. 3
( 12)


the asteroid belt. However, it never gets brighter than magnitude 6.9 as its
¤ albedo is only 9 percent. Ceres rotates in just over 9 hours. Its color and
brightness vary only slightly, suggesting that it is almost spherical and
uniformly gray. Information from its spectrum indicates that the composition
of the surface may be similar to that of ¤ carbonaceous chondrite meteorites.
Cerro Tololo Inter-American Observatory An observatory in Chile, forming
part of the ¤ National Optical Astronomy Observatories of the USA. The
headquarters are at La Serena, 480 km (300 miles) north of Santiago.
The mountain site, 70 km (45 miles) inland, is at an altitude of 2200 m
(7200 feet). The largest instrument is the 4-m (160-inch) Victor M. Blanco
Cetus (The Whale or Sea Monster) A large constellation in the region of the
celestial equator. It is said to represent the mythological sea monster that
threatened Andromeda, though Cetus is frequently translated as the ˜˜whale.™™
All but one of its stars are fainter than third magnitude. The most notable star
in Cetus is the variable ¤ Mira.
Chaldene A small outer moon of Jupiter discovered in 2000. Its diameter is about
4 km (2.5 miles).
Chamaeleon (The Chameleon) A small, faint, southern constellation, probably
invented by sixteenth century navigators and included by Johann Bayer in his
1603 star atlas Uranometria. None of its stars is brighter than fourth magnitude.
Chandler wobble Small variations in the position of Earth™s geographical poles
believed to result from seasonal changes in the distribution of mass on the
Earth and movements of material within the Earth.
Chandrasekhar, Subrahmanyan (1910“1995) Chandrasekhar was one of the
most distinguished astrophysicists of the twentieth century. He was awarded
the Nobel Prize in 1983 for his theoretical work on the structure of stars and
¤ stellar evolution. He was born in Lahore (then in India, now in Pakistan) and
studied physics in Madras and at Cambridge University in the UK. In 1937, he
went to the USA to work at the University of Chicago and remained there for
the rest of his life. He became an American citizen in 1953. He was known by
everyone simply as ˜˜Chandra,™™ and NASA named its orbiting ¤ Chandra X-ray
Observatory in his honor. ¤ Chandrasekhar limit.
Chandrasekhar limit The maximum mass a ¤ white dwarf star can have. The
astrophysicist Subrahmanyan ¤ Chandrasekhar proved theoretically that a
white dwarf can be no more than 1.4 solar masses. When more massive stars
exhaust their sources of nuclear energy, they must continue to collapse to a
size much smaller than a white dwarf to form a ¤ neutron star or a ¤ black hole.

Chandra X-ray Observatory

Subrahmanyan Chandrasekhar.

Chandra X-ray Observatory A NASA orbiting X-ray telescope launched from
the ¤ Space Shuttle Columbia on July 3, 1999, then put into a very elliptical,
high-altitude orbit. Its distance from Earth ranges between 10 000 and
140 000 km (6200 and 87 000 miles). It has instruments for making images and
for recording spectra. It was named in honor of Subrahmanyan (˜˜Chandra™™)
¤ Chandrasekhar. ¤ X-ray astronomy.
charge-coupled device ¤ CCD.
Charon The largest moon of ¤ Pluto. It was discovered by James Christy in 1978,
when he noticed a slight elongation of the image of Pluto on a photograph
taken at the US Naval Observatory. With a diameter of 1200 km (740 miles),

An artist™s impression of the Chandra X-ray Observatory in space.


it is about half the size of Pluto. Charon orbits Pluto in 6.39 days, which is also
the rotation period of both Pluto and Charon. There is evidence in the
spectrum of Charon for water ice, but not methane, which is present on Pluto.
Chicxulub Crater A large terrestrial impact crater, 170 km (106 miles) across,
located by the northern coast of the Yucatan Peninsula in Mexico. It has been
identified as the crater excavated by an impact event 65 million years ago that
appears to be linked with the mass extinction of living species, including the
dinosaurs. The crater is now buried under a kilometer of sediment and was
discovered in 1990 by a geophysicist looking for oil.
2060 Chiron An unusual asteroid discovered in 1977 by Charles Kowal. Its orbit lies
between the orbits of the planets Jupiter and Uranus, well outside the main
¤ asteroid belt. It was the first of several asteroids found in orbits of this kind, which
are now known collectively as ¤ Centaurs. Chiron™s diameter is about 180 km (112
miles). According to infrared observations, it has a moderately dark, rocky or dusty
surface and is nearly spherical. In 1989, a ¤ coma was discovered around Chiron, as
a result of which it was also designated as a periodic comet (95P/Chiron).
chondrite A common kind of stony ¤ meteorite, characterized by the presence of
¤ chondrules. About 85 percent of meteorites are chondrites, as opposed to
¤ achondrites.
chondrule A small sphere of silicate minerals found in many stony ¤ meteorites.
Chondrules range in size from less than 1 mm to more than 10 mm.
¤ chondrite, achondrite.
chromosphere The layer of the Sun (or of any star) above the ¤ photosphere.
Chromosphere literally means ˜˜sphere of color.™™ It is seen as a pinkish glow
when the light of the photosphere is hidden at a total solar eclipse.
Circinus (The Compasses) A small, insignificant constellation of the southern sky.
It was introduced by Nicolas L. de Lacaille in the mid-eighteenth century.
circumpolar star A star that never sets below the horizon as seen by a particular
observer. For a star to be circumpolar, its ¤ declination must be greater than 90
minus the latitude of the observer™s location. From the equator no stars are
circumpolar; at the Earth™s poles all stars are circumpolar.
circumstellar disk A disk of gas or dust surrounding a star. Newly forming stars
develop disks as part of the normal process. In some cases at least, planetary
systems form within circumstellar disks. ¤ accretion disk, proplyd.
Clavius A large lunar crater, 225 km (140 miles) in diameter, near the southern
limb of the Moon.
Clementine A mission to the Moon undertaken by the US Department of Defense
principally to test space hardware. Launched on January 25, 1994, it spent
two months in orbit around the Moon. Using digital imaging with
ultraviolet“visible and near-infrared cameras, it made better geological maps
than any previous lunar mission and returned a large amount of scientific data.


