observatory, Heidelberg (1912-57) on the astrometry of asteroids. Reinmuth discovered 270 asteroids, including many Trojan asteroids, and Hermes, which in 1937 came within 800,000 km (500,000 mi) of Earth. He also discovered two short-period comets.

relativity See general relativity; special relativity

relative sunspot number Daily index of sunspot activity, R, defined as R = k (10g + f), where k is a factor based on the estimated efficiency of observer and telescope (usually 1), g is the number of groups of sunspots (irrespective of the number of spots each contains), and fis the total number of individual spots in all the groups. The relative sunspot number has also been called the Wolf number and the Zurich number, after the pioneering sunspot records begun in 1848 by Rudolf wolf at the Zurich Federal Observatory. While empirical, the formula gives a good indication of overall solar activity.

relaxation time (RT) Interval during which individual stars' orbits within a star cluster will be changed significantly by the gravitational perturbations of other stars within the cluster. As a result, some stars will then have orbits that take them out of the cluster, and 1% of the stars will so escape over the relaxation time. In about 40 RT, the stars of the cluster will either escape or collapse down to the centre to form a black hole, and this is therefore the maximum lifetime of a star cluster. For a galactic cluster like the pleiades, the relaxation time is around 10 million years, for a globular cluster it is a few hundred million years, while for a galaxy like the Milky Way it is a few hundred thousand million years.

Renaissance astronomy Astronomy as practised in Renaissance Europe, from the recovery of authentic Greek astronomical texts in the mid-15th century, to the transition from 'classical' to Newtonian and telescopic astronomy in the mid-17th century. Renaissance astronomy grew out of the astronomical traditions of medieval european astronomy, with its underlying concern with the calendar, tabular computation, and the cosmologies of Ptolemy and Aristotle. Its initial point of departure was the recovery of authentic Greek astronomical texts, to replace the corrupted Latin translations of the medieval universities, following the influx of Byzantine Greek scholars and books into Italy, especially after the fall of Constantinople to the Ottoman Turks in 1453.

In Rome in the 1460s, Cardinal Johannes Bessarion (1395-1472) encouraged astronomers such as Georg von purbach and regiomontanus to study Ptolemy in the original Greek, and to undertake a programme of observations. Indeed, the run of solar declination observations made by Bernhard Walther (1430-1504) at Nuremberg with a large set of Ptolemy's rulers between 1475 and 1504 effectively began observatory research in northern Europe. Reverential to the past as all of these astronomers were, they were active not only in producing accurate digests of Almagest, but also in comparing the Greek and Arab observations with positional measurements in their own day. Indeed, it was a concern with checking the ancients that initiated original, observation-based research in Renaissance Europe.

Problems with the retrograde motions of Mars, Jupiter and Saturn led Nicholas copernicus, who had previously made original planetary position observations in Bologna and Rome, to devise his heliocentric system of the heavens in 1513, though his monumental De revolutionibus was not published until 1543. And, like a good classical scientist, Copernicus searched for heliocentric schemes amongst the ancient Greeks before publishing his own model. De revolutionibus, however, marked a decisive turning-point in astronomical history, because of the inevitable challenge put up by the heliocentric theory. If the Earth, against reason and common sense, both rotated on its axis and revolved around the Sun, then that motion should be detectable through the slight seasonal discrepancies displayed by astronomical bodies.

The need to settle the question of a moving or stationary Earth by observation gave rise to the great enterprise of Tycho brahe. Uraniborg, his observatory on Hven island, Denmark, was Renaissance Europe's greatest centre of scientific research, with its graduate students, craftsmen, laboratories, workshops and printing press. Yet even Tycho's superb instruments were unable to detect an annual parallax. What they did produce, however, was a body of observational data from which Johannes kepler would derive his three laws of planetary motion based on elliptical orbits, which demanded the abandonment of the ancient pre-requisite of uniform circular motion.

Renaissance astronomy came to its greatest fruition in galileo's revolutionary telescopic observations after 1610. He used his observations of the lunar 'seas', Jupiter's four large satellites, sunspots and the stars of the Milky

Way to launch a full-scale assault upon the limitations of the ancients, in order to advance the Copernican heliocentric theory. Yet before Galileo forced the issue after 1616, the Catholic Church was not opposed to Copernicanism. By 1640, however, astronomy had moved away from being a purely 'classical' to an instrument-based observational science.

reseau Reference grid of dots or small crosses superimposed on an astronomical image to facilitate the measurement of positions.

residuals Differences between predicted and observed values of some quantity, such as those that arise in the analysis of astronomical observations. Residuals of the measured positions of Solar System bodies compared with predicted positions are used in the process of improving the predicted orbit. For example, the predicted orbit of a natural satellite is computed from a mathematical model, or theory, which consists of a number of periodic PERTURBATION terms affecting the orbital elements, caused by the Sun, other satellites and the oblateness of the planet. The theory contains a number of parameters, the numerical values of which are determined from observations and have to be re-determined occasionally as the time base of the observations is extended. Typical parameters are the six ORBITAL ELEMENTS, the masses of the planet and other perturbing satellites, coefficients defining the oblate gravity field of the planet, and the orientation of the planet's equator. Residuals of the observations from the theory are formed, using the current best set of parameters. The process of least squares is then used, which determines corrections to the parameters by minimizing the sum of the squares of the residuals. New residuals are formed using the corrected parameters. Mostly these residuals will be caused by the inevitable measurement errors that arise in making the observations. Ideally they should be scattered randomly, but there are always biases due to systematic errors of the observation method. There may also be signatures in the residuals as a result of inadequacies of the perturbation theory used, and this is the real incentive for the celestial mechanician - to produce a better theory, and perhaps to discover some new unexpected source of perturbation.

resolution Smallest detail visible in the image formed by an optical system. The resolution of a TELESCOPE and its instruments is limited by many factors. The over-riding one is the theoretical RESOLVING POWER of the telescope, which is determined by its APERTURE. Other limitations are the quality and design of the optics and the effects caused by the atmosphere. If the image is a direct representation of an object in the sky then the resolution will determine the level of spatial detail that can be discerned. If the image is produced by a SPECTROGRAPH then the resolution will determine the level of wavelength detail produced.

resolving power Ability of an optical system to distinguish objects close to one another, such as the two components of a binary star, or a single small object. Resolving power is measured in angular units. A telescope's theoretical resolving power in arcseconds is given by dividing 115.8 by its aperture in millimetres (or 4.56 by the aperture in inches). This measure is called DAWES' LIMIT. A 150-mm (6-in.) telescope should be able to resolve objects separated by 0".8, but in practice ABERRATION, SEEING and imperfection in the observer's eye prevent this limit from being achieved. The unaided human eye can resolve objects separated by about 1 ".

