palimpsest Roughly circular ALBEDO spot found on the icy surfaces of Jupiter's satellites GANYMEDE and CALLISTO, presumably marking the site of a former impact CRATER and its rim deposit. Most, if not all, of the topographic structure has disappeared, but the visual distinction from adjacent areas remains. The topographic flattening of craters on their transformation into palimpsests might be brought about by their filling by water and/or the viscous flow of ice on the satellite surface layers.

Pallas Second MAIN-BELT ASTEROID to be discovered, and so numbered 2. It is the second-largest asteroid, after CERES. Pallas' mean density is about 4.2 g/cm3, which is greater than that of stony meteorites or common terrestrial rocks, indicating that it contains a metallic component similar to nickel-iron meteorites. Despite its size, Pallas is by no means spherical in shape, measuring about 570 X 525 X 482 km (354 X 326 X 300 mi). Its mass is about 3.2 X 1020kg (0.43% the mass of the Moon). Pallas is also noteworthy in that its orbital plane is tilted substantially relative to the ecliptic (inclination almost 35°).

pallasite One of the two subdivisions of STONY-IRON METEORITES, the other being the MESOSIDERITES. Pallasites are an approximately equal mixture of iron-nickel metal and silicates, predominantly olivine. They are presumed to represent material from the core-mantle boundary of their parent bodies. There are currently 50 known pallasites, almost all of which are Main Group (MG) pallasites. MG pallasites have oxygen isotope compositions similar to those of the HOWARDITE-EUCRITE-DIOGENITE ASSOCIATION achondrites.

Palomar Observatory Formerly known as Mount Palo-mar Observatory, this world-famous astronomical facility is at an elevation of 1706 m (5597 ft) in the San Jacinto Mountains 80 km (50 mi) north-east of San Diego, California. It was planned in 1928, when MOUNT WILSON OBSERVATORY's success with the 100-inch (2.5-m) HOOKER TELESCOPE was fresh in the minds of US astronomers. The Rockefeller Foundation was persuaded to fund a 200-inch (5-m) instrument, eventually named the HALE TELESCOPE, to be owned by the CALIFORNIA INSTITUTE OF TECHNOLOGY (Caltech).

The observatory was founded in 1938, but construction was delayed by World War II, and it was not until 1948 that the Hale Telescope began work, and another year before it was declared finished. Also in 1948, the 1.2-m (48-in.) Palomar Schmidt telescope (now known as the OSCHIN SCHMIDT TELESCOPE) became operational. During the 1950s and 1960s, these two instruments proved a formidable combination, giving the USA an unassailable position in world astronomy. Several smaller telescopes were also built at Palomar. The observatory is still owned by Caltech. While the LIGHT POLLUTION from San Diego limits the observational programmes, the Hale and Oschin telescopes remain effective instruments.

Palomar Observatory Sky Survey (POSS) Systematic photographic SURVEY of the sky north of -30° made with the 1.2-m (48-in.) Oschin Schmidt telescope of the Palomar Observatory, jointly funded by the National Geographic Society. The First Palomar Observatory Sky Survey (POSS I) consists of about 2000 glass plates (1000 in each blue and red colours) observed between 1948 and 1957; it was completed in the 1970s by the ESO Schmidt Telescope in Chile (for the red part) and UK Schmidt Telescope in Australia (for the blue part). The Second Palomar Observatory Sky Survey (POSS II) was carried out in the 1990s in three colours (blue, red and infrared). Both surveys have been digitized and are known as the Digitized Sky Surveys (DSSI and DSSII).

Pan Innermost satellite of SATURN, discovered by Mark Showalter (1957- ) in 1990 amongst images obtained a decade earlier by the VOYAGER 2 spacecraft. Pan is about 20 km (12 mi) in size. It has a circular equatorial orbit within the ENCKE DIVISION in Saturn's rings. It takes 0.575 days to complete a circuit of the planet, at a distance of 133,600 km (83,000 mi) from its centre.

The name Pan is also applied to a large crater, c.90 km (c.60 mi) long, on AMALTHEA.

Pandora One of the inner satellites of SATURN, discovered in 1980 by S.A. Collins and others in VOYAGER 1 images. It is irregular in shape, measuring about 110 X 85 X 60 km (68 X 53 X 37 mi). Pandora has a near circular equatorial orbit at a distance of 141,700 km (88,100 mi) from the planet's centre, where its orbital period is 0.629 days. It appears to act as a SHEPHERD MOON to the outer rim of the F RING. See also PROMETHEUS

panspermia Theory that life on Earth did not originate here, but arrived from the depths of space. It originated with Svante ARRHENIUS, who in 1908 suggested that light from stars could blow microscopic germs from a world orbiting one star to another world orbiting another star. Panspermia never met with much support, but the idea never quite disappeared, and was rekindled in 1996 when a team of NASA scientists claimed to have found evidence for the presence of fossilized ancient microorganisms in a Martian meteorite (see ALLAN HILLS 84001).

Computer models have shown that rocks launched from Mars by a large meteorite impact can immediately enter Earth-crossing orbits if they are ejected just marginally faster than Mars' escape velocity, and that about 1 in 10 million of Martian meteorites that reach the Earth may have spent less than half a Martian orbital period, about one year, in space. Any microorganisms aboard such meteorites might just have survived the rigours of space travel and entry through Earth's atmosphere (microorganisms known as extremophiles are now known to have extraordinary survival attributes - see LIFE IN THE UNIVERSE). Over the lifetime of the Solar System, the inner planets are calculated to have exchanged tens of thousands of tonnes of material in this way.

Fred HOYLE and N. Chandra Wickramasinghe (1939- ) have reformulated the idea of panspermia to suggest that life was brought to the Earth by a comet, and that interstellar dust 'grains' have a bacterial component. Their main argument is that the origin and evolution of life involves too many steps, each in itself inherently improbable, to have happened on Earth - instead, it needs all the 'resources of space'. Life will be carried around, mainly by comets, and take root wherever conditions are suitable.