Circumstellar disk around two nearby stars. Data from the Hubble Space Telescope is in
false color. Both stars are about 60 light years away. (The black circles are disks used to
cut out the glare from the stars.)

The intention had been for Clementine to travel on to a flyby of the asteroid
¤ Geographos. However, that part of the mission was cancelled after the
spacecraft malfunctioned on May 5.
Cloverleaf A ¤ quasar with a quadruple image cause by a ¤ gravitational lens effect.
The quasar has a redshift of 2.55.
Cluster A European space mission to study the interaction between Earth™s
magnetic field and the ¤ solar wind and processes in Earth™s ¤ magnetosphere. It
consists of four separate satellites flying in formation. The distance between
them varies between a few hundred kilometers and 20 000 kilometers (12 000
miles). The original four craft were destroyed when the first Ariane 5 launcher
exploded on June 4, 1996. Four further craft were subsequently constructed for
launch into highly elliptical polar orbits around Earth by two Soyuz rockets
in 2000. The new mission was named Cluster II.
cluster of galaxies An assembly of galaxies , held together by gravity.
The distribution of galaxies in space is not uniform: they tend to be in
clumps on distance scales of millions of light years. Clusters of galaxies take a
variety of forms. They can be spherical and symmetrical, or ragged with no
particular shape. They may contain a handful of galaxies or thousands. There
may or may not be a concentration towards the center. Our own Milky Way
Galaxy belongs to a small association known as the ¤ Local Group.
Clusters containing a concentration of many large galaxies are described as
˜˜rich.™™ The nearest rich cluster is the ¤ Virgo Cluster, which has thousands of
members. Even larger is the ¤ Coma Cluster, which is at least ten million light
years across. The center of a rich cluster is usually dominated by a giant
elliptical galaxy. The most massive galaxies known are at the centers of large
rich clusters. Galaxies tend to merge at the centers of clusters “ a process called
˜˜galactic cannibalism.™™ The cannibal galaxies often look distended and some

Coma Cluster

The Fornax cluster of galaxies contains
vast clouds of hot gas at 10 million
degrees between and around the
galaxies, which is revealed in this
X-ray image.

seem to have more than one nucleus. They are usually strong sources of radio
Tenuous hot gas ¬lls the space between the galaxies in rich clusters. Its
presence was revealed because of the X-rays it emits. In some clusters, there is
as much matter in the gas between the galaxies as there is in the visible parts
of the galaxies. ¤ galaxy, intracluster medium.
cluster of stars ¤ star cluster.
Coalsack An ¤ absorption nebula of interstellar dust in the ¤ Milky Way. It lies in the
constellation Crux and is about 4 across.
COBE ¤ Cosmic Background Explorer.
Cocoon Nebula The diffuse nebula IC 5146 in the constellation Cygnus. It is a
region of glowing hydrogen gas surrounding a sparse cluster of twelfth-
magnitude stars, at an estimated distance of 3000 light years. It is thought to be
a complex region of gas and dust where star formation is taking place.
Columba (The Dove) A small, faint constellation in the southern sky, introduced
by Augustin Royer in 1679. It is sometimes said to represent the dove that
followed Noah™s Ark.
coma A cloud of gas and dust surrounding the nucleus of a ¤ comet. Typically, a
coma reaches its maximum size of up to a million kilometers across when the
comet has just passed ¤ perihelion in its orbit around the Sun.
Coma Berenices (Berenice™s Hair) A small, faint constellation next to Bootes, ¨
which was introduced by Tycho ¤ Brahe in about 1602. It is supposed to
represent the tresses of Queen Berenice of Egypt who cut off her hair and
presented it to the gods in gratitude for the safe return of her husband from
battle. The constellation is notable for the large number of galaxies it contains,
members of both the Coma and Virgo clusters of galaxies.
Coma Cluster A rich cluster of galaxies in the constellation Coma Berenices. It
covers several degrees of sky and contains more than a thousand bright


galaxies. At an estimated distance of 300 million light years, it is one of the
nearer rich clusters. Most of the galaxies are concentrated towards the center
of the roughly spherical cluster, where the average distance between galaxies
is three times smaller than the distance between the Milky Way and the
Andromeda Galaxy in the ¤ Local Group.
comet An icy body orbiting in the solar system, which develops a ¤ coma and tails
when it nears the Sun.
The nucleus of a comet is like a ˜˜dirty snowball™™ a few kilometers across. It
is composed of water, carbon dioxide, methane and ammonia “ all frozen “
with dust and rocky material embedded in the ice. As a comet approaches the
Sun, solar heating starts to vaporize the ice. A visible sphere of gas and dust,

The Coma Cluster of galaxies.