resonance Increased perturbing effect that occurs when an external force on a dynamical system has a frequency close to one of the natural frequencies of the system. In astronomy the prominent example is the large perturbing effect on an orbit as a result of a close COMMENSURABILITY of orbital period with another body. In recent work in CELESTIAL MECHANICS it has become common to use the word 'resonance' in place of the word 'commensurability'. Thus one would refer to 'the 2:1 resonance' rather than 'the resonance effect caused by the 2:1 commensurability'. Using this terminology, recent work in celestial mechanics commenced with efforts to explain the large number of resonances that occur between pairs of satellites. It is likely that TIDAL EVOLUTION of the orbits has caused one of the orbits to spiral outwards until a resonance was encountered. In many cases the satellite would pass straight through the resonance, but in some circumstances capture into the resonance can occur. The intricate detail of Saturn's rings revealed by the VOYAGER spacecraft, and the tenuous rings of Jupiter, Uranus and Neptune, have many features that have been explained by resonances with satellites (see also SHEPHERD MOON). The same variety of types of resonance occur as those between pairs of satellites, but because they are acting on a ring of particles rather than a single body their effects are rather different, causing clusters around a ring, and radial density variations. Several types of resonance have been given particular names taken from the field of galactic dynamics, including linblad, vertical and corotation resonances.

rest mass Mass of a body when it is stationary. Strictly, it is the mass of a body measured when it is at rest in an inertial frame of reference. According to the theory of RELATIVITY, if a body has a finite rest mass, its mass increases as its speed increases and would become infinite if it could be made to travel at the speed of light. Photons (which travel at the speed of light) have zero rest mass.

resurfacing Renewal of a portion of a planetary surface by covering with fresh material, which is usually erupted from beneath the surface. Rocky bodies are resurfaced by the eruption of lava, whereas icy bodies are resurfaced by floods of water freezing on the surface. Resurfacing erases previously existing features and 'resets the clock' for accumulation of craters, by which the new surface may be dated.

resurfacing Alpha Regio on Venus, as imaged by radar from the Magellan orbiter. Venus has undergone extensive resurfacing in comparatively recent geological time as a result of volcanic activity in areas such as Alpha Regio.

retardation Change in moonrise times on successive nights. As the Moon orbits Earth, it moves eastwards across the sky, rising later (being 'retarded') each night.

RETICULUM (gen. reticuli, abbr. ret) Small southern constellation near the Large Magellanic Cloud, introduced by Lacaille in the 18th century to commemorate the reticle, a grid-like device in his telescope's eyepiece used for measuring star positions. a Ret, its brightest star, is visual mag. 3.33, distance 163 l.y., spectral type G7 or G8 III. Ret is a wide double for binoculars or even the naked eye, consisting of two yellow dwarfs similar to the Sun, spectral types G1 V and G2 V, mags. 5.2 and 5.5, both 39 l.y. away.

retrograde motion (2) The cause of the apparent reversal of superior planets in their motion along the ecliptic close to the time of opposition is explained in this schematic diagram. Here, Earth (inner orbit) is seen catching up on Mars (outer orbit). Before opposition, Mars appears to move eastwards (top). As Earth, moving more rapidly on its smaller orbit, catches up, Mars' eastwards motion apparently slows, then reverses relative to the background stars as Earth overtakes around opposition. After opposition, the apparent eastwards motion of Mars resumes.

retrograde motion (2) The cause of the apparent reversal of superior planets in their motion along the ecliptic close to the time of opposition is explained in this schematic diagram. Here, Earth (inner orbit) is seen catching up on Mars (outer orbit). Before opposition, Mars appears to move eastwards (top). As Earth, moving more rapidly on its smaller orbit, catches up, Mars eastwards motion apparently slows, then reverses relative to the background stars as Earth overtakes around opposition. After opposition, the apparent eastwards motion of Mars resumes.

Reticulum See feature article

retrograde asteroid asteroid with an orbital inclination of greater than 90, so that it orbits the Sun in the opposite sense to the planets and most other Solar System bodies. By late 2001 four such asteroids were known, all with long orbital periods. These may be cometary nuclei that have exhausted their volatile constituents. See also damocles; long-period asteroid

retrograde motion (1) Orbital or rotational motion in the opposite direction to that of the Earth around the Sun, that is, clockwise when viewed from above the Sun's north pole. It is the opposite of direct motion (prograde motion). Comet halley, together with some of Jupiter's outermost satellites, moves in a retrograde motion. Venus, Uranus and Pluto have retrograde rotational motion.

retrograde motion (2) Temporary apparent east-to-west movement relative to the background stars of a superior planet before and after opposition. Because the planets orbit the Sun at different velocities, those closer to it travelling faster than those farther away, the Earth periodically catches up with and 'overtakes' a superior planet when it is at opposition. This has the effect of causing that planet to appear to stand still against the stellar background and to describe a loop (most prominently in the case of Mars) before continuing its normal apparent west-east movement. The path it traces out is known as a retrograde loop, and the places at which it appears to change direction are known as stationary points.

revolution Movement of a planet or other celestial body around its orbit, as distinct from the rotation of the body around its axis.

RFT Abbreviation of rich-field telescope

Rhaeticus (1514-74) Name by which German mathematician-astronomer Georg Joachim von Lauchen was known. He was an early supporter of the Copernican model of the Solar System, and held mathematics positions at the Universities of Wittenberg and Leipzig. He wrote Narratio prima (1540) as a popular exposition of the theories of Copernicus, whom he later persuaded to publish de revolutionibus orbium coelestium with the patronage of Duke Albert of Prussia.

Rhea Second-largest satellite of saturn; it was discovered in 1672 by G.D. cassini. Rhea's icy surface is very heavily scarred by impact craters, with no clear evidence of cryovolcanism or other resurfacing processes. It is thus one of the few large satellites of the outer planets to have a surface that resembles what was expected before the role that tidal heating can play in driving these processes came to be appreciated.

Rhea This Voyager 1 image of Saturns satellite Rhea was obtained from a distance of 128,000 km (79,500 mi) in 1980 November. Features as small as 2.5 km (1.5 mi) in diameter are visible on Rheas heavily cratered surface.

The voyager image coverage of Rhea was patchy. Details as small as about 2 km (1 mi) were revealed in the north polar region, which is heavily cratered, whereas most images of the Saturn-facing hemisphere are only good enough to show details down to 20 km (12 mi). The resolution of the best images of the opposite hemisphere is even poorer. There appear to be a few fractures and some variations in crater density, but Rhea has little in common with its neighbouring satellite, dione, other than that observations by the Hubble Space Telescope have revealed ozone trapped within the ice on both bodies. The ozone is probably formed by the breakdown of water molecules in the same manner as on Jupiter's icy Galilean satellites. See data at saturn

Rho Cassiopeiae Puzzling semiregular variable star near p Cassiopeiae. Usually it is around mag. 5, with slight fluctuations and a periodicity of about 320 days. On rare occasions it has fallen to below mag. 6, most notably between 1945 and 1947. Its type is uncertain; the spectrum is F8.

