. 8
( 12)


take place between atomic nuclei and elementary particles.
Nunki (Sigma Sagittarii) The second-brightest star in the constellation Sagittarius.
It is a ¤ B star of magnitude 2.0 and is 225 light years away.

44 Nysa

nutation A relatively short-period oscillation superimposed on the ¤ precession of
the rotation axis of a spinning body. The nutation of Earth™s axis amounts to a
maximum of 15 arc seconds over a period of about 18.6 years and is caused by
changes in the Moon™s orbit over this time.
44 Nysa An asteroid discovered in 1857 by Herman Goldschmidt. Its diameter is
68 km (42 miles) and it is notable for its high ¤ albedo, which is nearly 40
percent. It is one of two large members of the Nysa ¤ Hirayama family, the other
being 135 Hertha.

Oberon The second largest moon of Uranus, diameter 1523 km (946 miles) which
was discovered by William ¤ Herschel in 1787. Its surface is covered by
numerous impact craters and seems not to have been altered very much.
objective The main light-collecting lens at the front of a refracting telescope.
obliquity of the ecliptic The angle between the plane of Earth™s equator and the
¤ ecliptic or the ˜˜tilt™™ of Earth™s axis. Its present value is approximately 23 260 .
The effects of ¤ precession and ¤ nutation cause it to change between extreme
values of 21 550 and 24 180 .
Observatorio del Roque de los Muchachos An observatory on the island of La
Palma in the Canary Islands Group. The observatory site, regarded as one of the
best in the world, is operated by the Instituto de Astro¬sica de Canarias, which
is host to a number of different organizations and countries that have
telescopes there. It is the site of the 10-m ¤ Gran Telescopio Canarias. Other
telescopes there include the ¤ William Herschel Telescope and the ¤ Telescopio
Nationale Galileo. The observatory occupies an area of nearly two square
kilometers at an altitude of 2400 m (7900 feet).
observatory A place where astronomical observations are (or were formerly)
made, or an administrative center for astronomical research.
occultation The passage of one astronomical object directly in front of another.
¤ grazing occultation, eclipse.
Octans (The Octant) A faint and obscure constellation containing the south
celestial pole. It was introduced in the mid-eighteenth century by Nicolas L. de
Lacaille and contains only one star brighter than fourth magnitude.

A Voyager 2 image of Oberon taken
in 1986.

Olympus Mons

Olbers™ Paradox The question ˜˜Why is the sky dark at night ?™™ In 1826, Heinrich
W. M. Olbers (1758“1840) drew attention to the fact that the sky should be a
continuous blaze of light if the universe is in¬nitely old and ¬lled more or less
uniformly with stars because every line of sight from an observer would
ultimately encounter a star. In fact of course, the sky is dark at night. This
turns out to be an important cosmological observation. It supports the idea
that the universe is not in¬nitely old but began with the event called the ¤ Big
Bang. The ¬nite age of the galaxies means that there has been insuf¬cient time
to ¬ll the universe with light. In addition, the expansion of the universe means
that the observed sky brightness due to remote objects is reduced.
2201 Oljato A small asteroid, diameter 2.8 km (1.7 miles), discovered in 1947 by
Henry Giclas then lost until recovered in 1979. It is in a highly elliptical
Earth-crossing orbit and has a unique spectrum that does not resemble that of
any other known asteroid, meteorite or comet. Its nature is not known, but it
could be the ˜˜dead™™ nucleus of a comet that has ceased to be active.
Olympus Mons The highest mountain on Mars, and the largest volcano in the
solar system. It rises to a height of 27 km (17 miles). This gigantic ¤ shield
volcano is 700 km (435 miles) across and is similar in nature to volcanoes on
Earth but its volume is at least 50 times greater than its nearest terrestrial
equivalent. The caldera at the summit is 90 km (60 miles) across and a cliff at

The huge martian volcano Olympus Mons.

Omega Centauri

An infrared image of the globular star
cluster Omega Centauri.

least 4 km high rings the mountain. Older volcanic rocks, fractured and eroded
by the wind, surround the main peak.
Omega Centauri (NGC 5139) A particularly bright ¤ globular cluster of stars in the
southern constellation Centaurus. With a diameter of 620 light years it is the
largest globular cluster known.
Omega Centauri lies at a distance of 16 500 light years. It spreads over
about 1 of the sky and its total magnitude is 3.6, making it easily visible to the
naked eye. Its shape is distinctly elliptical with axes in the ratio 5:4.
The name Omega Centauri is of a type normally given to single stars. The
cluster was mistaken for a single star by early observers at Mediterranean
latitudes, who could never see it more than about 10 above the horizon.
Omega Nebula (M17; NGC 6618) A luminous nebula in the constellation
Sagittarius, also known as the Horseshoe Nebula and the Swan Nebula. It is
4800 light years away and 27 light years in diameter. It is a region of ¤ ionized
hydrogen, stimulated to glow by a group of at least ¬ve hot stars. A dark dust
cloud lies on the western edge of the luminous region.
Oort, Jan Hendrik (1900“1992) The Dutch astronomer Oort proved that galaxies
rotate, and he was a pioneer in using radio astronomy to trace the spiral
structure of our ¤ Galaxy and probe its center. In 1950, he proposed that the
solar system is surrounded by a huge, distant swarm of comets, which came to
be called the ¤ Oort Cloud. He became a professor at the University of Leiden in
the Netherlands in 1935 and was director of the Leiden Observatory from 1945
to 1970.
Oort Cloud (Oort“Opik Cloud) A spherical shell of billions of ¤ comets surrounding
the solar system at a distance of about 1 light year (50 000 AU). The total mass
of the objects in the cloud is thought to be about that of the Earth.


An infrared
mosaic of the
Omega Nebula.

Though it is impossible to observe directly with present technology, there
are strong theoretical and practical reasons to believe it exists. The idea was
¬rst put forward on theoretical grounds by Ernst Opik in 1932, and developed
by Jan ¤ Oort in the 1950s. It is likely that the comets were formed near the
present location of the outer planets and subsequently ejected to their much
greater distance by gravitational interactions.
open cluster A type of star cluster containing several hundred to several thousand
stars distributed in a region a few light years across. The member stars are
much more spaced out than in ¤ globular clusters.
Open clusters are relatively young, typically containing many hot, highly
luminous stars. Open clusters within our own ¤ Galaxy are located within the
Galaxy™s disk, so appear to lie within the ¤ Milky Way on the sky. Well-known
open clusters include the ¤ Pleiades, the ¤ Hyades and the ¤ Jewel Box.
Ophelia A small moon of Uranus, 43 km (27 miles) in diameter, discovered by the
¤ Voyager 2 spacecraft in 1986. Together with the moon Cordelia, Ophelia acts
as a ˜˜shepherd™™ of Uranus™s Epsilon ring.
Ophiuchus (The Serpent Bearer) A large constellation straddling the celestial
equator. The mythological ¬gure of the serpent holder is sometimes identi¬ed
with the healer Aesculapius. Though Ophiuchus is not traditionally a zodiacal
constellation, the ¤ ecliptic passes through its southern part. It contains ¬ve
stars of second magnitude and seven of third magnitude.