Evidence is growing that the composition of giant molecular clouds, particularly in star-forming regions, is far richer that previously supposed (see INTERSTELLAR MOLECULES). Whether or not any prebiotic molecules that are formed survive long enough to be incorporated subsequently in planets orbiting newborn stars is not yet clear. However, only a tiny amount of such prebiotic material needs to survive to seed planets with the chemical basis of life. Life is more likely to have disseminated in this manner than by the transfer of living organisms by panspermia. Panspermia theories have, however, received support from the detection in 2001 of microorganisms in the stratosphere, at a height of 41 km (25 mi).

parabola Open curve, one of the conic sections, obtained by cutting a cone in a plane parallel to the side of the cone. It can be regarded as an ellipse with only one focus, an infinite major axis and eccentricity of 1. A parabolic orbit is used for some of the long-period comets. These comets are observable over only a short arc of their orbits near perihelion, and it is often not possible to distinguish between exactly parabolic orbits and extremely elongated elliptical orbits.

paraboloid Three-dimensional surface generated by rotating a parabola about its own axis. This shape is commonly used for the primary mirror in newtonian telescopes in preference to a simple spherical surface. Images formed by a spherical mirror suffer from spherical aberration because incident light at different distances from the axis come to a focus at different points. A paraboloid overcomes this problem and produces a diffraction-limited image for a point object, such as a star, that is in the centre of the field of view. Incoming light that is not parallel to the axis of the paraboloid (that is, from objects away from the centre of the field) produces images that suffer from coma. Newtonian telescopes often have long focal lengths to reduce this effect.

parallax (symbol it) Change in apparent position of a celestial object, relative to its background, caused by a shift in the position of the observer. When observed six months apart from opposite sides of the Earth's orbit around the Sun, the position of a nearby star relative to the more distant background stars, will appear to have shifted by an angle AO. Half of this angle, it, is the annual parallax (heliocentric parallax), which is also a measure of the angular size of the radius of the Earth as seen from that star. If a star's parallax can be measured, then its distance can be determined. A unit of stellar distance is the parsec - the distance at which a star would have a parallax of one second of arc; it is equivalent to 3.2616 l.y.

This is an example of trigonometric parallax, where a baseline of known length is used to make separate observations of a celestial object, the choice of baseline depending on the distance of the object in question. For objects in the Solar System, the radius of the Earth is used as a baseline, producing a measure of diurnal parallax (geocentric parallax). solar parallax is the Sun's geocentric parallax, in other words the angular size of the Earth's equatorial radius from a distance of one astronomical unit (1 AU). Another method of determining stellar distances uses a star's spectral type and is known as spec-troscopic parallax.

The astrometric satellite hipparcos, which operated between 1989 and 1993, extended the range of accurate parallax distances by roughly 10 times, at the same time increasing the number of stars with good parallaxes by a much greater factor. Of the 118,218 stars in the Hippar-cos catalogue, the distances of 22,396 are now known to better than 10% accuracy. Prior to Hipparcos, this number was less than 1000. The companion Tycho database, from another instrument on the satellite, provides lower accuracy for 1,058,332 stars. This includes nearly all stars to magnitude 10.0 and many to 11.0. See also dynamical parallax; secular parallax

Paranal Observatory Second of the two observing sites of the european southern observatory (ESO), inaugurated in the early 1990s. Situated about 500 km (300 mi) north of its fellow ESO facility, la silla observatory, the site is at an elevation of 2635 m (8640 ft) on Cerro Paranal in northern Chile and offers superb atmospheric conditions for astronomy. It is home to the four instruments that comprise the very large telescope (VLT). A nearby 2.5-m (100-in.) VLT Survey Telescope (VST) will provide imaging data at optical wavelengths, while the 4-m (157-in.) UK-ESO Visible-Infrared Survey Telescope for Astronomy being built at Paranal will provide infrared imaging data.

parhelion (mock Sun, sundog) Atmospheric phenomenon produced by refraction of sunlight by ice crystals in cirrus clouds at altitudes of 10-15 km (6-9 mi), near the top of the troposphere. A parhelion appears level in elevation above the horizon with the Sun, at an angular distance of 23° from it. There may be a single parhelion, or two parhelia lying to either side of the Sun in the sky. Sometimes parhelia are visible as brighter patches on a halo around the Sun. As with the halo, parhelia can show strong colour, with red towards the Sun, blue away from it. Parhelia commonly appear as elongated bars of light.

Close to full moon, the equivalent lunar phenomenon, called a parselene, may be visible.

Paris Observatory Second-oldest observatory in the world, after copenhagen observatory. The 'Observa-toire de Paris' was commissioned by King Louis XIV principally for geodesy, and built in 1667. Many famous 17th-century astronomers worked there, including Jean Picard, Ole romer, Christiaan huygens, Nicolas-Louis de lacaille and Urbain le verrier. Four generations of the cassini family were observatory directors in the 17th and 18th centuries, and such was its importance to geodesy that the Paris meridian remained the origin of longitude until 1884. The observatory helped to create the metric system and instigated the carte du ciel project. Today, Paris Observatory houses a collection of astronomical instruments from the 16th century to the 19th; it is also a modern research institution, operating meudon observatory and the nancay radio telescope.

Parkes Observatory See parkes radio telescope

Parkes Radio Telescope Major Australian radio astronomy facility 25 km (16 mi) north of Parkes in central New South Wales. The 64-m (210-ft) fully steerable dish was completed in 1961 and has undergone several upgrades, the most recent of which provides a unique multi-beam facility that allows the telescope to carry out direct radio imaging. Part of the australia telescope

Parkes Radio Telescope An evening view of the Parkes 64-m (210-ft) radio telescope in New South Wales, Australia. Constructed in 1961, the dish can record radio wavelengths between 5 mm and 2 cm.

PAVO (gen. pavonis, abbr. pav)

Constellation of the southern sky representing a peacock, intrody are very large and cool, lying to the right of the main sequence on the hertzsprung-russell diagram (HR diagram). As they evolve, they contract and their cores start to heat up. With their large size, the change in radius dominates any change in luminosity. They therefore follow almost vertical lines, known as hayashi tracks, on the HR diagram. At this point, the star's energy is almost entirely transported by means of convection, and it has only a small radiative core.