Rho Ophiuchi Dark Cloud Complex group of nebulae on the Ophiuchus-Scorpius border. It is about 500 to 600 l.y. from the Earth and its apparent size on the sky extends over an area about 300 times larger than the Moon. Nearby nebulosities around a Scorpii (Antares) and Scorpii add to the complexity of the region. The cloud contains a large amount of irregularly distributed dust, which blocks the visible light of the stars within it and behind it. The apparently empty dark areas represent the regions where the absorption of light is the greatest (see dark nebulae and pipe nebula). Nevertheless, the cloud can be penetrated to the core at radio and infrared wavelengths, and this has revealed evidence for the existence of more than 60 young stars buried in the cloud and several compact hii regions. It is estimated that about 10 per cent of the mass of the cloud has already condensed to form new stars. The thick parts of the cloud are very cold (around 10 K) and consist mainly of hydrogen in the molecular form (H2), with the densest NAGLER EYEPIECE designed to show a wide (2-5) field of view at a relatively low magnification. Such an instrument is ideal for studying starfields and hunting for novae and comets.

regions containing more than 1010 molecules m~3 (see GIANT MOLECULAR CLOUD). The molecular hydrogen cannot be observed directly, but the presence of molecules such as formaldehyde, hydroxyl (OH) or carbon monoxide (CO) can be mapped through their absorption at radio wavelengths. The regions of strongest absorption correlate well with the visibly thick parts of the obscuring cloud, confirming that the dust and the molecules cohabit in the cloud. The cloud is remarkable because it contains so many REFLECTION NEBULAE, showing a range of colours from blue to red, and so few extended HII regions.

Riccioli, Giovanni Battista (1598-1671) Italian Jesuit astronomer and topographer whose Almagestum novum (1651) included a lunar map based on many telescopic observations by him and Francesco Maria Grimaldi (1618-63). The map named major craters after Ptolemy and his followers, while relegating the supporters of Copernicus to minor features. Riccioli also observed Jupiter's coloured belts, and may have been the first to see the ashen light on Venus.

Riccioli Lunar crater (3S 75W), 160 km (100 mi) in diameter. In its original state, Riccioli was a complex crater with a central peak and terraced walls. It then underwent intense bombardment, notably by the ejecta from the ORIENTALE basin-forming event. Later yet, lava flooded the inner region, and meteorite erosion further degraded the crater. Now Riccioli appears as a deeply degraded rim with a central lava plain. It is crossed by numerous rilles.

Rigel The star p Orionis, seventh-brightest in the sky, with a visual mag. of 0.18. It is a blue supergiant of spectral type B8 Ia lying 773 l.y. away, and is 40,000 times as luminous than the Sun. Rigel has a companion of mag. 6.8, difficult to see in small telescopes because of the glare from the primary. The name comes from the Arabic rijl, meaning 'foot'.

right ascension (RA) Measure of angular distance along the CELESTIAL EQUATOR and one of the two coordinates of the EQUATORIAL COORDINATE system, the other being DECLINATION. Right ascension is the equivalent of LONGITUDE on the Earth and is measured eastwards from the FIRST POINT OF ARIES to where the HOUR CIRCLE of a celestial body intersects the celestial equator. It is occasionally expressed in degrees but is usually measured in hours, minutes and seconds of sidereal time and is equal to the interval between the TRANSIT or CULMINATION of the First Point of Aries and that of the celestial body. One hour of right ascension is equal to 15 of arc. See also CELESTIAL SPHERE

Rigil Kentaurus Popular name for the star ALPHA CENTAURI, sometimes abbreviated to Rigil Kent.

rille Linear or curvilinear surface depression on the Moon and other planetary bodies. Rilles have a variety of appearances and causes, and so are subdivided into the following major types: SINUOUS RILLES, arcuate rilles and floor fractures.

Rhea This Voyager 1 image of Saturn's satellite Rhea was obtained from a distance of 128,000 km (79,500 mi) in 1980 November. Features as small as 2.5 km (1.5 mi) in diameter are visible on Rhea's heavily cratered surface.

Rho Ophiuchi Dark Cloud The region of the Milky Way just north of Antares (the orange-red star at bottom left) is laden with dust, some of which can be seen reflecting starlight in this image. Running across this region at top left is the obscuring dusty material of the Rho Ophiuchi Dark Cloud. It cuts off the light from the rich starfields beyond. Our view in this direction is towards the centre of the Galaxy.

Richard of Wallingford (c. 1292-1336) English churchman-astronomer, abbot of St Albans and a pioneer of mechanical clockwork. He designed a clock for the abbey whose dial was a planispheric astrolabe showing the daily rotation of the sky, and the positions of the Sun and Moon. The most sophisticated device of its kind, it was essentially a model of the medieval cosmos.

Richer, Jean (1630-96) French astronomer, explorer and surveyor whose work during a scientific expedition to Cayenne, French Guyana (1671-73), helped to establish the scale of the Solar System and the shape of the Earth. From Richer's observations of Mars from South America, Giovanni Domenico CASSINI derived a good approximation for the ASTRONOMICAL UNIT. Richer also found that a pendulum at Cayenne, near the equator, ran more slowly than at Paris, which meant that gravity at the equator was slightly weaker than at far northerly latitudes, a result later used by Isaac Newton and others to show that the Earth is flattened at its poles.

rich-field telescope (RFT) Low-power telescope that is equipped with a wide-angle eyepiece (such as a Sinuous rilles are characterized as meandering channels of relatively small width, with sloping sides; they terminate in mare regions.

Arcuate rilles have flat floors between steep sided walls (see graben). They occur in parallel sets that are roughly concentric with a basin ring. Examples of arcuate rilles are seen around Mare Serenitatis and Mare Humorum.

Floor fractures are characterized by radial or concentric fractures in crater floors. Such craters occur along mare boundaries, suggesting that the lava that filled the basin also tracked up faults beneath these craters and collected beneath their floors, fracturing them into plates.

ring, planetary Band of particles orbiting a planet inside its roche limit, where tidal forces prevent them from accreting into a satellite. All four of the large outer planets are surrounded by ring systems, which have complex and varied structures.

jupiter has a narrow, tenuous ring of small particles, probably of silicate composition, at about 1.8 planetary radii. It is surrounded by a halo of charged grains of micrometre size, which are levitated by the planet's magnetic field. The outer 'gossamer' ring has a very low density and extends out to 2.9 radii.

saturn has three broad main rings, named C, B and A, in order of distance outwards from the planet. They extend from 1.2 to 2.3 planetary radii. There is also a faint inner D ring, which may extend down to the top of the planet's atmosphere. Despite their great width, these rings are less than 100 m (330 ft) thick. They consist of particles of sizes from about 1 cm to 10 m (0.3-33 ft), which are composed mostly of water ice. The main rings have radial variations in density and several gaps, such as the cassini division, which are produced by satellite perturbations. Saturn also has a narrow F ring confined between two small satellites, pandora and prometheus. The tenuous E and G rings consist mainly of micrometre-sized grains; the E ring extends out to about 8 planetary radii.

uranus has 11 narrow rings of widths 5 to 100 km (3-60 mi) in the region 1.45 to 2 radii. They are separated by wide gaps containing dust and believed to be occupied by unseen small satellites. neptune has two broad and four narrow dusty rings between 1.65 and 2.4 radii; the outermost contains denser regions known as ring arcs. The rings of Uranus and Neptune consist of dark material, probably of carbon-rich composition; both sets of rings contain bodies of metre to kilometre size.