The open cluster of stars, M39, in the constellation Cygnus.

opposition The position of one of the planets orbiting farther from the Sun than
Earth when it is opposite the Sun in the sky, that is, when its ¤ elongation is
180 . When a planet is at opposition, its disk is fully illuminated and it reaches
its highest point in the sky at midnight. It also comes closest to the Earth
around opposition. As the orbits of the planets are elliptical rather than
perfectly circular, some oppositions bring planets closer to Earth than others.
This is particularly the case with Mars.
optical double star A pair of stars that lie close to each other in the sky by
chance, but are not physically associated with each other as is the case for a
true ¤ binary star.
orbit The path followed by a body moving in a gravitational ¬eld.
90482 Orcus A large ¤ Kuiper Belt object discovered in 2004. It has an orbit similar
to Pluto™s and is approximately 1600 km (1000 miles) across.
Orientale Basin A huge impact feature on the Moon™s extreme western limb as
viewed from Earth. It is visible only at times of favorable ¤ libration.
Photographs taken from lunar orbit by spacecraft show a structure of at least
three concentric rings. Unlike many other impact basins on the Moon, it is not
extensively ¬lled by dark ¤ mare material.

Orion Nebula

A radar image of the Orientale Basin on
the Moon.

Orion (The Hunter) A brilliant constellation straddling the celestial equator,
widely considered to be the most magni¬cent and interesting in the sky. Its
pattern is interpreted as the hunter brandishing a raised club and a shield.
Three bright stars mark his belt, and several fainter ones a sword hanging from
it. Orion contains ¬ve stars of the ¬rst magnitude or brighter and a further ten
brighter than fourth magnitude. The most spectacular diffuse nebula in the
sky, the ¤ Orion Nebula, is faintly visible to the naked eye in the ˜˜sword.™™
Orion Arm The spiral arm of the ¤ Galaxy in which the Sun is located.
Orionids An annual ¤ meteor shower, with multiple radiants, which lie on the
border of Orion and Gemini near the star Gamma Geminorum. The peak of the
shower occurs around October 22, and its normal limits are October 16“27. The
shower is produced by meteoroids that have come from ¤ Halley™s Comet.
Orion Molecular Cloud ¤ Orion Nebula.
Orion Nebula (M42 and M43; NGC 1976 and NGC 1982) A bright emission nebula
surrounding the multiple star Theta1 Orionis in the ˜˜sword™™ of Orion.
The luminous nebula is just part of a complex region of interstellar matter
at a distance of 1 300 light years occupying much of the constellation of Orion.
The Orion cloud is the largest dark cloud of its kind known in the Galaxy.
Millimeter-wave observations of the emission from the molecules CO
(carbon monoxide), HCHO (formaldehyde), and many others, reveal the
presence behind the visible part of a large ¤ molecular cloud, known as the
Orion Molecular Cloud (OMC-1). This is an important region of star formation,
and the four young hot stars that make up Theta1 Orionis, also known as ˜˜the

Orion Nebula

6h 5h


2175 U


ξ ο1
µ +10º
± •2 •1
γ π4
ρ π5
0º M78
µ 0º
ι „

κ Rigel



Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Orion.

Trapezium,™™ are believed to be less than 100 000 years old. The ¤ Becklin“
Neugebauer object and the ¤ Kleinmann“Low Nebula, both detected through their
infrared emission, are sites of current star formation in the complex.
The Trapezium stars are creating an expanding spherical cavity near the
edge of the dark cloud. Their ultraviolet radiation is ionizing the gas and
blowing away the dust. Relatively recently, in astronomical terms, the bubble
broke through on our side of the dark cloud revealing the stars and ionized
hydrogen within. The sharper edges of the nebula are produced by remnants of
dust. M43 (NGC 1982) is a northern section of the nebula separated from the
larger part (M42; NGC 1976) by a lane of dust.
In color images, the dominant color of the luminosity is red from the ¤ hydrogen
alpha light. Observed visually, the nebula appears greenish because of the eye™s low
sensitivity to red light. The green emission is due to oxygen. The nebula occupies
an area of sky about one degree across and is faintly visible to the naked eye.

O star

The Orion Nebula. This mosaic was created from images taken by the Hubble Space Telescope.

orrery A working model of the solar system showing the planets, possibly with
some of their moons, in their orbits around the Sun. The term ˜˜orrery™™ comes
from a model made in 1713 for an Irish nobleman, the Fourth Earl of Cork and
Orthose A small outer moon of Jupiter discovered in 2001. Its diameter is about
2 km (1 mile).
Oschin Telescope The 1.2-m (48-inch) ¤ Schmidt camera at ¤ Palomar Observatory. It
has been in operation since 1948.
O star A star of ¤ spectral type O. O stars have surface temperatures in the range
28 000“50 000 K and are bluish white in color. Their spectra are characterized
by lines of both neutral and ionized helium; emission lines are also commonly
present. The four brightest O stars in the sky are Delta and Zeta Orionis, the

outer planets

easternmost stars in Orion™s belt, and the southern stars Zeta Puppis and
Gamma2 Velorum.
outer planets The planets beyond the ¤ asteroid belt, namely Jupiter, Saturn,
Uranus, Neptune and Pluto.
Owens Valley Radio Observatory The radio astronomy observatory of the
California Institute of Technology (Caltech), located 400 km (250 miles) north
of Los Angeles at an altitude of 1200 m (4000 feet). The instruments in use are a
40-m (130-foot) dish built in 1965 and an array of dishes used to observe the
Sun. The Millimeter Wavelength Array, consisting of six 10.4-m (34-foot)
dishes, was moved in 2005 from the original site to a new, nearby location at a
higher altitude to form part of CARMA, the Combined Array for Research in
Millimeter-Wave Astronomy. The interferometer is used particularly for solar
observation and the 40-m dish is used for ¤ very-long-baseline interferometry.
Owl Nebula (M97; NGC 3587) A ¤ planetary nebula in the constellation Ursa Major.
It is one of the largest planetary nebulae known, with a diameter of 1.5 light
years, and lies at a distance of 1600 light years.