A comet in the process of
disintegrating. The Hubble Space
Telescope imaged these fragments of
Comet Schwassmann“Wachmann 3,
which broke up as it approached the
Sun in 2006.

called the coma, develops around the nucleus. The coma may be up to a
million kilometers wide. The coma is surrounded by a huge invisible cloud of
hydrogen, many millions of kilometers in size. The hydrogen comes from
the breakdown of water molecules by solar radiation. In 1996, X-ray emission
was discovered from Comet ¤ Hyakutake, and it was subsequently found that
other comets are also X-ray sources.
Dust and gas leave the comet nucleus from jets on the side facing the Sun,
then stream away under the Sun™s in¬‚uence. Electrically charged atoms are
swept away directly by the magnetic ¬eld of the ¤ solar wind and form a
straight tail called an ion, Type I, plasma or gas tail. Variations in the solar
wind cause structures to form in the ion tail. Sometimes the tail breaks off.
Small dust particles are not carried along by the solar wind but get ˜˜blown™™
gently away from the Sun by radiation pressure. Dust tails (also called Type II
tails) are often broad and ¬‚at. Comet ¤ Hale“Bopp was found to have a third
distinct tail, made of sodium.
As a comet approaches the Sun, its tails grow. When it recedes from the
Sun, the tails shrink again. Whatever the direction of the comet™s motion, its
tails are always directed away from the Sun. They can be as much as a hundred
million kilometers long. Large dust particles get strewn along the comet™s orbit
and form ¤ meteor streams. Despite their often dramatic appearance, comets
contain very little material, perhaps only a billionth the mass of the Earth.
Their tails are so tenuous that only one ¬ve-hundredth of the mass of the
nucleus may be lost in a passage around the Sun.
New comets are discovered regularly. Some are short-period comets, in
elliptical orbits that take between about 6 and 200 years to complete. Most are

cometary globule

long-period comets, in orbits so elongated that they do not return for many
thousands of years.
It is now generally accepted that most comets come from a spherical cloud
that surrounds the solar system at a distance of around 50 000 AU. This
˜˜reservoir™™ of comet nuclei is called the ¤ Oort Cloud. Other comets appear to
come from the ¤ Kuiper Belt, located beyond the orbit of Neptune. The short-
period comets have been captured within the planetary system by a close
encounter with Jupiter.
When a new comet is discovered, or a known periodic comet is recovered,
it is given a designation consisting of the year followed by a letter for the half-
month period when it was found. The letters are A for January 1“15, B for
January 16“31 and so on, up to Y for December 16“31. The pre¬x P/ is added for
short-period comets or C/ for long-period comets. Periodic comets that have
disappeared or been destroyed are pre¬xed by D/. New comets are named after
their discoverers, though no more than three names are permitted when there
are several independent claimants. A few comets have been named after
individuals who calculated their orbits (Halley and Encke, for example) and
after observatories or satellites, where discovery was essentially through the
efforts of a team. When a short-period comet has been fully established as
such, it is also allocated a number. This system for designating and naming
comets was introduced in 1995.
cometary globule A small interstellar cloud resembling a comet. A cometary
globule is what remains of a dense clump within a cloud of interstellar gas and
dust after the more tenuous gas around it has been swept away by the action of
strong ultraviolet radiation from nearby stars. The clump is the head of the
globule. Its tail is like a shadow, where some of the material of the original
cloud has been protected from the ultraviolet light. The famous ¤ Horsehead
Nebula is a cometary globule in the process of formation.
comet group A class of comets that have orbits with similar characteristics.
Members of a group (or family) of comets are not necessarily close to each
other in space. Short-period comets are one example. These have come under
the influence of Jupiter™s gravity and have orbital periods typically between 6
and 8 years. The ¤ sungrazers, are another group. These long-period comets
skim the Sun™s outer layers and have other features of their orbits in common.
Compton Gamma Ray Observatory A NASA orbiting observatory with four
¤ gamma-ray astronomy experiments on board. It was launched from the
¤ Space Shuttle in April 1991 and operated for 9 years. Originally known simply
as the ˜˜Gamma Ray Observatory,™™ it was later named in honor of the American
physicist Arthur Holly Compton (1892“1962). One of the things it achieved was
a catalog of gamma-ray sources, including ¤ supernova remnants, stellar OB
¤ associations and ¤ active galactic nuclei.


Cone Nebula A dark, cone-shaped, dust nebula in NGC 2264, a complex region of
nebulosity and stars near the fifth-magnitude star S Monocerotis.
conjunction An alignment of two bodies in the solar system with the Earth, so
that they appear to be at the same place in the sky “ or very close to each
other “ as seen from Earth.
A planet is at conjunction when it is approximately in line with the Sun
and Earth. The planets Mercury and Venus have conjunctions when they are
between the Sun and Earth and when they are behind the Sun as seen from
Earth. These alignments are called inferior conjunction and superior
conjunction, respectively. The planets farther from the Sun than Earth only
have superior conjunctions.