Much of our knowledge of these ring systems comes from the voyager spacecraft. Their varied configurations are due primarily to gravitational perturbations by satellites in their vicinity. Saturn's rings may either be primordial or the result of tidal disruption of a captured large cometary body or centaur. The other ring systems, which are much less massive, are probably the result of disruption of small satellites by cometary impacts. The fine dust in these systems is constantly renewed by mete-oroid bombardment of the larger ring particles.

ring arcs Longitudinal structures within the outermost of Neptune's rings over which the ring's brightness varies by about a factor of three. They extend for between 1 and 10. Five such regions were discovered by the voyager 2 spacecraft. As orbital motions of ring particles would quickly even out such features, they must be maintained by some mechanism, most likely resonant perturbations by one or more small satellites. See also resonance

ring galaxy galaxy, clearly distinct from a spiral or elliptical, in which a ring of stars surrounds the nucleus like the rim of a wheel. Ring galaxies are thought to arise as a result of a density wave produced when a small galaxy passes through a larger one. Ring galaxies are also distinct from ringed galaxies, in which normal stellar dynamics (often in the presence of a bar) have channelled stars into a prominent ring about the galaxy's centre. Collisional rings often have an off-centre nucleus, or none at all. See also cartwheel galaxy

ring galaxy NGC 4560A in Centaurus, as shown in this Very Large Telescope (VLT) image, appears to consist of a lenticular galaxy surrounded by a ring of dust and stars. The structure is almost certainly the result of a collision between two galaxies.

Ring Nebula (M57, NGC 6720) Bright planetary nebula located midway between p and y Lyrae (RA 18h 53m.6 dec. +3302'). The nebula has an elliptical profile, with a diameter across the major axis of 71" and a darker central region. Although the Ring Nebula, at overall magnitude + 8.8, is an easy object for small telescopes, even large instruments struggle to reveal the magnitude + 15.3 central star. The nebula is toroidal in shape and is seen almost end-on. It was formed about 5500 years ago and lies at a distance of 4100 l.y. The Ring Nebula was discovered by the French astronomer Antoine Darquier (1718-1802) in 1779 January.

Ring Nebula The planetary nebula M57, the Ring Nebula in Lyra, has been produced by ejection of material from the hot white dwarf star at its centre.

Ring-Tail Galaxy See antennae

Ritchey, George Willis (1864-1945) American optical designer and craftsman, co-inventor of the ritchey-chretien telescope design. He served under George Ellery hale as chief optician and then head of instrument construction at Yerkes Observatory (1899-1904) and Mount Wilson Observatory (1905-09).

Ritchey designed and built both the optics and mounting for Mount Wilson's 60-inch (1.5-m) reflector, and figured the optics for the 100-inch (2.5-m) hooker telescope. He used these instruments to take photographs of nebulae and galaxies that showed more detail than could be observed visually, and concluded that spiral galaxies are similar to our own Milky Way. In 1917 he made some of the first distance estimates to M31, using novae and supernovae as 'standard candles'. In 1923 Ritchey emigrated to France, becoming director of Paris Observatory's astrophotographic laboratory (1924-30). With the Frenchman Henri Chretien (1879-1956), he designed a type of telescope, now named after them, that minimizes spherical aberration and coma.

Ritchey-Chretien telescope Modified form of cassegrain telescope, designed originally by George Willis ritchey and Henri Chretien. The primary and secondary mirrors are hyperboloidal, although there are variants with near-hyperboloidal, paraboloidal or ellip SOIDAL components. The system is free from COMA over a wide field, though there is some ASTIGMATISM and FIELD CURVATURE. See also VERY LARGE TELESCOPE

Rittenhouse, David (1732-96) American surveyor, instrument-maker and astronomer. Rittenhouse made many precise clocks and surveying instruments, including the vernier compass, of which he was the purported inventor. To astronomy he introduced the use of spider-web crosshairs and gratings. For the 1769 transit of Venus he constructed a transit telescope, quadrant and pendulum clock. Rittenhouse also made ORRERIES and other instruments to demonstrate astronomical phenomena.

Roche, Edouard Albert (1820-83) French mathematician, professor of pure mathematics at Montpellier, who studied the shapes of rotating fluid masses, deriving the ROCHE LIMIT for planetary satellites. He found that for a parent and satellite of equal density the equilibrium point is about 2.5 times the radius of the parent; a satellite venturing any closer than this will be broken apart by tidal gravitational forces.

Roche limit Minimum distance from a planet at which a satellite can remain intact, without being torn apart by gravitational forces. It was noted in 1848 by Edouard ROCHE that a satellite orbiting close to its planet is subjected to great stresses because the nearer parts of its body try to orbit faster than the more distant parts. If close enough, these stresses will exceed the mechanical strength of the satellite, and it will be ripped apart. Also, at this distance a ring of particles will never be able to form into a satellite by ACCRETION. The Roche limit is usually quoted as 2.5 times the planet's radius, though it is now recognised that it depends strongly on the internal strength of the satellite, and also slightly on the densities of the planet and satellite. For a body in hydrostatic equilibrium (that is, little internal strength) the limit is about 2.46 radii, and for a small rocky satellite it is about 1.44 radii. The ring systems of Jupiter, Saturn, Uranus and Neptune all lie within their planet's hydrostatic Roche limit.

Roche lobe Surface that defines the maximum sizes of stars in a BINARY STAR system relative to their separation. If both components are well within their Roche lobes, the system is termed a DETACHED BINARY. If one component fills its Roche lobe, material escapes through the inner LAGRANGIAN POINT, L1, on to its companion, and the system is termed a SEMIDETACHED BINARY. If both stars fill their Roche lobes, the system is a CONTACT BINARY. In contact binaries, material can escape entirely from the system through the outer Lagrangian points, L2, creating common envelope binaries.

Roche lobe In a binary star system, the stars (here marked M1 and M2) have their own gravitational sphere of influence, defined by the Roche lobe. Dependent on the evolutionary state and relative masses of the partners in a binary system, material from one star, or both, may fill the respective Roche lobes. Overspill of material from a distended, highly evolved star from its Roche lobe into that of a smaller more massive partner is an important driving mechanism in several forms of cataclysmic variable stars.


rocket astronomy Astronomical research carried out using instruments flown on suborbital rockets. Today, most astronomical observations are undertaken either from ground-based observatories or from orbiting satellites, but important discoveries have been made using instruments carried on balloons (see BALLOON ASTRONOMY) and suborbital sounding rockets. Sounding rockets provide the only means of making in situ measurements between the maximum altitude for balloons, about 50 km (30 mi), and the minimum altitude for satellites, about 160 km (100 mi). Larger rockets can be flown to altitudes of more than 1300 km (800 mi) and can carry payloads of up to 550 kg. Rocket-borne experiments are valuable for studies of the upper atmosphere and the Earth's radiation belts, and have also led to breakthroughs in our knowledge of the Sun, stars and galaxies.

During the first half of the 20th century, astronomers became increasingly aware that, although visible light from cosmic sources penetrates to the Earth's surface, most other wavelengths are absorbed or reflected by the atmosphere. The solution was to lift instruments above the blanket of air, but the enabling technology became available only after World War II, when captured German V-2 missiles were put to more peaceful uses.

On 1946 October 19, V-2 carried a spectrometer above the ozone layer to make the first detection of ultraviolet radiation from the Sun. Proof that X-rays were emitted by the Sun came in 1949, when Herbert FRIEDMAN flew Geiger counters on a modified V-2 launched from White Sands, New Mexico. Another early pioneer of rocket astronomy was James VAN ALLEN, who flew experiments from White Sands on the Aerobee rocket to observe cosmic rays before they collided with Earth's upper atmosphere.