The Owl

PA (p.a.) Abbreviation for ¤ position angle.
Paaliaq A small outer moon of Saturn, in a very elliptical orbit. It was discovered
in 2000 and is about 19 km (12 miles) across.
2 Pallas A large asteroid discovered by Heinrich W. M. Olbers in 1802. With a
diameter of 533 km (331 miles), it is the second largest asteroid. It is of the
carbonaceous type, similar to the largest asteroid, ¤ Ceres. Its orbit is at the
unusually steep inclination of 35 to the plane of the solar system.
Pallene A small moon of Saturn discovered in 2004 by the ¤ Cassini team. It orbits
Saturn at a distance of 211 000 km (131 000 miles) between Mimas and
Palomar Observatory The observatory on Palomar Mountain in California where
the 5- m (200-inch) ¤ Hale Telescope is sited. It is owned and operated by the
California Institute of Technology (Caltech). The other principal instruments at
the observatory are the 1.2- m (48-inch) Oschin Telescope (a ¤ Schmidt camera), a
46- cm (18-inch) Schmidt camera and the 1.5- m (60-inch) re¬‚ector owned
jointly by Caltech and the Carnegie Institution of Washington.
Pan A small moon of Saturn orbiting in the ¤ Encke Division in the planet™s ring
system at a distance of 133 583 km (83 005 miles) from Saturn. It was found by
Mark R. Showalter in 1990 from studies of images taken by the spacecraft
¤ Voyager 1 and ¤ Voyager 2. Its existence had been predicted as an explanation
for observed structure in the rings around the Encke Division. Pan is about
20 km (12 miles) in diameter.
Pandora A small moon of Saturn, measuring about 110 · 88 · 62 km (68 · 55 · 39
miles). It was discovered by ¤ Voyager 2 in 1980. With ¤ Prometheus, it acts as a

This close-up view of Pandora from the
Cassini spacecraft shows that the
surface is coated in ¬ne dust-sized icy


˜˜shepherd™™ to keep the F-ring in place, orbiting 141 520 km (88 050 miles) from
parallax The apparent change in the relative positions of objects at different
distances when they are viewed from different places. In astronomy, there is a
formal de¬nition of the annual parallax of a star, based in the change in its
apparent position over a period of six months, when Earth moves from one
side of its orbit to the other. Only the parallaxes of relatively nearby stars are
measurable with any accuracy. However, the determination of parallaxes is
important since it is the most direct method of ¬nding a star™s distance.
Paranal Observatory The site in Chile of the ¤ European Southern Observatory™s
¤ Very Large Telescope (VLT). It is situated in the Atacama desert, 120 km
(75 miles) south of Antofagasta, at an altitude of 2632 m (8500 feet).
Paris Observatory The French national astronomical research institute, based at
the original site in Paris where it was founded in 1667. This is the oldest
astronomical observatory still in use for research. There is an astrophysics
section, located at the Observatoire de Meudon, just outside Paris, and a radio
astronomy station at Nanc ¸ay.
At the Paris site, there are some historical nineteenth century instruments,
including the telescope built for the ¤ Carte du Ciel project. The Observatoire de
Meudon was founded in 1876. It became the Astrophysics Section of the
Observatoire de Paris in 1926 when the two institutions were merged. The
Nanc radio astronomy station, established in 1953, is a large site with many

The Parkes Observatory radio dish.

P Cygni

Parkes Observatory An Australian radio astronomy observatory located 20 km
(12 miles) north of Parkes, New South Wales. The main telescope there
is an altazimuth mounted 64- m (210-foot) single dish, originally
commissioned in 1961. In 1988, it became a unit of the ¤ Australia Telescope
National Facility. It can serve as a stand-alone telescope or as a member of a
long-baseline ¤ array.
The Parkes dish was the ¬rst radio telescope to be built in the southern
hemisphere. It identi¬ed the ¬rst quasar in 1963, and has discovered many
¤ interstellar molecules and ¤ pulsars. It has also been used as part of the ¤ Deep
Space Network for tracking spacecraft.
parsec (symbol pc) A unit of distance used in professional astronomy. It is de¬ned
as the distance at which an object would have an annual ¤ parallax of one arc
second. It is equivalent to 3.0857 · 1013 km, 3.2616 light years or 206 265
astronomical units.
Pasiphae One of the small outer moons of Jupiter, with a diameter of 60 km
(37 miles). It was discovered in 1908 by Philibert J. Melotte.
Pasithee A small outer moon of Jupiter discovered in 2001. Its diameter is about
2 km (1 miles).
451 Patientia An asteroid 230 km (143 miles) across, discovered by Auguste
Charlois in 1899.
Pavo (The Peacock) A southern constellation introduced in the 1603 star atlas of
Johann Bayer. It contains one first-magnitude star, sometimes itself called
Pavonis Mons One of the three giant ¤ shield volcanoes on the ¤ Tharsis Ridge on
Mars. It is about 400 km (250 miles) in diameter and rises to a height of 27 km
(17 miles), 17 km above the level of the surrounding ridge.
Payne-Gaposchkin, Cecilia Helena (1900“1979) In 1925, Payne-Gaposchkin
became the first person to receive a doctorate in astronomy from Harvard
University. In her thesis she established for the first time that hydrogen is by
far the most abundant chemical element in the Sun™s atmosphere, with helium
the second most abundant.
Payne-Gaposchkin was born in England but moved to Harvard University
after graduating from the University of Cambridge. She spent all her
professional life at Harvard. For many years she had no formal position but in
1956 she was appointed to a professorship. She also became the ¬rst female
Chair of the astronomy department at Harvard.
pc Abbreviation for ¤ parsec.
P Cygni An unusual variable star, which is a ¤ recurrent nova. It was recorded as
third magnitude in August 1600 and stayed at this brightness for six years
before fading slowly. A second outburst occurred in about 1655, which was
again followed by slow fading. It subsequently fluctuated in brightness around

P Cygni

Collapsed lava tubes on the side of the Pavonis Mons volcano imaged by the Mars Express

Cecilia Payne-Gaposchkin.

sixth magnitude and has been about fifth magnitude, with only small
variations, since 1715.
The lines in the spectrum of P Cygni are all double, consisting of a broad
¤ emission line with a narrower ¤ absorption line on the blue side. The absorption
comes from starlight passing through surrounding shells of material, while the
emission comes from the portions of the shells either side of the central star as


0h 23h 22h 21h

±(And) Matar


+20º χ

γ Markab

σ µ


EC 0º
0º TIC
h h

Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Pegasus.

viewed from the Earth. The emission and absorption lines are displaced from
each as a result of the ¤ Doppler effect because the shells are expanding. There
are three distinct shells. The outermost one is pulsating with a 114-day period.
Peacock The brightest star in the constellation ¤ Pavo.
peculiar galaxy Any ¤ galaxy that does not obviously fit into the ¤ Hubble
classification of galaxies, shows signs of unusual energetic activity or is
interacting with neighboring galaxies.
Pegasus (The Winged Horse) A large northern constellation. The prominent
Square of Pegasus is formed by its three brightest stars and Alpha Andromedae
(Alpheratz), all of which are second magnitude. Alpha Andromedae used to be
considered as belonging to Pegasus and was known as Delta Pegasi.
Pele One of the most active volcanoes on Jupiter™s moon ¤ Io.
Pelican Nebula Popular name for the diffuse nebulae IC 5067 and 5070 in the
constellation Cygnus. They form part of the ¤ North America Nebula (NGC 7000)
penumbra (1) A region of partial shadow. During a solar ¤ eclipse, when the
Moon™s shadow sweeps across the surface of the Earth, observers in the
penumbral zone see only a partial eclipse.