The Cone Nebula imaged by the Hubble Space Telescope.


Conjunctions can also occur between planets or between the Moon and a
constellation One of 88 designated areas in the sky or the pattern of stars in it.
From antiquity, different civilizations have given their own names to
patterns of bright stars. Many of the constellations used by astronomers today
originated in Mesopotamia and were further developed by the Greeks.
Forty-eight were listed by ¤ Ptolemy in the second century ad and the rest have
been added since about 1600. Some old star maps include constellations that
are no longer used.
Originally, constellations were regarded simply as star patterns, but they
gradually became a way for astronomers to specify stars and their positions. As
the science of astronomy developed, the fact that the constellations were not
precisely de¬ned led to confusion. This was resolved in 1930 when there was
international agreement among astronomers to de¬ne the boundaries of 88
constellations along lines of ¤ right ascension and ¤ declination.
Each of the 88 constellations has an entry in this dictionary under its
of¬cial Latin name.
contact binary A pair of stars touching each other or surrounded by a common
envelope of gas. As stars evolve, they expand and grow into giants. If an
expanding star is close to another star in a binary system, the pair can come
into contact. There may be ¤ mass transfer between the stars and their outer
layers can even merge to make one sphere of gas around the two cores.
The brightness of close binary stars often varies as one star crosses in front
of the other from our point of view on Earth. The shape of the ¤ light curve of
some pairs, such as W Ursae Majoris, are best explained if the two stars are in
contact. ¤ Roche lobe.
continuous spectrum A ¤ spectrum in which the strength of radiation varies only
gradually with wavelength, in contrast with the sharp peaks of intensity in an
¤ emission line spectrum. Any object warmer than ¤ absolute zero emits a
continuous spectrum. A continuous spectrum may be crossed by narrow
¤ absorption lines, as in the Sun™s spectrum, for example. ¤ black body radiation.
convection The transport of heat energy by currents of gas or liquid. Convection is
one of the ways in which heat reaches the surface of a star from its core where
nuclear energy is generated. Huge circulating currents are created as gas
rises because it is heated from below, then falls again after cooling.
coordinate system A way of specifying the position of something, either in space
or on a surface, by giving a set of linear or angular distances from particular
planes, lines or points. Latitude and longitude, for example, are the
coordinates used to specify the location of a place on the surface of the Earth.
Two coordinates are enough to identify a place on a surface but three are
needed to fix a position in three-dimensional space.

Cor Caroli

In astronomy, several different systems of celestial coordinates are used to
give positions on the ¤ celestial sphere. Each has its own special use. For example,
¤ galactic coordinates are most useful for studies of our own Galaxy.
Copernican system A description of the solar system in which the planets orbit
around the central Sun, as proposed by Nicholas ¤ Copernicus and published in
his book De Revolutionibus in 1543. The theory Copernicus set out did not find
favor at first since it predicted planetary positions no more accurately than
the ¤ Ptolemaic system, which had been used for hundreds of years.
Furthermore, it displaced the Earth from the center of the universe, which
many people regarded as unacceptable from a religious point of view.
Copernicus made the basic planetary orbits circles. He could not predict the
movements of the planets correctly because their orbits are in fact ellipses. To
reproduce the actual observations, Copernicus had to resort to ¤ epicycles.
Later, Johannes ¤ Kepler developed a more accurate heliocentric theory after
discovering that the planetary orbits are elliptical rather than circular.
Copernicus, Nicolaus (1473“1543) Copernicus™s book, ˜On the Revolutions of the
Heavenly Spheres™ (usually called by its shortened Latin title, De Revolutionibus) is
one of the most influential books of all time. In it, he set out his theory that
Earth is in orbit around the Sun, along with the other planets. This was a truly
revolutionary idea. Virtually everyone at that time believed that Earth was
stationary at the center of the universe and that the Sun, Moon and planets
circled around it. This view was deeply rooted in Christian beliefs as well as
being the picture that had been accepted by astronomers for over a thousand
years. Copernicus™s work marked the start of a complete change of perspective
on the universe.
Copernicus was born in the Polish city of Torun. He went to the university
at Krakow, where he became interested in astronomy, and also studied law and
medicine in Italy. His lifelong career was as a canon (an administrative position
in the church) at Frauenburg. Between about 1510 and the early 1530s,
Copernicus worked on his heliocentric theory, and produced the manuscript of
De Revolutionibus. Though news of his ideas spread among scholars, he did not
formally publish his book. Only years later was he persuaded to publish it. The
¬rst printed copies reached him just before he died in 1543. ¤ Copernican system.
Copernicus A large and conspicuous crater on the Moon at the center of a ray
system that extends for more than 600 km (370 miles). Terraced walls and
multiple central peaks are features of the crater, which is 93 km (58 miles) in
Cor Caroli (Alpha Canum Venaticorum) The brightest star in the constellation
Canes Venatici. This Latin name means ˜˜Charles™s heart™™ and is a reference to
the execution of King Charles I of England in 1649. It is said to have been
named by Charles Scarborough in 1660. Cor Caroli is 97 light years away and is


A close-up view of the terraced walls of the lunar crater Copernicus as seen by the Hubble
Space Telescope.