The use of sounding rockets for scientific research received a boost during the INTERNATIONAL GEOPHYSICAL YEAR (IGY) of 1957-58, which coincided with a period of enhanced solar activity. Almost 300 suborbital rockets were launched during the IGY by the United States, and another 175 by the Soviet Union. By the late 1950s, X-ray observations from sounding rockets had shown that the temperature of the solar corona reaches several million degrees. Such observations revolutionized our understanding of nuclear reactions inside the Sun, but it was more difficult to capture the small amounts of short-wave radiation from more distant cosmic sources.

The initial breakthrough came when a rocket-borne spectrograph developed by Donald Morton (1933- ) and Lyman SPITZER of Princeton University made the first detection of ultraviolet light from two stars in the constellation Scorpius. Within a short time, several hundred stars as faint as magnitude 6.5 had been observed in the ultraviolet.

The first detection of cosmic X-rays came on 1962 June 18, when a team led by Italian physicist Riccardo GIACCONI discovered a distant source in Scorpius, which they called SCORPIUS X-1, and a completely unexpected diffuse glow of X-rays known as the cosmic X-ray background. The following year, Friedman found a second source, Taurus X-1, which was later shown to be the X-ray counterpart of the CRAB NEBULA. Over the next 10 years, rocket surveys revealed more than 40 X-ray sources, two of which were extragalactic (the active galaxy M87 and the nearest quasar, 3C 273).

Advances in infrared astronomy suffered from the difficulties of developing instruments and operating them at very low temperatures. The first success was the project HISTAR to launch a 166-mm (6 i-in.) telescope cooled by liquid helium. Seven flights were made from White Sands between 1972 April and December, with two more from Australia in 1974 to survey the southern sky. Although the total observing time was only about 30 minutes, the project generated a catalogue of more than 2000 celestial infrared sources.

Much of the research previously undertaken with sounding rockets has since been taken over by orbiting observatories, but sounding rockets are still widely used in the United States, Europe, Japan, India and Brazil. NASA currently uses 13 different suborbital launchers and conducts about 25 launches annually from sites at White Sands, Wallops Island and Poker Flat in Alaska, although many of these are devoted to microgravity experiments rather than astronomy.

Their rapid launch capability and recovery time (a typical flight lasts no more than half an hour) and relatively low cost also ensure that suborbital rockets still have an important role to play. Their quick response capability was demonstrated by the launch of six NASA rockets from Australia in 1987-88 to monitor supernova 1987a.

Sounding rockets also serve as low-cost test-beds for new techniques, instrumentation and technology intended for future satellite missions. For example, the payloads flown to study Supernova 1987A included the first photon-counting CCD X-ray camera ever flown. NASA missions that have benefited from precursor flights include the Compton Gamma Ray Observatory, the Solar and Heliospheric Observatory (SOHO) and the Transition Region And Coronal Explorer (TRACE).

Current research areas for rocket astronomy include observations of the ionosphere, the aurora and processes in the magnetosphere, together with X-ray studies of the Sun and supernova remnants. These frequently take place as part of coordinated observational programmes involving ground-based instruments and satellite overpasses, for example during total solar eclipses.

Romer, Ole Christensen (1644-1710) Danish astronomer, the first to obtain a reasonable approximation for the value of the speed of light (c). In 1671 Romer, then professor of astronomy at the University of Copenhagen, was offered a post at paris observatory by Jean Picard (1620-82), who was impressed with the accuracy of the Dane's measurements. At Paris, the director Giovanni Domenico cassini had instigated a broad programme of observations with the object of drawing up tables that could be used by navigators to find longitude at sea. It had been suggested that the periodic eclipses by Jupiter of the Galilean satellites would provide a suitable standard.

In 1675 Romer, checking eclipse times based on predictions by Cassini, found that they did not match the observed times: intervals between successive eclipses decreased as the orbital motions of the Earth and Jupiter brought them closer together, and increased as the two planets moved farther apart. Romer correctly deduced that the differences were caused by light taking a finite time to travel the intervening distance. The next year, he was able to announce a value for c equivalent to about 225,000 km/s (140,000 mi/s), three-quarters of the true value.

Romer returned to his homeland in 1681, becoming the Danish Astronomer Royal in Copenhagen, where he invented the transit telescope.

roof prism Optical component often used in compact binoculars to produce an image that is the right way up and the right way round as seen by the user. The objectives and eyepieces in a pair of binoculars would produce inverted images if used alone, so another component such as a roof prism must be placed between them to cancel out this inversion. Unlike the porro prisms that they are beginning to replace where size is important, roof prisms permit the objectives and eyepieces to be in line, leading to a more compact shape.

Roque de los Muchachos Observatory (Observato-rio del Roque de los Muchachos, ORM) Major optical astronomy facility on the island of La Palma in the Canary Islands at an elevation of 2400 m (7870 ft). It occupies some 2 sq km (0.8 sq mi) on the highest peak of the Caldera de Taburiente and is named after the 'Rock of the Companions' at the very highest point. The observatory belongs to the instituto de astrofisica de canarias (IAC), and was developed jointly by Spain, the United Kingdom, Denmark and Sweden. The site was established as an observatory in 1979 and inaugurated in 1985. Its clear skies, atmospheric stability and freedom from light pollution make it one of the northern hemisphere's best locations for optical astronomy.

The ORM's facilities include the 4.2-m (165-in.), 2.54-m (100-in.) and 1.0-m (39-in.) telescopes of the isaac newton group, the 3.5-m (138-in.) galileo national telescope and the 2.6-m (102-in.) nordic optical telescope. Other instruments include the Carls-berg Automatic Meridian Circle for positional astronomy, the HEGRA gamma-ray facility and a solar tower operated by Sweden. In 2001 the 1.2-m (48-in.) Mercator Telescope of the Catholic University of Leuven, Belgium, was completed, and the 2.0-m (79-in.) liverpool telescope went into operation the following year. When it is completed in 2003, the largest conventional telescope on the site will be the 10.4-m (34-ft) gran telescopio canarias, although a 17-m (56-ft) Cherenkov radiation telescope is also under construction.

Rosalind One of the small inner satellites of uranus, discovered in 1986 by the voyager 2 imaging team. Rosalind is about 58 km (36 mi) in size. It takes 0.558 days to circuit the planet, at a distance of 69,900 km (43,400 mi) from its centre, in a near-circular, near-equatorial orbit.

Rosat (Roentgen Satellite) Joint German/UK/US mission launched in 1990; it produced the first high-resolution, all-sky astronomical surveys at X-ray and extreme-ultraviolet (EUV) wavelengths. It detected about 150,000 X-ray sources and 600 EUV sources, and obtained more than 9000 observations of objects such as quasars, galaxy clusters, black holes, supernova remnants and protostars. Discoveries included X-ray emissions from comets. The mission ended on 1999 February 12.

Rosat This all-sky map of diffuse X-ray emission was obtained from Rosat. The brightest sources are supernova remnants and the central region of the Galaxy.