The volcano Pele deposited sulfur to
create the large red ring seen in this
image of Io taken by the Galileo

Part of the Pelican Nebula.

penumbra (2) The outer zone of a ¤ sunspot, surrounding the darker ¤ umbra. In
the penumbra the magnetic field is horizontal and spreads out radially from
the sunspot.
Perdita A small inner satellite of Uranus, which is about 27 km (17 miles) across. It
was first imaged by ¤ Voyager 2 in 1986 and seen again in 1999 but the
discovery was not confirmed until further observations in 2003 made with the
¤ Hubble Space Telescope.
periastron In the orbital motion of a ¤ binary star system, the point where the two
stars are closest.
perigee In orbital motion, the point where the Moon or an artificial satellite is
closest to Earth.


6h +60º 5h 4h 3h 2h 1h

· Cluster

M76 º


Mirphak ±
µ θ
+40º º
California Nebula
ξ Menkib

+30º +30º
ζ Atik

5h 4h 3h 2h

Magnitudes: 5 4 3 2 1 0 brighter than 0 Variable stars

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

A map of the constellation Perseus.

perihelion (pl. perihelia) In orbital motion in the solar system, the point where a
body is closest to the Sun.
period The time after which a cyclical phenomenon repeats itself.
periodic comet A ¤ comet in a closed, elliptical orbit within the solar system.
Periodic comets are observed at their regular returns to Earth™s vicinity, if they
come near enough to be seen. This term is usually applied to comets with
periods of less than 200 years, more strictly called ¤ short-period comets.
Perseids A major annual ¤ meteor shower. Its radiant lies near the star Eta Persei and
it peaks on August 12. The normal limits are July 23“August 20. The meteor
stream responsible is associated with Comet 109P/Swift“Tuttle. The Perseids are
one of the best, most reliable annual showers, with peak rates typically between
50 and 100 meteors per hour. Records of it date back around 2000 years.
Perseus A large constellation of the northern hemisphere lying in a rich
part of the ¤ Milky Way. It contains 10 stars brighter than fourth magnitude,
including the variable star ¤ Algol. Perseus also includes a magnificent
pair of ¤ open clusters visible to the naked eye, known as the ¤ Double Cluster in

Perseus Arm

Perseus Arm One of the spiral arms of the Milky Way ¤ Galaxy. It winds around
from the far side of the galactic center to the region of the Galaxy beyond
the Sun.
Petavius A large lunar crater, 176 km (110 miles) in diameter, near the south-east
limb of the Moon. A prominent rille runs across the crater floor between the
multiple central peak and the terraced walls.
3200 Phaethon An asteroid, diameter 6 km (4 miles), discovered in 1983 by the
¤ Infrared Astronomical Satellite (IRAS). It is in a highly eccentric, Earth-crossing
orbit and appears to be the parent body of the ¤ Geminid meteor shower. It is
probably the inactive nucleus of a former comet.
phase The ratio between the illuminated area of the disk of a celestial body as it
presents to an observer on Earth and the area of its entire disk, taken as a
circle. The phases of the Moon are the repeating cycle of illuminated shapes
presented by the Moon.
The Moon and planets show phases because they emit no light of their
own and shine only by re¬‚ected sunlight. The hemisphere of a moon or planet
facing the Sun is bright, its other hemisphere dark. The phase as seen from
Earth depends on the relative positions of Earth, the Sun and the body in
question since this determines what proportion of the body™s illuminated half
is visible from Earth.
Phekda (Phecda; Gamma Ursae Majoris) The third-brightest star in the
constellation Ursa Major. It is an ¤ A star of magnitude 2.4 located 84 light
years away.
Phobos The nearer to Mars of its two small moons, which were discovered by
Asaph Hall in 1877. Images from the ¤ Viking spacecraft of 1977 showed
Phobos to be ellipsoidal in shape (28 · 20 km, 17 · 12 miles) and covered with
craters. The largest crater, Stickney, is 10 km (6 miles) in diameter, one third of
the moon™s largest dimension. A series of striations emanating from Stickney
appear to be fractures caused by the impact that created the crater. Data
collected by ¤ Mars Global Surveyor suggests the surface may be covered by fine
dust up to a meter (3 feet) thick.
Phocaea group A group of asteroids at a distance of 2.36 AU from the Sun, with
orbits inclined at 24 to the plane of the solar system. The group is separated
from the ¤ main belt of asteroids by one of the ¤ Kirkwood gaps and is not a true
family with a common origin. The group is named after 25 Phocaea, which has
a diameter of about 70 km (45 miles).
Phoebe The largest of Saturn™s outer moons, discovered by William Pickering in
1898. Phoebe is 220 km (137 miles) in diameter and roughly spherical. Its
surface is largely covered by very dark material that reflects only 6 percent of
incident light, though there are also bright spots. Phoebe™s orbit is very
elliptical, tilted, and ¤ retrograde, which suggests it was captured by Saturn.


Phobos imaged by the Mars Express spacecraft.

Phoenix A southern constellation introduced in the 1603 star atlas of Johann
Bayer. Though not particularly conspicuous, it does contain seven stars
brighter than fourth magnitude.
Phoenix Mars Scout A spacecraft NASA proposes to launch in August 2007 and
land in Mars™s polar region in May 2008. It is expected to operate for three
months, and will be equipped to dig into the ground. It will particularly be
looking for organic material and sub-surface water. Its name includes
˜˜Phoenix™™ because the spacecraft uses instruments constructed for planned
Mars missions that have been cancelled.
5145 Pholus An asteroid with a diameter of 190 km (118 miles), discovered in 1991
by David L. Rabinowitz. It follows a highly unusual, remote orbit, on which it
ranges between 8.7 and 32 AU from the Sun. With ¤ Chiron, and five other
asteroids in orbits with similar characteristics, it forms the group termed
¤ Centaurs. Pholus has a low ¤ albedo of 4.4 percent, and is very much redder in
color than typical asteroids.
photometry The accurate determination of the ¤ magnitudes of stars, or other
astronomical objects, over specified wavelength bands in their spectra.
Photometric measurements are used to find out broad physical characteristics


A high-resolution mosaic of Phoebe made from images taken by the Cassini spacecraft.

of stars, without the need to study their spectra in detail. Several systems are
used for this employing standard filters. Photometric measurements are also
important for determining the ¤ light curves of variable stars.
photosphere The visible ˜˜surface™™ of the Sun or a star. About 500 km (300 miles)
thick, the photosphere is a zone where the character of the gaseous layers
changes from being completely opaque to being transparent. It is the layer
that emits the light we actually see. The temperature of the Sun™s photosphere is
about 6000 K on average, falling to about 4000 K at the base of the ¤ chromosphere.
Pic du Midi Observatory An observatory in the French Pyrenees at an altitude of
2877 m (9350 feet). The largest telescope is a 2- m (79-inch) reflector, which
started operation in 1980.