The star then starts to become radiative, and the luminosity starts to increase as the effective temperature increases. The star's evolution at this phase is represented by the henyey track on the HR diagram. The core temperature rises until nuclear fusion can start in its core, at which point it becomes a main-sequence star. The pre-main-sequence phase of the Sun lasted several hundred million years. ttauri stars are examples of pre-main-sequence stars. See also stellar evolution

pre-nova Putative interacting binary star system that is in a quiescent state but undergoing active mass transfer prior to erupting as a nova. Technically such a system may be classified as a cataclysmic variable, and presumably exhibits characteristics similar to those of such a system, most probably appearing as a nova-like variable. Although images of nova precursors have been detected after the event, no previously known system has yet undergone a nova outburst. The reasons for this undoubtedly lie in the moderately low number of systems known, the relatively short period during which they have been studied, and the extremely long recurrence interval (thousands of years) between nova outbursts. A few pre-outburst light-curves have been obtained, and these appear to show a general rise in the magnitude of the system by some 1-5 magnitudes in the years immediately preceding the outburst.

pressure broadening Broadening of spectrum lines as a result of electromagnetic 'collisions'. Spectrum lines are broadened by mass motions that shift the lines by the Doppler effect, and by the smearing of energy levels by quantum uncertainty, magnetic fields (the zeeman effect) and electromagnetic effects induced by close-passing atoms. The incidence of electromagnetic collisions depends on the closeness of the atoms and, therefore, on the pressure of the gas. Pressure broadening causes dwarfs to have wider hydrogen lines than do giants and supergiants.

primary Largest of a system of celestial bodies. The Sun is the primary member of the Solar System. The term is also used to describe the more massive component in a binary or multiple star system, the primary member of such a system being the one about which the others rotate. It can also be used to describe a planet with respect to its moons, for example, the primary body of the Moon is the Earth.

primary Main, light-collecting mirror in a reflecting telescope.

prime focus Position at which an objective lens or a primary mirror brings starlight directly to a focus (without the intervention of any additional lenses or mirrors). In large research telescopes astronomers use the prime focus of the main mirror to feed light to instruments that need a wide field of view. Some astrophotographers use the term to refer to images formed directly by a schmidt-cassegrain telescope rather than from the arrangement where an eyepiece is also used to form the image. The term 'principle focus' would be more correct in this case.

Principia Shortened name of Isaac Newton's Philosophi-ae naturalis principia mathematica ('Mathematical Principles of Natural Philosophy'), summarizing his researches into physics and astronomy and published in three volumes in 1686-87. Most of the work it describes had been completed years before, and it was the persistence of Edmond halley that persuaded the reclusive Newton to put it into print. Its appearance immediately established Newton's international fame, even though he wrote it (in Latin) in a deliberately abstruse manner 'to avoid being bated by little smatterers in mathematics', as he put it.

The Principia is a milestone in science, bringing together systematic observation and rigorous mathematical analysis. Its 'Newtonian mechanics' set the agenda for two centuries of enquiry into the physical world. It sets out newton'slawsofmotionand newton'slawofgrav-itation, and demonstrates their universality, from earthly motions to the celestial mechanics of the Solar System. It shows that an inverse-square law of gravitation leads inevitably to kepler'slawsgoverning planetary orbits. The motion of the Moon is analysed in detail, as is the phenomenon of the tides.

prism Transparent optical element with flat, polished sides used to bend or disperse a beam of light. Prisms can have a uniform cross-section, typically triangular as in the porro prism, or they can have a more complex shape, such as the penta prism used in a single-lens reflex camera. Porro prisms and penta prisms are both used to reflect light with little or no dispersion. Light enters one face of the prism at right-angles and after two reflections leaves again, also at right-angles to the prism surface. The porro prism reflects the light through 180° whilst the penta prism reflects it through 90°. Prisms are often used in place of mirrors to produce a simple 90° reflection, for example in star diagonals.

If light enters and/or leaves at any angle other than at right-angles to the surface then it will be dispersed into its component spectrum. This dispersion allows the composition of the light to be measured and analysed. Early spectrographs used prisms as their main dispersing elements but modern ones are more likely to use diffraction gratings because these can provide greater dispersion. Prisms are sometimes used in conjunction with echelle gratings to separate the different orders of spectra produced in high-dispersion spectrographs.

Large, thin prisms with a very small angle (typically one degree) between the two main sides are used as objective prisms to produce low-resolution spectra of all the objects in a telescope's field of view simultaneously. Provided the field is not so crowded that the spectra overlap, this technique can be used to survey a large number of objects in a short period of time. It is normally used to identify potential targets for further study at higher resolution.

Procellarum Oceanus (Ocean of Storms) Lunar lava plain in the western region of the Moon. While called an 'ocean', it is actually a vast mare region. However, unlike the majority of maria, which formed within impact basins, Oceanus Procellarum formed in a topographically low region of the Moon. Many volcanic features are found here, including mare (wrinkle) ridges, domes and sinuous rilles. Unusual features include the magnetic Reiner Gamma and the volcanic aristarchus Plateau.

Proclus Lunar crater (16°N 47°E), 29 km (18 mi) in diameter, with rim components reaching 2440 m (8000 ft) above its floor. Proclus is intermediate in type between simple bowl-shaped craters and complex craters with central peaks. On its floor are a few small mounds, along with extensive non-terracing slumps. The bright ejecta are irregular because of an oblique impact, which produced a zone without bright ejecta to the south-west. The angle of impact was not low enough, however, to produce an elongated crater.

Proctor, Richard Anthony (1837-88) English astronomy writer and popularizer who collated and analysed the work of others, particularly in the areas of Solar System and stellar astronomy. By carefully reviewing drawings of the albedo features of Mars made between 1666 and 1878, he derived a rotation period of 24h 37m 22s.7, very close to the currently accepted value. Proctor studied the distribution of the Milky Way's stars, inventing graphical techniques to illustrate the arrangement of stars, clusters and nebulae. He charted the 324,000 stars in FW.A. Argelander's Bonner Durchmusterung and constructed a map of the Milky Way and its structure.