Rosetta european space agency (ESA) spacecraft to be launched in 2003 May. It will become the first spacecraft to orbit a comet, Wirtanen, and to deploy a lander to make the first landing on the nucleus of the comet, in 2012. At the time Wirtanen will be in an active phase, close to the Sun. Rosetta will require a series of planetary gravity-assisted flybys to pick up enough speed to reach Wirtanen. It will reach Mars in 2005 May and then fly back towards a rendezvous with the Earth in 2005 October and 2007 October, before being directed towards its path for Wirtanen. En route, Rosetta will fly past two asteroids, 4979 Otawara and 140 Siwa (the largest asteroid to be explored by a spacecraft), at a distance of 1000 km (600 mi), in 2006 July and 2008 July respectively. After ajourney of 5.3 billion km (3.3 billion mi), Rosetta will rendezvous with Wirtanen in 2011 November, eventually entering orbit around the comet the following May, while a small lander will fly 2 km (1.2 mi) to the surface of the nucleus, anchoring itself by a small harpoon.

Rosette Nebula (NGC 2237-2239) Large, visually rather faint emission nebula in northern Monoceros (RA 06h 32m.3 dec. +0503'). Covering an area of 80'X 60', the Rosette emits strongly in the wavelength of hydrogen-alpha and is well recorded on red-sensitive films. The Rosette lies 4900 l.y. away and has an actual diameter of 90 l.y. Embedded within it is the bright star cluster NGC 2244, the stars of which formed from the nebulosity about 500,000 years ago. Ultraviolet emission from NGC 2244's hot O-class stars has cleared a central cavity of about 30 l.y. in diameter in the nebula. Star formation is probably still going on here: detailed images reveal the presence of numerous Bok globules in the Rosette. Several sections of the outer ring have been assigned their own NGC numbers.

Ross, Frank Elmore (1874-1960) American astronomer whose work covered celestial mechanics, optical design and astrophotography. In 1905, while at the Carnegie Institution, he computed the orbit of Saturn's satellite Phoebe. As director of the International Latitude Observatory in Maryland (1905-15), he designed and built the first photographic zenith tube (1911) for use in a programme to determine precisely the wobble of the Earth's rotational axis. Ross spent the next nine years at Eastman Kodak, where he designed wide-angle photographic lenses and developed highly sensitive photographic emulsions for imaging faint astronomical objects. He then moved to Yerkes Observatory (1924-39), where he carried out photographic surveys of our Galaxy that resulted in Atlas of the Northern Milky Way (1934-36) and New Proper-Motion Stars (1929-39), a catalogue of stars he discovered with high proper motions.

Rosse, Third Earl of (William Parsons) (1800-1867) Irish amateur astronomer who constructed large telescopes used to discover the spiral structure of galaxies. After serving as a member of parliament (1821-34), he determined to build reflecting telescopes to rival those of William herschel. His largest instrument, known as the Leviathan of Parsonstown, had an aperture of 72 inches (1.8 m). Working at special workshops built at his estate in Birr, Ireland, Parsons developed a new method of casting mirrors from a solid disk; he invented a ventilator that solved the common problem of the metal cracking during the cooling process. The Leviathan's primary mirror was cast in 1842; Parsons built a steam-driven machine to grind and polish its surface to the desired shape. The rest of the telescope was completed in 1845.

Parsons used his big reflectors to survey the star clusters and nebulous objects of the northern sky; they clearly showed the spiral arms of galaxies such as M51, the Whirlpool Galaxy, which he sketched. He discovered 15 spiral galaxies and gave the Crab Nebula (M1) its name. His son Laurence Parsons (1840-1908), the fourth earl, continued to work with the Leviathan, but after he died it was dismantled. Another son, Charles Algernon Parsons, was an engineer who continued Howard Grubb's telescope-making business under the name grubb, parsons & co. See also birr castle astronomy

Rossi, Bruno Benedetto (1905-93) Italian-American physicist regarded as the founder of high-energy astrophysics. After chairing the physics department (1932-38) at Padua, Italy, Rossi emigrated to Denmark, England and finally the USA, where he held the chair of physics at the Massachusetts Institute of Technology (1946-71). In the 1930s Rossi had shown that cosmic rays are highly energetic, positively charged particles that constantly bombard the Earth; in the late 1950s he made pioneering studies of the interplanetary medium, finding it to consist of highly ionized gases. In 1963 Rossi and his team discovered Scorpius X-1, the first known extraterrestrial source of X-rays besides the Sun, opening up the field of high-energy astronomy. The Rossi X-Ray Timing Explorer (RXTE) satellite, launched in 1995, was named after him.

rotating variable variable star that exhibits variations in light output as it rotates; the variations are a result of its ellipsoidal shape (with consequent changes in apparent surface area) or its non-uniform surface luminosity. The latter may arise through starspots or through magnetic effects, as in stars where the rotational axis does not coincide with the magnetic axis ('oblique rotators'), or through reflection. See also alpha2 canum venaticorum star; by draconis star; ellipsoidal variable; reflection variable

rotation Spinning motion of a body around an axis as opposed to orbital motion about another body. The time it takes a celestial body to complete one revolution about its rotational axis is a measure of the length of its day. Some degree of rotation seems to be a general property of all classes of celestial body.

rotation curve Plot of the way in which the average linear velocities through space of stars and nebulae within a galaxy vary with their distances from the centre of the galaxy. If stars orbited in a manner similar to the planets in the Solar System, then the rotation curve would show a steady decrease with increasing distance from the centre. Instead, for many galaxies the velocity is more-or-less constant away from the central regions of the galaxy. This is probably due to large amounts of matter being present in the galaxy's halo and corona.

Rowland, Henry Augustus (1848-1901) American physicist who devised a method of ruling diffraction GRATINGS more finely than previously, achieving nearly 1700 lines per millimetre (43,000 lines per inch). Using his own gratings, Rowland also made a detailed map of the solar spectrum that was named after him.

Royal Astronomical Society (RAS) Britain's main organization for professional astronomers (though amateurs are admitted), founded in 1820 and receiving its Royal Charter in 1831. The Society's aims are the encouragement and promotion of astronomy and geophysics, which it achieves by publishing the results of scientific research and holding regular meetings. Its main publications are Monthly Notices and Geophysical Journal International, both of which have international reputations. The RAS headquarters are at Burlington House, Piccadilly, London.

Royal Astronomical Society of Canada (RASC) Principal astronomical body in Canada, with a membership of around 4500 amateur and professional astronomers. Originally established in the mid-19th century, the organization was granted its Royal Charter in 1903. The Royal Astronomical Society of Canada (RASC) is based in Toronto, with 25 regional centres across the country. A bi-monthly Journal publishes observational reports and analyses, and the annual RASC Observer's Handbook is a useful guide to astronomical phenomena.

Royal Astronomical Society of New Zealand (RASNZ) New Zealand's main organization for amateur and professional astronomers, who work in close collaboration principally on variable star observations and their analysis. Founded in 1920, the RASNZ received its Royal Charter in 1967 and is based in Wellington. It has around 200 members drawn from the islands. Observational work is coordinated by special interest groups and sections, with reports published in a quarterly journal Southern Stars.