The Pinwheel

Pico An isolated mountain peak in the Mare Imbrium on the Moon.
Pictor (The Painter™s Easel or Painter) An inconspicuous southern constellation
introduced in the mid-eighteenth century by Nicolas L. de Lacaille. Its original
name was Equuleus Pictoris, the painter™s easel, but this was subsequently
shortened to Pictor. Its two brightest stars are third magnitude.
Pierre Auger Observatory A pair of installations for detecting ¤ cosmic rays, run
by an international consortium. The first is in western Argentina. A second is
expected to be constructed later in Colorado. At each site there will be 1600
particle detectors in an area of about 4800 km2 (1860 square miles). The
detectors are 3000-gallon water tanks equipped with instruments. By 2006, the
installation in Argentina was very nearly complete.
Pinwheel Galaxy (M101; NGC 5457) The popular name for a large spiral galaxy in
Ursa Major, which we see face on. It is estimated to be at a distance of 15
million light years.
Pioneer A series of 11 American spacecraft, launched between 1958 and 1973.
Pioneers 1 to 4 were directed towards the Moon and all failed. Pioneers 5 to 9
were put in orbits around the Sun and used to study the Sun and conditions in
interplanetary space. Pioneers 10 and 11 were highly successful flyby missions,
Pioneer 10 to Jupiter and Pioneer 11 to both Jupiter and Saturn.
Pioneer 10 was launched on March 3, 1972 and passed Jupiter on
December 4, 1973 at a distance of 132 000 km (82 000 miles), returning the best

Pioneer Venus

Pioneer 11 returned this image of
Jupiter in December 1974.

pictures of the planet up to that time. It continued to transmit information
continuously while heading out of the solar system. On March 30, 1997, when
it was more than 66 AU from the Sun, its scienti¬c program was ended. The last
signals from it were received in February 2003. Pioneer 11 was launched on
April 6, 1973 to encounter Jupiter on December 3, 1974 at a distance of
42 800 km (26 600 miles) and Saturn on September 1, 1979 at 20 800 km
(13 000 miles). The last signals from it were received on September, 30, 1995.
Pioneer Venus Two American spacecraft sent to Venus in 1978. Pioneer Venus 1
was an orbiter, launched on May 20, 1978, which obtained radar maps of the
surface and returned visual images and other data. Pioneer Venus 2, launched
on August 8, 1978, was an atmospheric probe, with five small landers not
intended to transmit data after impact.
Pisces (The Fishes) A large but faint constellation in the ¤ zodiac. Its three brightest
stars are only fourth magnitude.
Piscis Austrinus (or Piscis Australis; The Southern Fish) A small southern
constellation. It contains the first-magnitude star ¤ Fomalhaut, but no others
brighter than fourth magnitude.
Pistol Star A ¤ luminous blue variable star near the galactic center surrounded by
nebulosity where it has ejected large amounts of material. Its name describes
the shape of the nebula. It is one of the most luminous stars known, thought to
have had an initial mass around 200 times the Sun™s.
Piton An isolated mountain peak in the Mare Imbrium on the Moon.
plage A bright emission region in the solar ¤ chromosphere. Plages coincide with
¤ faculae in the photosphere beneath them and enhancements of the ¤ corona


The Pistol Star.

above, from which there is increased X-ray, extreme ultraviolet and radio
emission. They occur in ¤ active regions on the Sun.
Planck A European Space Agency spacecraft to image anisotropies in the ¤ cosmic
background radiation over the whole sky with unprecedented sensitivity and
resolution. Launch is scheduled for 2007, jointly with the ¤ Herschel telescope. It
is named in honor of the German physicist Max Planck (1858“1947). Planck will
operate in an orbit around the ¤ Lagrangian point L2 between Earth and the Sun.
planet An astronomical body, with not enough mass to become a star or a ¤ brown
dwarf. The upper mass limit for a planet is about 0.013 solar masses (equivalent
to about 13 Jupiter masses). Though planets have traditionally been considered
as objects in orbit around parent stars, isolated bodies of very low mass
discovered in regions of star formation have also been described as
˜˜free-floating planets.™™ To qualify as a planet in the solar system, a body must
be in orbit around the Sun, and massive enough both to take on a shape close
to spherical and to have swept away most smaller objects from the vicinity of
its orbit. Under this definition, there are eight planets in the solar system.
Planets may be basically rocky objects, such as the inner planets “
Mercury, Venus, Earth and Mars, or primarily liquid and gas with a small solid
core like the outer planets “ Jupiter, Saturn, Uranus and Neptune. These eight
are regarded as the major planets of the solar system. Historically, Pluto was


also considered to be a major planet, but that categorization was called into
question by the discovery of other ¤ transneptunian objects similar in size to
Pluto, or even larger. In 2006, the ¤ International Astronomical Union adopted the
term ¤ dwarf planet to describe Pluto, the largest asteroid Ceres, and other
similarly sized bodies orbiting the Sun. Within the solar system, there are large
numbers of minor planets, or ¤ asteroids, and a population of small icy bodies
in the region beyond Neptune, known as the ¤ Kuiper Belt. Pluto is one of the
larger known members of this population.
So far, the presence of planets around stars other than the Sun has largely
been inferred, either from the measurement of small cyclical changes in
¤ radial velocity, as revealed through the ¤ Doppler effect, or by the small dip in
light observed when the planet crosses in front of its parent star. Observations
of disks around newly forming stars, which could provide the material to form
planetary systems, strengthen the argument that planetary systems probably
accompany at least some stars comparable with the Sun. In addition, there is
strong evidence (from small variations in pulse frequency) that at least two
¤ pulsars have planetary-sized companions. ¤ extrasolar planet.
planetarium A dome-shaped building housing a special projector that is used to
simulate the appearance of the night sky. Planetaria are widely used for both
educational purposes and entertainment. An ¤ orrery is sometimes called a
planetary nebula An expanding shell of gas surrounding a star in a late stage of
¤ stellar evolution. It was ¤ William Herschel who coined the term. He thought
that some circular nebulae he saw looked rather like the disks of the planets as
seen through a small telescope. There is no other connection between planets
and planetary nebulae.
Planetary nebulae are formed when ¤ red giant stars lose mass in the
process of becoming ¤ white dwarfs and form a gaseous shell. Typically,
a planetary nebula contains a few tenths of a solar mass of material and is

A planetary nebula nicknamed the
˜˜Spirograph Nebula™™. This false color
image from the Hubble Space
Telescope is assembled from views
through three different color ¬lters.