Procyon The star a Canis Minoris, visual mag. 0.40 (the eighth-brightest in the sky), distance 11.4 l.y., spectral type F5 IV or V. Procyon has a white dwarf companion of mag. 10.7 which orbits it every 41 years, but it is far too close to be seen without a large telescope. The name Pro-cyon comes from the Greek meaning 'preceding the dog', from the fact that it rises before Sirius, which is known as the Dog Star.

Prognoz satellites Series of ten Soviet spacecraft; they were launched from 1971-85. Most were designed to monitor solar activity and the interaction of the solar wind with the Earth.

prograde See direct motion

Prometheus Long-lived 100-km-high (60-mi-high) volcanic eruption plume on Jupiter's satellite io. Its location shifted by about 70 km (44 mi) between its detection on voyager images in 1979 and its appearance on galileo images in the late 1990s. The source of the plume cannot therefore be a fixed vent and may instead be the end of a lava flow.

Prometheus One of the inner satellites of saturn, discovered in 1980 by S.A. Collins and others in voyager 1 images. It is irregular in shape, measuring about 150 X 90 X 70 km (90 X 60 X 40 mi). Prometheus has a near-circular equatorial orbit at a distance of 139,400 km (86,600 mi) from the planet's centre, where its orbital period is 0.613 days. It appears to act as a shepherd moon to the inner rim of the f ring. See also pandora

The name Prometheus is also applied to an active volcano on io.

prominence Relatively cool and dense plasma suspended above the Sun's photosphere and contained by magnetic fields. The insulating effects of the magnetic fields are such that material in a quiescent prominence may have a temperature of 10,000 K while the surrounding corona is at 2,000,000 K. Prominences appear as bright features extending from the solar limb at a total eclipse or can be seen in a spectrogram or spectroheliogram of the chromosphere taken in the hydrogen-alpha line or H and K lines of ionized calcium. When seen against the bright solar disk, prominences appear as dark filaments.

prominence This SOHO image from 1997 September 14 shows at lower left a huge eruptive prominence high above the solar limb. The image is a spectroheliogram in the light of HeII at 30.4 nm in the extreme-ultraviolet.

Prominences have been divided into two main classes - quiescent and active. Quiescent prominences occur mainly in two zones in either hemisphere, chiefly away from active regions. One zone lies just polewards of the sunspot-forming latitudes (migrating equatorwards behind the sunspots in accordance with sporer'slaw), while the other forms a high-latitude 'polar crown'. Quiescent prominences show no large motions, and can last for weeks or months. Typically, prominences are between 100,000 and 600,000 km (60,000-370,000 mi) in length, 5000 and 10,000 km (3000-6000 mi) wide, and can reach as high as 50,000 km (30,000 mi) above the photosphere.

Active prominences are short-lived features showing rapid motion; they are associated with active regions, sunspots and flares. They can appear as surges, sprays or loops, and can reach heights of about 700,000 km (400,000 mi) in just an hour. Quiescent prominences can develop from active prominences.

Quiescent prominences can become detached and disappear if disturbed by MORETON WAVES associated with flare activity. Such eruptive prominence events can arch a million kilometres outwards before bursting apart; their disappearance is often associated with a CORONAL MASS EJECTION.

proper motion (symbol |u) Apparent angular displacement of a star against the celestial sphere as a result of its motion over a year. Proper motion is a combination of a star's actual motion through space and its motion relative to the Solar System.

proper motion Five of the stars making up the familiar pattern of the Plough are members of a cluster showing a common proper motion relative to the more distant stellar background. The two non-members, Alkaid and Dubhe, have different proper motions, and over the course of hundreds of thousands of years, the plough pattern will become distorted. The view in the centre shows the stars’ relative positions today, with an earlier epoch represented at top, and the configuration in the distant future at bottom.

Stars in our Galaxy move relative to each other and relative to the Sun, but because of their great distances their apparent movements in the sky are very small. In fact most stars are so distant that their proper motions are negligible. The full space motion of a star is the combination of its proper motion, across an observer's line of sight, and its radial velocity, along the line of sight. The latter is usually measured from the Doppler shift in the star's spectrum.

Proper motion is usually denoted by the Greek letter Mu (u) and is measured in seconds of arc (") per year. About 300 known stars have proper motions larger than one arcsecond per year, but most proper motions are smaller than 0.1 arcsecond per year. Barnard's Star in the constellation Ophiuchus has the largest known proper motion, at 10".27 per year.

Proper motions are measured by comparing the accurate positions of stars obtained at two or more epochs. This gives the components of the proper motion in right ascension, lux, measured in seconds of time per year, and in declination, |u§, measured in seconds of arc per year. The combined proper motion, u , measured in seconds of arc per year.

Proper motions have traditionally been determined using instruments such as transit circles or from photographic astrometry, but the astrometric satellite HIPPAR-COS, which operated between 1989 and 1993, measured the proper motions for 118,218 stars with positional accuracies averaging hardly more than a thousandth of an arcsecond.

The detailed analysis of many observations are tabulated in a series of FUNDAMENTAL CATALOGUES, the most recent being called FK5 Part II, which contains the positions and proper motions of 3117 FUNDAMENTAL STARS brighter than magnitude 9.5. This catalogue defines the basic reference frame to which the kinematics of all stars in our Galaxy are ultimately referred.

Proper motions of stars have been measured with respect to galaxies. The proper motions of galaxies are negligible, so they provide a fixed frame of reference against which to measure stellar proper motions. This method is therefore independent of the positions and proper motions of the bright REFERENCE STARS measured by instruments such as meridian circles.

The local motion of the Sun relative to stars within a 20-parsec radius can be derived from proper motions. The average proper motion of the local stars depends on the angle between the direction of the Sun's motion and the direction to the stars. If the stars lie in the direction of the APEX (or ANTAPEX) of the Sun's motion, the average of their proper motions will be zero. The calculated position of the apex, and the solar velocity towards it, depends to some extent on the number and type of stars used in the analysis. The average result is that relative to the nearby stars the Sun is travelling about 20 km/s (12 mi/s) in the direction of the constellation Hercules. When the solar motion is subtracted from the observed proper motion of a star its peculiar proper motion is obtained. The apparent displacement of a star arising from the solar motion is called its parallactic motion. By measuring the parallactic motion of a homogeneous group of stars, the so-called SECULAR PARALLAX of the group is obtained. This method has been applied to estimate the average distances of groups of variable stars, such as RR LYRAE and CEPHEIDS, which are beyond the range of TRIGONOMETRIC PARALLAX.