Royal Greenwich Observatory (RGO) Britain's former national astronomy institution, so named after its move from GREENWICH OBSERVATORY in 1948. The new location at Herstmonceux Castle, near Eastbourne, Sussex, offered much-improved observing conditions, and by 1958 the move was complete. A comprehensive suite of telescopes at the new site included the 0.91-m (36-in.) Yapp Reflector (which was formerly at Greenwich), the largest instrument at Herstmonceux until the 2.5-m (98-in.) ISAAC NEWTON TELESCOPE was completed in 1967. In the 1970s, the RGO played a major role in developing the ROQUEDE LOS MUCHACHOS OBSERVATORY, eventually the home of the ISAAC NEWTON GROUP OF TELESCOPES. With the shift in emphasis to overseas observing, there was no need for telescopes on British soil, and the RGO underwent a controversial and protracted move to Cambridge in 1990. Further controversy over the location of a UK ASTRONOMICAL TECHNOLOGY CENTRE led to the closure of the RGO in 1998 October, bringing to an end 323 years of distinguished astronomical achievement.

Royal Observatory, Cape of Good Hope Originally a southern-hemisphere outstation of the GREENWICH OBSERVATORY, the observatory was proposed in 1821 and completed in 1828 near Cape Town, South Africa. During the 19th century, it generated most of the accurate star positions measured in the southern skies. Under David GILL the observatory's work was expanded to include the compilation of star catalogues from photographic surveys (see CAPE PHOTOGRAPHIC DURCHMUSTERUNG). At the beginning of 1972 the observatory broke its 140-year-long association with the ROYAL GREENWICH OBSERVATORY and became the SOUTH AFRICAN ASTRONOMICAL OBSERVATORY. The original observatory buildings serve as the new organization's headquarters.

Royal Observatory, Edinburgh (ROE) Scotland's national observatory from 1822 until the formation of the UK ASTRONOMICAL TECHNOLOGY CENTRE (ATC) in 1998. It began its life at Calton Hill, Edinburgh, where it rose to prominence under the direction of Charles Piazzi SMYTH, Astronomer Royal for Scotland 1846-1888. A move in 1896 to its present site at Blackford Hill, 3 km (2 mi) south of the city centre, allowed the observatory to escape the pollution of Edinburgh, and in 1928 a new 0.91-m (36-in.) reflector was installed. The ROE's outsta-tion at Monte Porzio Catone, Italy, was opened in 1967, followed by the UNITED KINGDOM SCHMIDT TELESCOPE in 1973 and the UNITED KINGDOM INFRARED TELESCOPE in 1979. Growing expertise in astronomical technology eventually led to ROE being selected as the UK's ATC in 1998.

With the transfer of its remaining astronomy support functions to the INSTITUTE FOR ASTRONOMY, UNIVERSITY OF EDINBURGH (with which the observatory had always had close ties), ROE formally ceased to exist. However, the name is retained for the site and the Visitor Centre.

r process Method of creating heavy stable nuclei within the interiors of stars by successive capture of neutrons. In the r process, neutrons are added rapidly before the nuclei has time to decay, a process that occurs in SUPERNOVAE. See also NUCLEOSYNTHESIS; PPROCESS; SPROCESS

RR Lyrae variable (RR) Type of pulsating VARIABLE STAR. RR Lyrae stars are radially pulsating GIANTS of spectral classes A-F, with periods that are generally fractions of a day. At visible wavelengths their amplitudes are 0.2-2 mag. These stars are often called short-period Cepheids or cluster variables. The latter name originated when Solon BAILEY, in 1895, discovered numerous examples in GLOBULAR CLUSTERS. It was soon found that some of the brightest globular clusters contained many of these stars, and within a few years hundreds had been found.

RR Lyrae variable This is the light-curve of a typical RR Lyrae star. Like Cepheids, these stars show a rapid rise to peak light followed by a slower decline. Commonly found in globular clusters, RR Lyrae stars have much shorter periods than Cepheids.

These rapidly pulsating stars have light-curves that differ from classical CEPHEID VARIABLES. Most rise very quickly to maximum in only a tenth or less of their total period. Their minima are comparatively prolonged, so that for a few hours their light remains constant. Their periods range from 1.2 to 0.2 days. In 1900 the first short-period Cepheid was found outside a globular cluster; it was discovered by Williamina Fleming, at Harvard, and was RR Lyrae, the type star of these variables. At first it was thought that this star must have escaped from a globular cluster, but as more and more such stars were discovered away from globular clusters the idea was dropped. There are now more than 6000 of these stars known (including subtypes), and roughly half are found in clusters. Most have periods between 9 and 17 hours, with many around 13 hours. In the Small Magellanic Cloud some RR Lyrae stars have periods as long as two days, but apart from these there is a fairly sharp cut-off in periods after 1.2 days.

RR Lyrae variables are giant stars; those in our Galaxy belong to the halo component. Some are known to have variable light-curve shapes as well as variable periods. If these changes are periodic, they indicate what is called the BLAZHKO EFFECT. The maximum expansion velocity of the stars' surface layers almost coincides with maximum light: in this respect they resemble the classical Cepheids. Some stars show two simultaneous operating pulsating modes - the fundamental and the first overtone; such stars are placed in a subtype of RR Lyrae variables designated RRB. Another subtype (RRAB) has stars with asymmetric light-curves, with steep rises and periods from 0.3 to 1.2 days. Their amplitudes range from 0.5 to 2 mags. The type star, RR Lyrae, belongs to the RRAB subtype. Stars of another subtype (RRC) have nearly symmetrical (sinusoidal) light-curves with periods of 0.2 to 0.5 days and amplitudes that do not exceed 0.8 mag. The absolute magnitude of RR Lyrae variables is about +0.5. They are too faint to be seen in any but the nearest external galaxies, such as the dwarf system in Sculptor. Those found in the Magellanic Clouds are at about the observable limit.

RR Lyrae variables may be used as distance indicators out to about 650,000 l.y. The method used depends on the star's motion in space. It is assumed that all the stars in a globular cluster are at about the same distance and therefore have the same velocities in space. The radial velocity is one-third of the total space velocity. With these assumptions, the approximate distance to a globular cluster is found and this, in turn, gives its absolute magnitude. From the absolute magnitudes of clusters the size of the Galaxy may be calculated, because the globular clusters form a halo around it. It is then possible to extend these measurements to the nearest external galaxies.

RS Canum Venaticorum star (RS) Type star for a distinctive set of binary stars in which one star interacts with the other to produce giant starspots, resulting in quasi-periodic variations in light with amplitudes of up to 0.2 mag. RS Canum Venaticorum stars are X-ray sources and generate radio emission from flares.

R star Warm carbon star. Class R stars, recognized by bands of C2, CN and CH, describe spectra that track oxygen-rich stars from class G4 to Ml (5000 to 3600 K). The class - along with the cooler nstars - has been subsumed into C, with subclasses R0-R9 becoming roughly C0-C5. The classical carbon stars are on the second-ascent asymptotic giant branch of the hertzsprungrussell diagram. The warmer R stars may be helium-burning red giants.