moving outwards with a velocity of 20 km/s. It lasts for perhaps 35 000 years
before the shell becomes too tenuous to be visible. The central star is
essentially a burnt-out core in the process of becoming a white dwarf.
Planetary nebulae are seen in a variety of forms, often with complex
structure resulting from the out¬‚ow of stellar winds at different rates and
different episodes of mass loss. Some planetary nebulae have been found to be
dumbbell-shaped rather than spherical, probably due to the central star being
a binary, or the existence of a ring around the star, constraining the direction
of out¬‚ow. Well-known planetary nebulae include the ¤ Ring Nebula, the
¤ Helix Nebula and the ¤ Dumbbell Nebula.
planetesimal A body smaller than about 10 km (6 miles) across made of rock
and/or ice, that formed in the primordial ¤ solar nebula. Planetesimals coalesced
to form larger planetary bodies.
planisphere A round star map, which is a projection of part of the celestial sphere
onto a plane. Planispheres usually have a rotating overlay with an aperture in
it that reveals the portion of the sky visible at a given date and time. A
planisphere is only useful over the latitude range for which it is designed.
plasma An ionized gas, consisting of a mixture of electrons and atomic nuclei. All
of the matter in the interior of stars is in the form of plasma, as is ¤ ionized
plasmasphere A region of ionized gas surrounding Earth above the ¤ ionosphere,
at altitudes greater than about 1000 km (600 miles). It extends out to between
25 000 and 40 000 km (about four and six Earth radii). The upper boundary
is marked by a sudden drop in plasma density, known as the plasmapause.
The particles in the plasmasphere are almost all protons and electrons.
plasma tail ¤ ion tail.
Plato (c. 428 bc“347 bc) The Greek philosopher Plato established the Academy in
Athens, which survived for over 900 years as the greatest institution of
learning in the ancient classical world. Plato™s contribution to science was to
establish that natural phenomena, such as the apparent motion of the planets,
could be analyzed mathematically, particularly by using geometry.
Platonic year The period of 25 800 years it takes the Earth™s rotation axis to sweep
out a complete cone in space as a result of ¤ precession.
Pleiades (M45; NGC 1432) An ¤ open cluster of stars in the constellation Taurus,
clearly visible to the naked eye. It is thought to contain about 1000 stars within
a sphere 30 light years across, and is 440 light years away. The stars are
embedded in a ¤ reflection nebula of cold gas and dust that appears blue in color
photographs. The cluster is young by astronomical standards “ only about 50
million years old “ and contains some very massive bright stars.
In Greek mythology, the Pleiades were the seven daughters of Atlas and
Pleione, who were called Alcyone, Asterope, Celaeno, Electra, Merope, Maia


A color-composite image of the Pleiades star cluster taken by the Palomar 48-inch Schmidt

and Taygete. These names, along with Atlas and Pleione, have been given to
brighter stars in the cluster. Though the popular name for the Pleiades is
the Seven Sisters, most people are able to distinguish only six stars with the
naked eye.
Pleione One of the brighter stars in the ¤ Pleiades. It is slightly variable in
brightness and was observed in 1938 and 1970 to throw off shells of gas.
plerion A ¤ supernova remnant with no clear shell structure. The ¤ Crab Nebula is
the chief example. About 10 percent of supernova remnants are like this.
Plough (Dipper or Big Dipper) A popular English name for the ¤ asterism formed
by the stars Alpha, Beta, Gamma, Delta, Epsilon, Zeta and Eta in the
constellation ¤ Ursa Major.
plume eruption A type of volcanic activity observed on ¤ Io. Plumes arise from
rifts or vents on the surface. The eruptive center is surrounded by a deposit of
white or dark red material. The eruption may be violent and short-lived or a
longer-lasting eruption of white material, rather like a geyser, consisting of
liquid sulfur or sulfur dioxide. Hot liquid from underground changes to a gas as
it rushes up through the eruptive center then condenses again as it falls. There
is evidence for similar eruptions on ¤ Triton in images returned by ¤ Voyager 2.


Pluto and its moon Charon, in a time sequence of images.

Plutino A ¤ transneptunian object which, like Pluto, follows an orbit in a 2:3
¤ resonance with Neptune so that it completes two orbits of the Sun in the time
it takes Neptune to make three orbits (248.5 years).
Pluto A ¤ dwarf planet in the solar system, discovered as a fifteenth-magnitude object
on February 18, 1930, from the ¤ Lowell Observatory by Clyde ¤ Tombaugh. Searches
for a planet beyond Neptune had started in 1905, stimulated by apparent
discrepancies between the calculated and observed orbits of Uranus and Neptune.
However, it is now known that the mass of Pluto is less than one-fifth that of the
Moon, insufficient to have any gravitational effect on Uranus and Neptune.
From its discovery until 2006, Pluto was categorized as the ninth major
planet of the solar system. However, discoveries of other objects beyond
Neptune made from the early 1990s onwards, revealed that Pluto is only one of
the larger known members of the ¤ Kuiper Belt. It was known from early on
that Pluto™s orbit is more highly inclined to the ecliptic and more eccentric
than that of any major planet. Its distance from the Sun ranges between 30 and
50 AU. Its most recent perihelion passage was in 1989 and between 1979 and
1999, Pluto was nearer to the Sun than Neptune.
The discovery of the moon ¤ Charon, in 1978, made it possible to obtain
improved values for Pluto™s diameter and mass. Its diameter is 2300 km
(1429 miles) and its mass is 1.27 · 1022 kg (0.0021 of Earth™s mass). Pluto™s overall
density is approximately twice that of water and it is thought likely to consist of
a thick layer of water ice overlying a core of partially hydrated rock. Charon and
Pluto are locked in synchronous rotation with a period of 6.39 days. Charon
keeps the same face towards Pluto and its orbital period is the same as Pluto™s
rotation period, so it is always over the same part of Pluto. Pluto™s rotation axis is
inclined at 122 to the plane of the ecliptic. As is the case for Uranus, Pluto™s
rotation is retrograde, so it seems to orbit while ˜˜lying on its side.™™
A rare series of mutual occultations and transits of Pluto and Charon took
place between 1985 and 1990. They happen only twice in Pluto™s 248-year
orbital period. They made it possible to distinguish the individual spectra of
Pluto and Charon and to construct the ¬rst rough maps of Pluto™s surface.
These con¬rmed previous suspicions of a highly non-uniform and variegated


surface. In contrast with Charon, which is gray, Pluto™s surface is reddish in
color. Methane ice was detected on Pluto in 1976. The occultation of a star by
Pluto in 1988 revealed the presence of an extended tenuous atmosphere.
Nitrogen and carbon monoxide ices were discovered on the surface in 1992.
The surface temperature is about 40 K. In 1996, observations with the Hubble
Space Telescope resolved broad light and dark features on Pluto™s surface for
the ¬rst time.
In 2005, observations made with the Hubble Space Telescope in
preparation for the launch of NASA™s ¤ New Horizons mission to Pluto revealed
two previously unknown moons orbiting Pluto between two and three times
farther away than Charon. They were estimated to be around 50 km (30 miles)
in size and were named Hydra and Nix.
Pointers The stars Alpha and Beta in the constellation Ursa Major, so called
because the line joining them points almost directly to the Pole Star.
polar A type of short-period, variable ¤ binary star that emits X-rays. The light from
polars is strongly polarized and the polarization varies over the orbital period,
which is between one and four hours. These close systems appear to consist of
a normal star and a strongly magnetic ¤ white dwarf. Matter is transferred from
the normal star to the white dwarf but, because of the strong magnetic field,
an ¤ accretion disk cannot form. Instead, the material is channeled along the
magnetic field lines and is deposited at the poles. Polars are also known as
AM Herculis stars after their prototype.
Intermediate polars are similar, but they have longer orbital periods
lasting several hours. A pulse of radiation comes from them each time the
white dwarf spins, which is typically in less than one hour. Their white dwarfs
are thought to have weaker magnetic ¬elds, making it possible for an outer
accretion disk to form, though material close to the white dwarf is channeled
onto the magnetic poles. The pulsed emission is a searchlight effect seen as the
accreting pole of the white dwarf sweeps across the line of sight. Intermediate
polars are also known as DQ Herculis stars after their prototype.
polar axis One of the two rotation axes about which a telescope on an ¤ equatorial
mount can turn. The polar axis must be accurately oriented parallel to Earth™s
rotation axis, which means that it has to be set at an angle to the horizontal
equal to the latitude of the place where it is located, and also in the north“south
plane. Rotation about the polar axis results in a change in the right ascension of
the direction in which the telescope is pointing, but not in declination.
polar cap A roughly circular area of limited extent around a pole of rotation of a
planet. In the case of Earth and Mars, the term is applied to the areas covered
by ice or frost in the two polar regions.
Polaris (Alpha Ursae Minoris) The brightest star in the constellation Ursa Minor. It
lies within one degree of the north celestial pole. Polaris is a ¤ Cepheid variable