Another important application of proper motions is in determining the distances of moving clusters. Since stars in a cluster have a common space motion, their proper motions appear to converge to a point on the celestial sphere. By combining the measurement of the convergent point for the cluster with measurements of its proper motion and radial speed, it is possible to calculate its distance.

Proper motion surveys also have an important application in investigating the distribution and luminosity of stars in the solar neighbourhood. Stars close to the Sun tend to have larger values of proper motion than more distant stars, so by selecting stars from proper motion surveys that are complete to some apparent magnitude, a statistical sample of the stars within a specified volume around the Sun is obtained. By combining this sample with the known parallaxes of some of the stars, and making allowances for the incompleteness of the sample, one can arrive at a statistical estimate of the distribution and luminosity of the stars in the vicinity of the Sun. This shows that most stars in the solar neighbourhood are intrinsically faint.

Proper motions have another important application in the subject of galactic rotation. They provide the only direct method of measuring the Oort constant B, which essentially describes how the rate of change of the angular velocity of rotation changes with distance from the centre of our Galaxy.

proplyd Acronym for PROTOPLANETARY DISK

Prospero One of the several small outer satellites of URANUS; it was discovered in 1999 by Matthew Holman (1967- ) and others. Prospero is c.20 km (c.12 mi) in size. It takes 2037 days to circuit the planet at an average distance of 16.67 million km (10.36 million mi). It has a RETROGRADE orbit (inclination near 152°) with a substantial eccentricity (0.439). See also CALIBAN Proteus Largest of the inner satellites of NEPTUNE, and the second largest of Neptune's satellites overall, after TRITON. Proteus was discovered in 1989 by the VOYAGER 2 imaging team. It has a low albedo (0.07). Proteus is squarish in shape, measuring about 436 X 416 X 402 km (271 X 259 X 250 mi), and is heavily cratered. The most prominent feature on its surface is the large crater Pharos, which is 250 km (160 mi) across. Proteus takes 1.122 days to circuit the planet, at a distance of 117,600 km (73,100 mi) from its centre, in a near-circular, near-equatorial orbit.

protogalaxy Matter consisting of primarily hydrogen and helium gas which collapses by gravitational interaction to form a GALAXY. It is not clear if there are galactic seeds such as hot dark matter or cold dark matter, or even black holes around which this matter gravitates to form galaxies.

Proton Russian workhorse satellite and planetary space probe launcher. It started life as a two-stage booster, first flown in 1965, and has since made more than 300 flights in different models. With four stages, the Proton K was first launched in 1967, later proving its capability to place satellites directly into GEOSTATIONARY ORBIT (GEO). A three-stage version of the Proton K has been used to launch SALYUT, MIR and INTERNATIONAL SPACE STATION modules. The Proton K is now being marketed as a commercial launcher by ajoint US-Russian company, International Launch Services, which also promotes the US Atlas booster. The launcher can place a satellite weighing 1.8 tonnes directly into GEO or a 4.9-tonne payload into geostationary transfer orbit (GTO). The new Proton M, with a powerful Breeze upper stage, was introduced in 2001, to place payloads weighing 2.9 tonnes into GEO or 5.5 tonnes into GTO.

proton Stable subatomic particle with a unit positive electric charge. Its mass is 1.672614 X 10~27 kg (1.007399 amu). It is also the nucleus of the H1 atom and with the NEUTRON forms the nuclei of other atoms. Protons thus form about 87% of the mass of the Universe. The number of protons in a nucleus determines which ELEMENT it forms. Modern GRAND UNIFIED THEORIES suggest that a proton should eventually decay, but its lifetime may lie between 1035 and 1045 years, and the decay has yet to be confirmed experimentally.

Protonilus Mensa Mesa area on MARS (44°.2N 309°.4W). It is some 600 km (370 mi) long.

proton-proton reaction (p-p reaction) Set of NUCLEAR REACTIONS that results in the conversion of hydrogen into helium. 26.8 MeV (4 X 10~12 J) of energy is released during the formation of a single helium nucleus. This comes from the conversion of mass into energy, since the helium nucleus has a mass that is about 0.7% less than that of the four hydrogen nuclei (protons) that go to form it.

Proton satellites Four very heavy (12-17 tonnes) Soviet satellites launched 1965-68. They were used for monitoring cosmic and gamma rays.

protoplanet Planet in the making. The planet-forming process begins with small solid bodies, called PLANETESIMALS, in a PROTOPLANETARY DISK orbiting a star. Their mutual gravitational perturbations cause their orbits to intersect, resulting in collisions and growth by ACCRETION. The rate of collisions is enhanced by gravitational attraction, which is stronger for more massive bodies, thus the largest planetesimal in some local region of the disk becomes more effective at sweeping up mass than its smaller neighbours, which allows it to become still larger. This process results in rapid 'runaway growth' of a large body, called a planetary embryo, which dominates the region of the disk near its orbit and eventually exhausts the supply of planetesimals in that region. This process occurs at various distances from the star, producing hundreds of such bodies, of the order of the Moon's mass, on separated orbits. Because of their perturbations, these orbits are unstable on somewhat longer timescales, and collisions occur between the embryos. As they become larger in size and fewer in number, the largest may be called protoplanets; the final bodies at the end of this process are planets. In the inner Solar System, formation of embryos took less than a million years, while the late stage of giant impacts lasted for tens of millions of years.