a ¤ visual binary star. The components have magnitudes of 2.9 and 5.5. The
brighter one is an ¤ A star.
Cordelia A small moon of Uranus discovered when ¤ Voyager 2 flew by the planet
in 1986. Cordelia is one of the two moons that act as ˜˜shepherds™™ of the
planet™s Epsilon ring (the other being Ophelia).
corona The outermost layers of the Sun, which become visible as a white halo
during a total solar ¤ eclipse. The solar corona extends out to many times the
Sun™s radius, until it merges with the ¤ interplanetary medium. It consists of a
number of components:
 The K corona (electron corona or continuum corona) is white light from the
Sun™s ¤ photosphere scattered by high-energy electrons, which are at a
temperature of a million degrees. The structure of the K corona is variable,
with streamers, condensations, plumes and rays.
 The F corona (Fraunhofer corona or dust corona) is light from the
photosphere scattered by slower-moving dust particles around the Sun.
¤ Fraunhofer lines are seen in its spectrum. The extension of the F corona
into interplanetary space is ¤ zodiacal light.
 The E corona (emission line corona) is light in the form of ¤ emission lines
produced by highly ionized atoms, particularly iron and calcium. It is
detected out to two solar radii only.
The solar corona also emits extreme ultraviolet light and soft X-rays. The
extent and shape of the corona changes during the course of the ¤ solar cycle,
mainly due to the streamers produced in ¤ active regions.
Corona Australis (The Southern Crown) A small constellation on the southern
border of Sagittarius. Though rather faint, it is quite conspicuous and is one
of the ancient constellations listed by ¤ Ptolemy.

coronal mass ejection

The Sun™s corona as seen during a total eclipse in 1970.

Corona Borealis (The Northern Crown) A small but noticeable constellation of the
northern sky. Its main stars form a rough semicircle.
coronagraph An instrument for observing the Sun™s ¤ corona, which is normally
seen only at a total solar eclipse. Invented by Bernard Lyot in 1930, the
coronagraph is a special telescope containing a disk at the prime focus to block
out the light from the Sun™s face and create an artificial eclipse. However, even
when a coronagraph is located where the sky is very clear, scattering of light
by Earth™s atmosphere is a problem. This is partly overcome by the use of
special filters or by observing the light from the corona with a spectrograph.
coronal hole A region in the solar ¤ corona where the density and temperature are
exceptionally low. They typically last for several rotations of the Sun, and
they are sources of strong ¤ solar wind.
coronal mass ejection (CME) An eruption of material from the solar ¤ corona.
CMEs are associated with features in the Sun™s magnetic field. During periods
of high ¤ solar activity, one or two occur each day and can happen anywhere in
the corona. In periods when the Sun is quiet, there is one CME about every
3“10 days and they are confined to lower latitudes. The average ejection speed
ranges from 200 km/s at minimum activity to twice that speed at solar
maximum. Most occur without an accompanying ¤ flare. If a flare does occur,
it usually follows the onset of the CME. CMEs are the most powerful kind of
transient solar phenomena and have a significant influence on the ¤ solar wind.


A coronal mass ejection imaged from space by SOHO. In each view, the white circle represents
the size of the Sun, which is hidden behind the occulting disk at the center.

Large CMEs that eject material directly towards Earth provoke ¤ magnetic
Corvus (The Crow) A small constellation on the southern border of Virgo. Its four
main stars are third magnitude and arranged roughly in the shape of a kite.
Cosmic Background Explorer (COBE) A NASA astronomy satellite launched in
1989 to map far-infrared and millimeter-wave radiation from the whole sky.
It measured the temperature of the ¤ cosmic background radiation precisely for
the first time and found it to be 2.73 K. COBE confirmed that the cosmic
background radiation is ¤ thermal radiation and consistent with the ¤ Big Bang
theory of the origin of the universe. COBE™s infrared detectors also mapped the
distribution of dust in the Milky Way.
cosmic background radiation Diffuse electromagnetic radiation that appears to
pervade the whole of the universe. Its discovery in 1964 by Arno Penzias and
Robert Wilson (publicly announced in 1965) was immensely important for
¤ cosmology because it was strong evidence in favor of the ¤ Big Bang theory.
The radiation received today is understood to be a relic of the radiation created
when our universe came into existence. The spectrum of the background
radiation is characteristic of a ¤ black body at a temperature of 2.73 degrees
above ¤ absolute zero (2.73 K) and is most intense in the microwave region. For
this reason, it is often called the cosmic microwave background (or CMB).
The Milky Way Galaxy is traveling through space at 600 km/s relative to the
background radiation.
The ¤ Cosmic Background Explorer satellite (COBE) in 1992 showed for the ¬rst
time that the radiation is not totally even across the sky. It discovered ripple-like