The north polar cap of Mars.

and its magnitude changes between about 1.95 and 2.05 over a period of four
days. Its distance is 430 light years.
polarization (of light) The non-random distribution of electric field direction in a
beam of ¤ electromagnetic radiation.
polar motion A small ˜˜wobble™™ in the position of Earth™s geographic poles
relative to its surface (not relative to the stars). It arises because the axis
around which Earth rotates does not quite coincide with Earth™s axis of
symmetry. The wobble typically amounts to about 0.3 arc seconds, and there
are regular variations over periods of 433 days and one year. Much smaller
variations also take place over short timescales, ranging between two weeks
and three months, due to surface air pressure changes. ¤ Chandler wobble.
polar plume A bright stream of gas flowing away from the Sun. Plumes
usually come from the Sun™s poles but they can appear wherever there
is a ¤ coronal hole. They follow the direction of the magnetic field out of
the hole.
polar-ring galaxy An ¤ elliptical galaxy or ¤ lenticular galaxy that has dust, gas and
stars orbiting in a ring around it, more or less at right angles to the main plane
of the galaxy. Polar rings can form when two galaxies merge or pass close to
each other.
Pole Star Popular name for the star ¤ Polaris.
Pollux (Beta Geminorum) The brightest star in the constellation Gemini. It is an
orange-colored, giant ¤ K star. Castor and Pollux were the twin sons of Leda
in classical mythology.
Polydeuces A small moon of Saturn discovered in 2004 by Carl Murray of the
¤ Cassini team. With ¤ Helene, it shares the orbit of the larger moon ¤ Dione,
377 400 km (234 500 miles) from Saturn. It measures 13 km (8 miles).

Population I

NGC 4450A is a rare example of a
polar-ring galaxy. It lies 130 million
light years away. This image was taken
by the Hubble Space Telescope.

Population I Stars and star clusters in our ¤ Galaxy that are relatively young and
are in the ¤ galactic plane, especially in the spiral arms. The terms Population I
and ¤ Population II (for older stars) were introduced by Walter ¤ Baade in 1944.
Hotter stars, ¤ open clusters, and stellar ¤ associations are all typical Population I
objects. Interstellar material is also associated with Population I. Population
I stars are relatively rich in heavier elements because the material from
which they formed includes elements formed by ¤ nucleosynthesis in earlier
generations of stars. ¤ astration.
Population II Old stars and star clusters in our ¤ Galaxy. They are found all
around the Galaxy throughout a spherical halo, rather than just in the
¤ galactic plane like ¤ Population I stars. Population II stars contain less of the
elements heavier than helium than Population I. They move at high speeds


around the Galaxy on very elliptical orbits inclined at steep angles to the plane
of the Galaxy. ¤ Globular clusters belong to Population II. Population II stars
formed when the Galaxy was spherical before its disk formed, and before the
gas and dust could be enriched with heavier elements from previous
generations of stars.
pore A small ¤ sunspot without a ¤ penumbra that lasts for about a day.
Porrima (Gamma Virginis) The second-brightest star in the constellation Virgo,
which is in reality a visual ¤ binary star made up of two almost identical
¤ A stars. Their combined magnitude is 2.8; individually they are each
magnitude 3.6. They orbit around each other in 170 years. At a distance of 38
light years, Porrima is relatively close to the solar system. Porrima was a
Roman goddess of prophecy.
Portia A small moon of Uranus, discovered in 1986 by the ¤ Voyager 2 spacecraft.
Its diameter is 135 km (84 miles).
position angle (PA, p.a.) An angle specifying the position of one astronomical
object or feature relative to another. The position angle of an object B relative
to another one, A, is the angle between the line from A pointing due north and
the line from A to B. It is measured in the sense north“east“south“west, on a
scale from 0 to 360 . For double stars, the position angle is given for the
fainter component relative to the brighter one.
potentially hazardous asteroid (PHA) An ¤ asteroid that can pass within 0.05 of
an astronomical unit (5 million miles) of Earth. The first to be discovered was
1862 Apollo. In practice, hardly any pose a threat to Earth over timescales of
many thousands of years.
Praesepe (M44; NGC 2632) An ¤ open cluster of at least 200 stars in the constellation
Cancer lying at a distance of 500 light years. Its brightest stars are about sixth
magnitude. In many ways it is similar to the ¤ Hyades, even sharing the same
direction and speed of motion through space; this suggests that the two
clusters originated in the same interstellar cloud.
Praxidike A small outer moon of Jupiter discovered in 2000. Its diameter is about
7 km (4 miles).
precession The uniform motion of the rotation axis of a freely rotating body when
it is subject to a turning force (torque).
Precession causes Earth™s rotation axis to sweep out a cone in space over a
period of 25 800 years. Earth™s axis is always tilted at an angle of about 23.5 to
the plane of Earth™s orbit (the ¤ ecliptic), but the direction it points in slowly
changes. The main torques acting on Earth are the gravitational pulls of the
Sun and Moon on Earth™s equatorial bulge. The Moon™s contribution is about
twice as large as the Sun™s, because it is so close. Precession would not occur if
Earth were a perfect sphere. However, the Earth™s equatorial radius is 0.3
percent greater than its polar radius because of its rotation.