The term protoplanet is also applied to self-gravitating condensations hypothesized to form by gravitational instability of the gaseous component of a protoplanetary disk. In some theories, this process is assumed to produce gas giant planets such as Jupiter and Saturn, after the gaseous protoplanet cools and contracts. See also COSMOGONY

protoplanetary disk (proplyd, protostellar disk) Material surrounding a newly formed star. A star forms by gravitational collapse of a cloud of interstellar gas. Almost invariably, such a cloud has some rotational motion, and conservation of angular momentum causes its spin rate to increase as it contracts. Material cannot fall directly into the centre, but instead forms a disk. Viscosity in the disk, produced by turbulence, magnetic fields, and/or gravitational interactions between regions of varying density, redistributes angular momentum towards the outer part, allowing most of the mass to flow inwards to form a protostar. The remainder of the mass, perhaps a few per cent to a few tenths of the total, is left orbiting the star. The disk of gas and dust, which may persist for millions of years before dissipating, supplies the material for formation of planets. Protoplanetary disks, many of them much larger than our Solar System, have been observed by the Hubble Space Telescope.

protostar Young star, still accreting material, said to exist once fragmentation of its parent molecular cloud has finished. A protostar is the earliest phase of stellar evolution. This phase may take from 105 to 107 years, depending on the mass of the star. The Sun's protostellar stage took about 0.1-1 million years.

On a hertzsprung-russell diagram, protostars would lie above and to the right of the main sequence. Protostars cannot be observed in the optical because they are embedded in the cloud of material from which they are forming, although their presence can be detected in the infrared. Once protostars become optically visible, they are known as pre-main-sequence stars.

Proxima Centauri The closest star to the Sun, 4.22 l.y. away. It is a red dwarf of visual mag. 11.0, spectral type M5 V, with just 0.005% of the Sun's luminosity. Named because of its proximity to us, Proxima is a member of the alpha centauri triple system but lies about 2° from the other two members. It is a flare star, which can brighten by up to a magnitude for several minutes at a time.

Ptolemaeus Ancient lunar crater which has deeply incised walls that show eroded rims (14°S 3°W); it is 148 km (92 mi) in diameter. Its features are the result of impact erosion, much of which came from the ejecta of the imbrium multi-ring impact basin to the north-west. The scars from Imbrium's ejecta all trend from northwest to south-east. The floor of Ptolemaeus is smooth, also as a result of the ejecta from the Imbrium impact. The floor contains several ghost craters and a few recent impact scars.

Ptolemaic system geocentric theory of the Universe as presented in Ptolemy's Almagest of ad 140. The Earth is taken to be the centre of the Universe, with the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn revolving around it. Outside lies the sphere of the fixed stars.

Ptolemaic system Placing Earth at the centre, with the Sun orbiting between Venus and Mars, the Ptolemaic system was a prevailing world view for over a thousand years. To account for the planets’ occasional retrograde (westwards) motion, an ever more complex pattern of epicycles – small circles superimposed on their orbits – had to be invoked. Eventually, the Ptolemaic system was superseded by the Copernican, which accounted more readily for observed planetary movements.

The orbit of a planet as described by eudoxus was such that it moves around in a small circle, called the epicycle. The centre of the epicycle itself revolves about the Earth on a larger circle called the deferent. Ptolemy added two further points, the eccentric and the equant, to each orbit. The eccentric is at the centre of a line joining the Earth and the equant; the deferent is centred on the eccentric point rather than on the Earth itself. The centre of the epicycle thus moves around the deferent with a variable velocity that makes it appear to be moving with uniform angular velocity when viewed from the equant point - in direct conflict with the Aristotelian doctrine that the celestial motions had to be perfectly uniform.

The Ptolemaic system allowed the future positions of the planets to be predicted with reasonable accuracy and remained 'state of the art' until ousted by the copernican system.

Ptolemy (Claudius Ptolemaeus) (2nd century ad) Egyptian astronomer and geographer. His chief astronomical work, the Almagest, was largely a compendium of contemporary astronomical knowledge, including a star catalogue, drawing on the work of hipparchus of Nicaea. It described the so-called ptolemaic system, a geocentric universe with the Earth fixed at the centre, and the Moon, Sun and planets revolving about it. Ptolemy's modification of the previous, simpler Greek theory based on epicycles and deferents reproduced the apparent motions of the planets, including retrograde loops, so well that it remained unchallenged until the revival of the heliocentric theory by Nicholas Copernicus in the 16th century.

Little is known about Ptolemy's life. His dates and even his name are uncertain - 'Ptolemy' merely indicates that he had Greek or Greek-naturalized ancestors and lived in Egypt, which was then ruled by the Ptolemaic dynasty. He is sometimes referred to as Claudius, but this probably means simply that Ptolemy's Roman citizenship goes back to the time of the emperor Claudius I. It seems likely, though, that he lived and worked at the famous library and museum in Alexandria.

Ptolemy was a prolific author, but his earliest large work, the Almagest, was not only his greatest, but also the greatest astronomical work of antiquity. It is essentially a basic textbook, expecting its reader to know only the fundamentals of Greek geometry and some familiar astronomical terms; the rest Ptolemy explains.

The first two books form an introduction. They present an outline of the Ptolemaic system - a spherical universe with spheres carrying the planets - and give persuasive arguments that the Earth is stationary at its centre. The basic mathematics, making use of chords (the Greeks did not develop the sines and tangents of modern trigonometry), is described.

Book III discusses the apparent motion of the Sun, using observations mostly by Hipparchus, for whose work Ptolemy shows such reverence that of his own observations he seems to have selected only those that agreed with his predecessor's. A table of the Sun's motion is then constructed. The motion of the Moon according to Hip-parchus is the subject of Book IV. The theory, based on eclipses, accounts for the Moon's motion at conjunction (new moon) and opposition (full moon), but is inadequate for intermediate positions in the orbit.

In Book V, Ptolemy develops his own lunar theory. Based on epicycle and deferent, it is extremely ingenious. His observations showed him that the Moon changed in apparent size, which he accounts for by a kind of crank mechanism operating on the centre of the Moon's epicycle. This theory accounts for the motion of the Moon at all positions of its orbit.

Book VI explains how to calculate every detail of eclipses. Books VII and VIII present tables of positions and pulsar This schematic illustration of a pulsar shows the two beams of radiation directed from the collapsed star along its magnetic poles, which need not coincide with the axis of rotation. If the rotational axis is so aligned that the radiation beams sweep across our line of sight, the rapidly pulsing radio signal characteristic of a pulsar will be detected.

Typically, Ptolemy tackled the problem in a truly scientific way. Using his own observations as well as those of Hipparchus and the Babylonians, he discovered that each planet had two irregularities or 'anomalies'. One depended on the planet's elongation, the other on its position along the ecliptic. Ptolemy had to explain these anomalies by a theory that fitted into the framework of the Greek geocentric universe.