variations in temperature amounting to about ten millionths of a degree. These
ripples show the ¬rst signs of structure emerging in the early universe.
cosmic rays Extremely energetic particles traveling through the universe at close
to the speed of light. They were discovered by Victor F. Hess in 1912 during a
balloon flight. Particles beyond Earth™s atmosphere are known as primary
cosmic rays. When they encounter the atmosphere, they collide with atomic
nuclei and produce air showers of particles, called secondary cosmic rays.
The mix of atomic nuclei in cosmic rays largely mirrors the abundance of
the different chemical elements making up stars like the Sun. Cosmic rays are
the only particles we can detect that have traversed the Galaxy. The ones with
most energy may even have come from remote ¤ quasars and ¤ active galactic
nuclei. Lower-energy cosmic rays are generated within the Galaxy in ¤ supernova
¤ supernova remnants and ¤ pulsars. Solar ¤ ¬‚ares are a source of the lowest-
energy cosmic rays, which are more intense during maximum ¤ solar activity.
cosmic shear Distortion of the shape of a remote galaxy as observed from Earth,
caused by the gravity of matter along the line of sight. This effect is predicted
by ¤ general relativity. It has been used to map the distribution of ¤ dark matter
in the universe. ¤ gravitational lens.
cosmic year (galactic year) A period of about 220 million years, which is the time
taken by the Sun to complete one revolution about the center of the ¤ Galaxy.
cosmogony The study of how celestial systems and objects in general are formed
and, in particular, the study of the origin of the solar system.
The modern view is that the solar system formed from a slowly rotating
cloud of gas. As the cloud collapsed, a dense opaque core formed “ ultimately
to become the Sun “ surrounded by a disk of gas and dust. The ¬rst such
nebular theories were suggested by Immanuel Kant (1724“1804) in 1755 and
Pierre Simon de ¤ Laplace in 1796. Ideas on how the planets actually formed
within the disk have changed greatly over that time. Current thinking is that
the smaller planets gradually accumulated by ¤ accretion while the giant
planets may have formed from condensations within the disk, developing
disks of their own which gave rise to their satellite systems. The fall-off in
temperature with distance from the Sun accounts for the main differences
between the inner rocky planets and the outer giants.
cosmology The branch of astronomy concerned with the origin, properties and
evolution of the universe. Physical cosmology is about making observations to
obtain information on the universe as a whole. Theoretical cosmologists create
mathematical descriptions of the observed properties of the universe.
Cosmology in the broadest sense covers physics, astronomy, philosophy
and theology because it aims to assemble a world picture explaining why the
universe has the properties it has. Ancient cosmologies were simple pictorial


models and myths. Greek cosmologists tried to describe the motion of the
planets through mathematics.
Modern observational cosmology depends on collecting information about
the universe as a whole, such as surveys of remote galaxies and studies of the
¤ cosmic background radiation. It is now generally accepted that the observational
evidence strongly supports ¤ Big Bang cosmology. Theoretical cosmology is
normally based on the theory of ¤ general relativity. Over large distances, gravity
is the dominant force affecting matter and it controls the large-scale structure
of the universe. General relativity describes the relationships between space,
time, matter and gravity.
cosmos An alternative word for the universe as a whole or the realm of space
beyond Earth.
coude focus A place where the image from a telescope on an ¤ equatorial mount
can be brought to a focus and remain stationary regardless of the orientation of
the telescope. The coude focus is at a point on the polar axis of the telescope
mount. Light from the telescope is directed there along a zig-zag route by
´ ´
of a series of mirrors. The word ˜˜coude™™ is French for ˜˜elbow.™™ The coude
focus was often used in the past for bulky equipment. Modern, large,
telescopes on computer-controlled ¤ altazimuth mounts use the ¤ Nasmyth focus
Crab Nebula (M1; NGC 1952) The nebulous remains in the constellation Taurus of
a ¤ supernova that exploded in ad 1054. In colored photographs it appears as a
network of red filaments surrounding an elliptical area of pale white light. This
white light is ¤ synchrotron radiation generated by hot ionized gas in a magnetic
field. The filaments are the outer layers of the star that were blown off in
the explosion and are traveling outwards at about 1500 km/s.
The core of the star that exploded remains at the center of the nebula. It is
now a ¤ pulsar. Electrons emitted by the pulsar are responsible for the
synchrotron radiation. The interval between ¬‚ashes from the pulsar is
33 milliseconds; ¬‚ashes are seen in visible light as well as the radio pulses.
The Crab Nebula is one of the strongest sources of radio waves in the sky.
This source was called ˜˜Taurus A™™ by radio astronomers before it was
identi¬ed with the known nebula. It is also a source of X-rays.
Crab pulsar The ¤ pulsar at the center of the ¤ Crab Nebula.
In 1942, astronomers speculated that a peculiar star in this nebula was a
¤ neutron star, created in the ¤ supernova of ad 1054. Radio astronomers
discovered in 1968 that the central star is a pulsar spinning 30 times a second
so that the period between pulses is 33 milliseconds. Shortly after discovery
of the radio pulsar, the visible light from the neutron star was also shown to be
coming in pulses. At the time of its discovery, this was the fastest pulsar
by far and its spin rate proved that the object must be a neutron star: any

Crab pulsar

The Crab Nebula.

object with a typical star™s mass but larger than a neutron star would fly apart
if it were rotating so quickly.
Historically, this discovery was of great importance because theorists
subsequently felt much more con¬dent that highly condensed objects really do
exist in the universe. This led directly to research on ¤ black holes.
The Crab pulsar is a source of electrons traveling at almost the speed of
light. They give rise to strong X-ray and radio emission from the Crab Nebula
surrounding the pulsar. The pulsar™s rotation period is increasing at
36 nanoseconds per day. At that rate, its rotation speed will have halved within
about 1200 years. The pulsar has random ¤ glitches, which may be caused by
¤ starquakes. Its mass is thought to be about half that of the Sun.