primary mirror

Because of precession, the celestial poles trace out circles in the sky over
25 800 years. So, for example, about 13 000 years from the present, the nearest
bright star to the north celestial pole will be Vega rather than Polaris.
The zero point of ¤ right ascension, one of the ¤ equatorial coordinates,
normally used to give the positions of celestial objects, is one of the two points
where the celestial equator and the ecliptic cross. For centuries this point has
been called ˜˜The First Point of Aries.™™ However, the equator is ˜˜sliding
around™™ the ecliptic because of precession so the intersection points are
constantly moving. As a result, the First Point of Aries is no longer in the
constellation Aries, but has moved into Pisces and will soon be in Aquarius.
This phenomenon is known as the precession of the equinoxes. Its effect on
the right ascension and declination of an object is noticeable from year to year.
Tabulated values of right ascension and declination are given for a particular
date or ¤ epoch, at which they were precisely correct. ¤ equinox, nutation.
primary mirror The main light-collecting mirror in a reflecting telescope.
prime focus The point at which the primary mirror in a reflecting telescope
brings light to a focus if there is no secondary mirror.
prime meridian The great circle on the surface of a planetary body adopted as the
zero of longitude measurement. On the Earth, the prime meridian is the circle
of longitude passing through Greenwich, London “ the Greenwich Meridian.
Procyon (Alpha Canis Minoris) The brightest star in the constellation Canis Minor.
At magnitude 0.38, it is the fifth-brightest in the sky, and it lies 11.25 light
years away. Procyon was discovered to be a binary system by John M.
Schaeberle in 1896. The primary is a normal ¤ F star and its faint companion
an eleventh-magnitude ¤ white dwarf. Their orbital period is 41 years. The
name Procyon comes from Greek and means ˜˜before the dog.™™ It refers to the
fact that Procyon rises before the ˜˜Dog Star,™™ ¤ Sirius.
Project Ozma The first serious scientific attempt to contact ¤ extraterrestrial
intelligence by radio waves. The experiment was conducted in 1960 at ¤ Green
Bank and unsuccessfully looked for unnatural radio signals from the nearby
stars Tau Ceti and Epsilon Eridani. ¤ SETI.
Prometheus (1) A small moon of Saturn, discovered in 1980 by the ¤ Voyager 1
team. Its diameter is about 148 · 100 · 68 km (92 · 62 · 42 miles). It is one of the
˜˜shepherd™™ moons that keep the F-ring in place, orbiting 139 350 km from
Prometheus (2) One of the most active volcanoes on Jupiter™s moon ¤ Io.
prominence A term used for a variety of flame-like streams of gas in the
¤ chromosphere and ¤ corona of the Sun, which have a higher density and lower
temperature than their surroundings. When seen at the edge of the Sun
they look bright but when seen against the Sun™s disk they appear as dark
¤ filaments.


Prometheus as seen by the Cassini

A solar prominence observed by the SOHO spacecraft on April 11, 2003. It extends for
over 30 times the size of the Earth.

Quiescent prominences occur away from active regions and are stable over
many months. They may extend upwards for tens of thousands of kilometers.
Active prominences are associated with ¤ sunspots and ¤ ¬‚ares. They appear as
surges, sprays and loops, have violent motions, and last up to a few hours.

proper motion

Proplyds in the Orion Nebula, imaged
by the Hubble Space Telescope.

proper motion The apparent annual motion of a star across the celestial sphere
due to its real motion through space relative to the solar system.
proplyd A recently formed star, surrounded by a cloud of gas and dust, which may
ultimately produce a planetary system. The word ˜˜proplyd™™ is a contraction of
˜˜protoplanetary disk.™™
Prospero A small outer moon of Uranus discovered in 1999. Its diameter is about
30 km (19 miles).
Proteus A moon of Neptune discovered during the flyby of ¤ Voyager 2 in August
1989. It measures about 436 · 416 · 402 km (271 · 259 · 250 miles).
protogalaxy The earliest stage in the formation of a galaxy from primordial gas
and dust.
proton“proton chain A series of nuclear reactions that convert hydrogen to
helium inside stars and are a major source of the energy stars generate.
protoplanet A body in the early stage of the process of becoming a planet.
protostar A star in the earliest observable stage of formation.
Proxima Centauri The nearest star to the solar system lying at a distance of 4.26
light years. It is an eleventh-magnitude dwarf red ¤ M star in the constellation
Centaurus. It appears to be associated physically with the bright binary star
Alpha Centauri, which is two degrees away in the sky, and about 0.11 light
years more distant from the solar system. It is estimated that Proxima may take
a million years to orbit its companions.
Psamathe A small moon of Neptune discovered by David Jewitt in 2003. Its
diameter is about 24 km (15 miles).
16 Psyche An asteroid with a diameter of 248 km (154 miles), discovered by
Annibale de Gasparis in 1852. It is of the metallic type and its surface appears
to be an almost pure alloy of iron and nickel.
Ptolemaeus A large shallow crater, flooded with dark lava in the Moon™s southern
upland area. Its diameter is 153 km (95 miles) and there are many smaller
craters within its walls.


The Ptolemaic system, showing the Sun, Moon and planets in orbit around the Earth.

Ptolemaic system A concept of the solar system with Earth at the center,
described by the Greek astronomer ¤ Ptolemy (c. ad 100“170) in his
book, the ¤ Almagest. It was the model generally accepted in the Arab
and Western worlds for more then 1300 years, until superseded by the
¤ heliocentric model.
Ptolemy, Claudius (c. ad 100“170) Little is known about the life of the Egyptian
astronomer Ptolemy who worked in the city of Alexandria between about
ad 127 and 151 but his great book, the Almagest, remained the most influential
work on astronomy for 1300 years until his assumption that the Earth is the
center of the solar system was overtaken by the idea put forward by
¤ Copernicus of a Sun-centered solar system. The Almagest included a star catalog
and rules for calculating the future positions of the planets based on
complicated geometry. It was not all original work by Ptolemy but a synthesis
of Greek astronomy from over 500 years.
Puck A moon of Uranus, discovered by the ¤ Voyager 2 spacecraft in 1985. Its
diameter is 162 km (101 miles), and a Voyager image shows several relatively
large craters on its surface.

Pulkovo Observatory

Pulkovo Observatory An observatory near St Petersburg in Russia, originally
established in 1718. The present site dates from 1835; the buildings were
destroyed during World War II and subsequently rebuilt in their old style. The
observatory is associated particularly with the ¤ Struve family, six members of
which became well-known astronomers. F. G. W. Struve was director from
1839 to 1862 and his son, Otto, director from 1862 to 1889.
pulsar A star from which we detect regular bursts of radio waves in rapid
sequence. The time between successive pulses ranges from milliseconds for
pulsars belonging to binary systems up to 4 seconds for the slowest. Some
pulsars emit radiation in other bands of the electromagnetic spectrum,
including X-rays, gamma rays and visible light, as well as radio waves.
A pulsar is a rotating, magnetic ¤ neutron star, with a mass similar to the
Sun™s but a diameter of only about 10 km (6 miles). The pulses occur because
the neutron star is rotating very rapidly and a beam of radio emission sweeps
by once per rotation. The pulses are very regular, apart from the occasional
¤ glitch, and all single pulsars are slowing down as they gradually lose energy.
Some X-ray pulsars are in binary systems where complex dynamical effects
cause their spin rate to speed up, and these millisecond pulsars are the fastest
known. Millisecond pulsars not currently in binary systems are thought to
have once belonged to pairs that have been split apart. Most have been
discovered in ¤ globular clusters, where stars are densely packed and
gravitational interactions can easily occur. At least one pulsar appears to have


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