Careful study showed him that the epicycle and deferent would account for the first anomaly. For the second he introduced the ECCENTRIC. Here the centre of the planet's epicycle moved around a point M whose distance from the Earth was determined by the planet's apparent eccentricity. It was another basic condition of Greek planetary motion that each planet should move at an unvarying rate about the centre of the Universe (even though observation showed that not one of them did). Ptolemy satisfied this condition by using another point, the EQUANT, on the opposite side of the Earth to M and an equal distance away. Motion with respect to the equant was uniform. This was an inspired solution, which, with an additional minor modification, explained the motions of all the planets then known.

The Almagest was not Ptolemy's only great text. He wrote the Tetrabiblos on astrology, much of it on 'natural astrology' - the physical effects of the Sun and Moon, for instance. Then he produced Planetary Tables, which were extracted from the Almagest, and a popular abridgement called Planetary Hypotheses, which, however, extends some of his theoretical ideas and in particular his measurements of the distances and sizes of the Sun and Moon. There was also his Phases of the Fixed Stars, which went into more detail about rising and setting of the stars just before dawn and just after sunset, and the Analemma, a book on constructing sundials. He also wrote on geography, music, mechanics, geometry and optics.

Puck Largest of the inner satellites of URANUS, discovered in images returned by VOYAGER 2 as it approached Uranus near the end of 1985. Puck is about 154 km (96 mi) across, roughly spherical in shape, and heavily cratered. It takes 0.762 days to circuit the planet, at a distance of 86,000 km (53,000 mi) from its centre, in a near-circular, near-equatorial orbit.

Pulcherrima Alternative name for the star e Bootis. See IZAR

Pulcova MAIN-BELT ASTEROID; number 792. In 2000 it was found to be accompanied by a small moon. Pulcova is about 145 km (90 mi) in size, ten times as large as its satellite, which takes four days to orbit it at a distance of about 800 km (500 mi).

Pulkovo Observatory One of the oldest observatories in Russia, founded in 1839 with a 15-inch (400-mm) refractor that was then the largest refractor in the world. Members of the STRUVE FAMILY number among its past directors. It is located near St Petersburg (formerly Leningrad) and was completely destroyed in the siege of the city during World War II. In a symbolic gesture by the Soviet government, the observatory was rebuilt between 1945 and 1954. However, the observing conditions there are poor, and Pulkovo astronomers today use the optical and radio telescopes of the SPECIAL ASTROPHYSICAL OBSERVATORY and other better-sited facilities. The observatory carries out research in most branches of theoretical and observational astrophysics as well as solar physics and astronomical instrumentation.

pulsar Rapidly spinning NEUTRON STAR, emitting two beams of RADIO WAVES that are seen as pulses. The beam of radio waves emitted by the rotating pulsar sweeps past the Earth in a manner similar to the flash of light from a lighthouse beam. The first pulsar was discovered by Anthony HEWISH and Jocelyn BELL in 1967, and there are now over 1000 known. A few pulsars have been detected at wavelengths other than radio (for example visible light, X-ray and even gamma-ray). The pulsar periods range from milliseconds (see MILLISECOND PULSAR) to around 5 seconds, with the most common period being around 1 second. The pulses are not always exactly the same shape from cycle to cycle, or the same strength, and the pulse at each wavelength has a slightly different appearance, but the period is always the same. Some pulsars are found in binary systems (see BINARY PULSAR) and there is one case ofa pulsar with planets (PSR B1257+12). The distribution of pulsars is concentrated towards the plane of the Galaxy, as is the concentration of supernovae, and there may be as many as 100,000 pulsars in the Galaxy.

pulsar This schematic illustration of a pulsar shows the two beams of radiation directed from the collapsed star along its magnetic poles, which need not coincide with the axis of rotation. If the rotational axis is so aligned that the radiation beams sweep across our line of sight, the rapidly pulsing radio signal characteristic of a pulsar will be detected

A pulsar is formed following a supernova explosion, but only a few pulsars can be seen in their associated SUPERNOVA REMNANT. Most pulsars are over a million years old, so the supernova remnant will have long since dispersed and faded from view. Only something as small as a neutron star can spin fast enough to be a pulsar and not break up. The beams of radio waves come out from both magnetic poles, and the magnetic axis need not be the same as the rotation axis. The beam is a hollow cone, with the gas at the centre in different conditions as compared to the edge of the cone. In one pulsar the pulse shape is very complicated, suggesting that the rotating cone is made up of several beams. The first pulsar had a period of 1.337 s and was picked out during a survey at radio wavelengths for objects with signals that changed rapidly on a very short timescale (see also SCINTAR). Soon after the first discovery Bell and Hewish found three more pulsars, with periods of 0.253 s, 1.188 s and 1.274 s. The CRAB PULSAR has a very short period (0.033 s); it is at the heart of the Crab Nebula and the obvious result of a supernova. It is one of the few pulsars to be detected at visible wavelengths, and it has also been detected in X-rays and gamma rays. The beams of radio waves are pointing almost directly at Earth, so a weak pulse from the second beam can be seen between the main pulses (called the inter-pulse). The Hubble Space Telescope picture of the Crab Pulsar suggests there may be a JET (with knots of material) coming out from one pole, and waves of gas rippling outwards along the equatorial plane. The short period of the Crab Pulsar shows that it is a young pulsar, and it is very gradually slowing down. Almost all pulsars slow down because of the strong magnetic field, the except being the VELA PULSAR, which occasionally speeds up suddenly (an event called a glitch), producing a faster period.

There are around 50 pulsars, known as MILLISECOND PULSARS, with very short periods. Most millisecond pulsars are thought to have a weak magnetic field, which is why they have not slowed down. For some other millisecond pulsars, the short period is explained by proposing that the pulsar was once in a binary star system. The pulsar slowed down, but then speeded up again when material from the companion was accreted on to the pulsar (see BINARY PULSAR). One millisecond pulsar in a binary star system is called the 'black widow pulsar' because it is thought to have consumed most of its companion star. The pulses from the black widow pulsar disappear for 50 minutes every 9 hours, when the pulsar is eclipsed by its companion, and the eclipses show that the companion is very small (the mass is 2% of the solar mass although the size is 1.5 times that of the Sun).