Craters on the Oceanus Procellarum on the Moon, including Schiaparelli C and
Schiaparelli E, photographed from lunar orbit by the crew of Apollo 15.

crater A circular pit on the surface of a planetary body. The word literally means
˜˜cup,™™ and craters are typically shaped like a shallow cup. They are lower in
the center than the surrounding terrain but have raised walls. They may
contain a central peak or depression. The vast majority of craters observed on
planets and their moons are now known to have resulted from the impact of
meteorites. However, volcanic craters (¤ calderas) also occur “ on Mars, for
Craters range in size from microscopic to hundreds of kilometers across.
The largest are often called basins. The exact shape of a crater depends on
many factors, including the composition of the planetary surface, the speed,
mass and direction of the body that caused it, and weathering or geological
activity that has modi¬ed it since it ¬rst formed.
Crater (The Cup) A small, faint constellation on the southern border of Virgo. It is
one of the ancient constellations and is said to represent the goblet of Apollo.
None of its stars is brighter than fourth magnitude.
Crepe ring One of the rings of ¤ Saturn, also called the C ring. It is fainter than the
prominent A and B rings. It lies closer to Saturn than ring B, extending about
halfway between the B ring™s inner edge and the planet. The name comes from


The Crescent

a description of its appearance given by the observer William Lassell after
William Cranch Bond announced its discovery in 1850.
crescent One of the ¤ phases of the Moon, Venus or Mercury when less than half
the disk is illuminated.
Crescent Nebula A popular name for NGC 6888, a diffuse shell of gas surrounding
the ¤ Wolf“Rayet star HD 192163. One crescent-shaped segment of the spherical
shell is relatively bright.
Cressida One of the small moons of Uranus discovered during the encounter
¤ Voyager 2 had with the planet in 1986. Its diameter is about 80 km (50 miles).
critical density In cosmology, the average density of matter and energy that
ensures the universe will not expand for ever and makes space flat rather than
curved. Its value is equivalent to about six atoms of hydrogen in a cubic meter,
but this is far larger than the amount of visible matter, such as stars and
galaxies, can account for. ¤ Inflationary universe theories predict that the
universe should have the critical density. Detailed studies of the ¤ cosmic
background radiation suggest that the average density of the universe is indeed
close to the critical value but ¤ dark energy and ¤ dark matter together account
for 98 percent of the density of the universe.
Crux (The (Southern) Cross) The best-known of all southern constellations. It used
to be part of Centaurus (which surrounds it on three sides) before Augustin
Royer introduced it as a separate constellation in about 1679. It is the smallest
constellation by area, yet one of the most distinctive and recognizable. It lies in
the ¤ Milky Way and contains a fine star cluster known as the ¤ Jewel Box.


14h 13h 12h 11h
“40 º



“50 º
β δ
10 h
Jewel Box
15 κ
± Acrux
θ 2,1
Coal Sack
(Dark nebula)
Rigil Kent

0º 0º


12 h
14 10 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 Crux.

culmination The moment when a star or other celestial object reaches its
maximum altitude above the horizon. Culmination occurs when the object
crosses the observer™s ¤ meridian. So, at culmination an object is either due
south or due north of the observer. Circumpolar stars cross the meridian
both below and above the pole. These events are called lower and upper
culmination, respectively.
Cupid A small inner moon of Uranus, discovered in 2003. It is about 18 km
(11 miles) across.
65 Cybele One of the larger asteroids, discovered by Ernst W. L. Tempel in 1861.
Its diameter is 308 km (191 miles).
Cygnus (The Swan) A conspicuous constellation of the northern ¤ Milky Way. It has
the shape of an elongated cross similar to a swan in flight and is sometimes
called the Northern Cross. It contains 11 stars brighter than fourth magnitude,
including the first-magnitude ¤ Deneb and a well-known double star, ¤ Albireo.
Cygnus A An elliptical galaxy that is one of the strongest radio sources in the sky.
Cygnus A, also known as 3C 405, is the strongest radio source in the
constellation Cygnus and was detected by the ¬rst radio telescopes. The source
consists of two similar clouds of radio emission, symmetrically located either

Cygnus A

23h +60º º
22h 21h 20h 19h 18h


+50 º
κ +50
π2 R
ω2 ω1
ρ δ
+40º º
North America
ν γ
σ Sadr

„ P

+30º +30º

Veil Nebula

22h 21h 20h 19h

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 Cygnus.

False color image of the radio jet and
lobes of radio galaxy Cygnus A.

side of a ¬fteenth-magnitude galaxy at a redshift of 0.057. Cygnus A is the
prototype of all powerful radio galaxies. It is one of the largest physical
structures in the universe, its clouds spanning about 300 000 light years. The
energy associated with the radio clouds is about ten million times greater than
the output of normal galaxies such as the Andromeda Galaxy (M31).
The galaxy has strong emission lines in its optical spectrum, indicating
that it contains an ¤ active galactic nucleus. It is widely accepted that the energy

Cygnus Loop

Artist™s impression of Cygnus X-1.