Rubin, Vera Cooper (1928 ) American astronomer who discovered the first persuasive observational evidence for the existence of dark matter in galaxies. Working at the Carnegie Institution from 1965, Rubin specialized in galaxy dynamics and clustering. In 1978 she and her colleagues observed that giant gas clouds in galaxies showed high rotational velocities that were consistent with a 'dark matter' model first proposed five years earlier by Jeremiah Ostriker (1937- ) and Jim peebles. She realized that only haloes of dark matter beyond the visible boundaries of these galaxies could keep the gas clouds orbiting. Rubin also discovered (1951) peculiar velocities of galaxies as they move through intergalactic space, suggesting that the Universe's large-scale structure is less uniform than previously thought.

runaway star Star moving with very high space velocity, typically hundreds of kilometres per second. They are usually of spectral class O or B. Some are thought to have been ejected from a close binary system when its companion exploded as a supernova. Some of the youngest are likely to have escaped from their birthplaces through the slingshot interactions of single and binary stars, and several can be traced to particular star-forming regions. For example, the massive stars j Columbae, AE Aurigae and the binary i Orionis all seem to have left the Orion Nebula about 2.5 million years ago. They may indeed have originally been members of two close binaries that underwent a close encounter, flinging two stars away in opposite directions and leaving the other two in a very eccentric mutual orbit. See also high-velocity star

runaway star O and B stars ejected by supernova explosions from star-forming regions show high proper motions and may produce a bow shock in the interstellar medium ahead of their direction of travel. Shown here is HD 77581, companion to the supernova remnant Vela X-1.

Russell, Henry Norris (1877-1957) American astronomer, famous for first publishing what is now called the hertzsprung-russell diagram (HR diagram). After graduating from Princeton University, he received his PhD there (1899) for a new way of determining binary star orbits. From 1902 he worked with Arthur Robert Hinks (1873-1945) at Cambridge University, determining stellar distances (and hence absolute magnitudes, M) by parallactic measurements on photographic plates. Russell returned to Princeton, where he became professor of astronomy (1911-27) and observatory director (1912-47), and a major figure in US astrophysics.

Back at Princeton, he built on his work at Cambridge and discovered a correlation between M and the spectral types that were being assigned by Annie cannon at Harvard. In 1913 he produced what was initially known as the Russell diagram, on which he plotted M against Harvard spectral type for over 300 stars. Immediately apparent were the main sequence running from top left to bottom right, and giant stars across the top. (Unknown to Russell, Ejnar hertzsprung had done the same some years previously, but had published his results in an obscure journal and in a form less easy to interpret.) The Hertzsprung-Russell diagram is the starting-point for all modern theories of stellar evolution, though Russell first thought, erroneously, that a star began its life as a giant and evolved down the main sequence to become a red dwarf.

Russell continued to study binaries throughout his life, developing a method for estimating the sizes and orbits of eclipsing binaries. He also studied the solar spectrum, using the saha equation, and in 1928-29 was able to confirm Cecilia payne-gaposchkin's earlier finding that hydrogen is the major constituent of the Sun and other stars.

Russell-Vogt theorem See vogt-russell theorem

Russian Aviation and Space Agency Organization controlling Russia's activities in space. The Agency's major goals are formally linked to the solution of social and economic problems at home as well as the implementation of Russia's international interests as the first space-faring power. They include the provision of communications and broadcast facilities across Russia and CIS territories, environmental monitoring, fundamental research including planetary science and astrophysics, and manned spaceflight.

Rutherfurd, Lewis Morris (1816-92) Pioneer American astrophotographer who subsequently turned to spectroscopy, using improved diffraction gratings of his own design and making. Beginning in 1858, he used an 11.5-inch (290-mm) refractor by Henry Fitz (1808-63) to take some of the earliest photographs of the Sun, Moon and planets, and of stars as faint as 5th magnitude.

runaway star O and B stars ejected by supernova explosions from star-forming regions show high proper motions and may produce a bow shock in the interstellar medium ahead of their direction of travel. Shown here is HD 77581, companion to the supernova remnant Vela X-1.

RV Tauri star The light-curve of a typical RV Tauri star shows alternating deep and shallow minima. The deep minima result from different modes of pulsation in the star's outer layers coming into phase with each other. At other times, the pulsation mechanism may apparently switch off, leaving only the shallow minima.

RV Tauri star The lightcurve of a typical RV Tauri star shows alternating deep and shallow minima. The deep minima result from different modes of pulsation in the stars outer layers coming into phase with each other. At other times, the pulsation mechanism may apparently switch off, leaving only the shallow minima.

In 1864 he built the first purely photographic telescope, designed to focus the violet wavelengths to which photographic emulsions were most sensitive. Rutherfurd obtained a solar spectrum that showed three times as many spectral lines as the best previous examples.

RV Tauri star (RV) Highly luminous, pulsating VARIABLE STAR. Members of this small group are mainly yellow or orange SUPERGIANT stars in spectral classes G and K, with some F stars. Examples are RV Tauri, R Sagittae and R Scuti. Their enormous, very extended atmospheres of gas emit infrared radiation. The light-curves of the RV Tauri stars are characterized by alternating deep and shallow minima, which may occasionally exchange places. Their periods range from about 30 to 145 days. Sometimes the light variations may become rather irregular, particularly for the stars having the longest periods. For this reason the RV Tauri stars are classified as SEMIREGULAR VARIABLES. They also have a variation in their COLOUR INDEX (an indication of the temperature of the star) that looks like the light-curve, but goes through its maximum shortly before the star reaches minimum brightness. There are two distinct photometric subtypes: one (RVA) exhibits the behaviourjust described; in the second subtype (RVB) the characteristic variations are superimposed on a longer-period fluctuation, with an amplitude of about two magnitudes and a typical period of 600-1500 days. R Sagittae and RV Tauri itself are both RVB stars.

RW Aurigae variable Very young star, a member of a subclass of NEBULAR VARIABLE or TTAURI STARS. The RW Aurigae stars have quite large amplitudes and are almost all G-type DWARFS. Very few are brighter than 10th magnitude at maximum. They show rapid and extremely irregular variations in their light-curves, although some stars appear to have a pseudo-periodic wave superimposed upon the primary light variations. Many RW Aurigae stars are found associated with the T ORIONIS VARIABLES.

Ryle, Martin (1918-84) English astronomer, the twelfth ASTRONOMER ROYAL, who established Cambridge as a centre for radio astronomy, and pioneered radio interfer-ometry and aperture synthesis. He joined Cambridge University's Cavendish Laboratory after working on radar during World War II, and became director of the MULLARD RADIO ASTRONOMY OBSERVATORY (MRAO) in 1957, and the university's first professor of radio astronomy two years later.

Ryle saw that the immediate task facing radio astronomers was to map the sky at radio wavelengths. He chose to do this by building the first RADIO INTERFEROMETERS, for which MRAO became famous. In 1950 he instigated the first of the Cambridge surveys, the most important of which was the third, completed in 1959, which yielded the third Cambridge catalogue ('3C') of some five hundred radio sources. He and his colleagues developed the technique of APERTURE SYNTHESIS in which an array of small instruments provide the sensitivity of a much larger single collector. The first such instrument, the One-Mile Telescope, was completed in 1963; its successor was the Five-Kilometre Telescope, since renamed the RYLE TELESCOPE.

The Cambridge surveys and radio telescopes advanced knowledge of PULSARS and QUASARS, and the number of distant radio sources lent support to the Big Bang theory. For his pioneering work in radio astronomy he shared the 1974 Nobel Prize for Physics with Antony HEWISH.

Ryle Telescope Eight-element, east-west radio interferometer operated by the MULLARD RADIO ASTRONOMY OBSERVATORY. The individual elements are steerable 13-m (43-ft) antennae, four of which are mounted on a 1.2-km (0.75-mi) rail track while the others are fixed at 1.2-km (0.75-mi) intervals.