Most pulsars have incredibly accurately determined periods, and the rate of change of the period is precisely known. This allows peculiarities to be identified easily. A pulsar was the first star to be identified as having planets orbiting around it. PSR B1257+12 has at least three planets in orbit, and there may be a fourth with a mass around that of Saturn. One pulsar was found to have a slightly irregular period, caused by the fact that it was in a binary system and the Doppler effect influenced the pulse period, by spreading the pulses apart slightly as the pulsar receded and jamming them together slightly as the pulsar approached.

pulsating star Star that expands and contracts, in particular, a class of VARIABLE STAR, including various types and subtypes. See also CEPHEID VARIABLE; IRREGULAR VARIABLE; MIRA STAR; RR LYRAE STAR; SEMIREGULAR VARIABLE

Puppis See feature article

Purbach (or Peurbach), Georg von (1423-61) Austrian mathematician-astronomer and student of Ptolemaic theory whose table of lunar eclipses, published in 1459, was still in use two centuries later. Purbach's real family name is unknown; he is identified by the name of the town in Austria where he was born. He revised and translated Ptolemy's Almagest, work completed after his death by his pupil REGIOMONTANUS. In his Theoricae novae planetarum ('New Theory of the Planets', 1460), Purbach attempted to explain the structure of the Solar System in terms of both Ptolemy's epicyclic orbits and the homocentric spheres of EUDOXUS and ARISTOTLE.

Purbach Lunar crater (25°S 2°W), 120 km (75 mi) in diameter, with rim components reaching 2440 m (8000 ft) above its floor. This ancient crater is from the earliest age of the Moon. Impact erosion has erased its ejecta blanket, rounded its rim, destroyed the north wall, and deeply incised its walls. Several ridges run through the central region of Purbach; these ridges represent the rims of dilapidated craters, perhaps in combination with central peak remnants.

Purcell, Edward Mills (1912-97) American physicist who won the 1952 Nobel Prize for Physics for his role in developing the technology of nuclear magnetic resonance. His major contribution to astronomy was his discovery (1951) of the TWENTY-ONE CENTIMETRE radio radiation from the spin-flip transition of neutral hydrogen atoms, predicted by Jan OORT. The 21-cm emission from H I regions enabled the structure of the Milky Way and other galaxies to be mapped.

Purkinje effect Change in the colour sensitivity of the eye that occurs when DARK ADAPTATION takes place. The normal level of sensitivity of the human eye lies in the 400-750 nm range, with the peak lying in the yellow to green region of the spectrum. As the level of illumination drops, this shifts more towards the green, sensitivity towards the red end of the spectrum decreasing correspondingly. The effect, which is named after the Czech physiologist Jan Purkinje (1787-1869), can also influence an observer's perception of star colours and magnitudes.

Purple Mountain Observatory Astronomical institution of the Chinese Academy of Sciences, also called Zijin Shan Observatory or Nanjing Observatory. It is located on Mount Zijin on the eastern outskirts of Nanjing, capital of Jiangsu Province, at an elevation of 267 m (880 ft). It was built in 1929 and began operation in 1934. Today, the observatory operates several optical telescopes up to 0.6 m (24 in.) in aperture and a 13.7-m (45-ft) millimetre-wavelength radio telescope at its Delinha station in Qinhai Province. The observatory runs research groups in celestial mechanics, astrophysics, radio and space astronomy and solar physics. It has four remote outstations withinbrightnesses of the 'fixed' stars, and precession is mentioned.

The final books, IX to XIII, are concerned with the thorny problem of planetary motion. There were fewer observations for Ptolemy to use here, and very little past theoretical work to help him. The epicycle and deferent were not totally satisfactory in the planetary context, because on their own they could not account for variations in the apparent retrograde motion of a planet, nor could they show why retrograde motion occurred at seemingly irregular intervals.

China, and possesses some of the Ming Dynasty instruments formerly at the ANCIENT BEIJING OBSERVATORY.

Putredinis Palus (Marsh of Decay) Lunar lava plain located in the north-central region of the Moon. It is an irregularly shaped region within the Mare IMBRIUM. It contains numerous volcanic features, including a dark mantling deposit, produced by fire fountaining, and Hadley Rille, a SINUOUS RILLE that is an ancient lava channel. An arcuate rille cuts through part of this province. Putredinis Palus was selected for the Apollo 15 mission, which returned samples of pyroclastic 'green glass'.

Pyxis See feature article


PUPPIS (gen. puppis, abbr. pup)

Large southern constellation, part of the old Greek figure of ARGO NAVIS, the ship of the Argonauts; Puppis represents the ship's poop, or stern. The brightest stars of Argo are now in neighbouring Carina and Vela, so the lettering of the stars in Puppis begins with Zeta, its brightest, known as NAOS. Ј Pup is a wide binocular double, mags. 3.3 and 5.3, while k Pup consists of two nearly equal 5th-magnitude stars divisible in small telescopes. L2 Pup is a semiregular variable ranging between 3rd and 6th magnitudes with a period of around 140 days; it forms a wide optical double with L1 Pup, mag. 4.9. V Pup is an eclipsing binary, a BETA LYRAE STAR of range 4.4—1.9, period 1.45 days. Lying in the Milky Way, Puppis is rich in open star clusters. Among the best are M46 and M47, both just visible to the naked eye. M93 is a binocular cluster. NGC 2451 includes a mag. 3.6 star, c Pup. Next to it lies NGC 2477, which resembles a globular when seen through binoculars.

PYXIS (gen. pyxidis, abbr. pyx) Small southern constellation introduced by Lacaille in the 18th century. It lies on the edge of the Milky Way adjoining Vela and Puppis, and represents a magnetic compass as used aboard ships. Its brightest star is a Pyx, visual mag. 3.68, distance 845 l.y., spectral type B1.5 III. T Pyx is a RECURRENT NOVA that has undergone five recorded eruptions, in 1890, 1902, 1920, 1944 and 1966, although it reaches only 6th magnitude at maximum.

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