generated by Cygnus A can be explained only by a central ¤ black hole that is
releasing very large amounts of energy as matter falls towards it.
Cygnus Loop A circular nebula 3 across in the constellation Cygnus. The
¤ Veil Nebula forms one section of the circular structure. Radio emission
confirms that the nebula is a ¤ supernova remnant. It is estimated to be 30 000
years old and to lie at a distance of 2500 light years. What we see as a loop is in
fact a spherical shell. It is currently expanding at a rate of 6 arc seconds every
hundred years.
Cygnus X-1 An intense sources of X-rays in the constellation Cygnus, discovered in
1966. It is thought to be a ¤ black hole and has been the subject of much study
and speculation.
Cygnus X-1 has been identi¬ed as a ¤ binary star system. The primary star is
a hot supergiant of ¤ spectral type O or B. Its orbit around its unseen companion
suggests that the invisible star is signi¬cantly more massive than the upper
limit for neutron stars. If so, the invisible star can only be a black hole. The
X-ray energy is generated by matter from the primary star streaming onto its
compact companion.
Cygnus X-3 A source of X-rays in the constellation Cygnus. It is a close ¤ binary star
system in which the two members orbit each other every 4.8 hours. The
primary star™s mass is about the same as the Sun™s and it appears to be a
¤ Wolf“Rayet star. The secondary star is a pulsed source of gamma-rays with a
period of 12.6 milliseconds. This star is likely to be a ¤ pulsar whose rate of
rotation has been increased because of interactions with its partner.
¤ Cosmic rays have been detected at ground level on Earth from the
direction of Cygnus X-3. This demonstrates that X-ray binary stars are
important sources of high-energy cosmic rays. ¤ X-ray astronomy.
Cyllene A small outer moon of Jupiter discovered in 2003. Its diameter is about
2 km (1 mile).

Dactyl ¤ Ida.
5335 Damocles An asteroid following an unusual, highly elliptical orbit. Its
distance from the Sun ranges between 1.6 and 22 AU. Damocles was discovered
in 1991 by the Australian astronomy Rob McNaught. Its diameter is about
10 km (6 miles).
Daphnis A small moon of Saturn discovered in 2005 by the ¤ Cassini team. It orbits
in the ¤ Keeler Gap in the ring system, 136 500 km (84 800 miles) from Saturn.
dark energy The unknown source of a repulsive force which appears to pervade
the universe. It opposes the tendency of gravity to slow down the rate at
which the universe is expanding. Because of its effects, the expansion of the
universe has been speeding up for the last 500 million years. Dark energy
accounts for about three-quarters of the total energy and mass in the universe.
¤ accelerating universe.
dark matter Matter in the universe that gives out no radiation and has so far been
detected only by its gravity. Some of it is likely to be ordinary matter made
from atoms containing neutrons and protons, but a large proportion is thought
to be in the form of exotic elementary particles that hardly interact at all with
radiation or with the neutrons and protons in ordinary atoms. About
23 percent of the total matter and energy in the universe is dark matter. Only 2
percent is ordinary matter. (The rest is ¤ dark energy.)
The motion of galaxies in clusters strongly suggests that many clusters are
at least 10 times more massive than the luminous parts of the galaxies in them.
It is also possible to estimate the masses of individual galaxies from the way
their rotation varies between their center and their edge. Giant spiral galaxies
studied in this way contain far more matter than can be accounted for by
glowing stars and gas. Dark matter is also important for explaining how
galaxies formed in the early universe.
dark nebula ¤ absorption nebula.
Darwin A project under study by the European Space Agency (ESA) for a ¬‚otilla
of space telescopes designed to search for ¤ extrasolar planets and to analyze
the composition of their atmospheres. The spacecraft would form an
¤ interferometer operating in the infrared and would be positioned
1.5 million km sunward of Earth, at the ¤ Lagrangian point. Darwin would be
launched in 2015 at the earliest.

511 Davida

511 Davida An asteroid 324 km (201 miles) across, discovered by Raymond S.
Dugan in 1903. It is one of the largest asteroids.
David Dunlap Observatory The observatory of the University of Toronto,
Canada, located 25 km (15 miles) north of the university campus. It was
presented to the university in 1935 by Mrs Dunlap as a memorial to her
husband. The main instrument is a 1.88-m (74-inch) re¬‚ector, the largest in
Dawn A NASA mission to investigate the asteroids ¤ Vesta and ¤ Ceres,
scheduled for launch in June 2007. The spacecraft is due to arrive at Vesta in
September 2011 and orbit there until April 2012. It will then goes on to orbit
Ceres from February 2015. The mission would end in July 2015 unless
extended with a visit to another asteroid.
day In astronomy, a unit of time de¬ned as 86 400 seconds. This period is close to
the average time Earth takes to rotate. However, Earth™s rotation is not
completely uniform. ¤ solar day, sidereal day.
De Cheseaux™s Comet An exceptionally bright ¤ comet discovered independently
by Dirk Klinkenberg from Haarlem on December 9 and by Jean Philippe Loys
de Cheseaux from Lausanne on December 13, 1743. It reached magnitude “7
and developed a fan of 11 separate tails.
declination (Dec.) One of the coordinate used to de¬ne positions on the ¤ celestial
sphere in the ¤ equatorial coordinate system. Declination is the equivalent of
latitude on the Earth. It is the angular distance, measured in degrees, north or
south of the ¤ celestial equator. Northerly declinations are positive and
southerly ones negative.
Deep Impact A NASA space mission to Comet Tempel 1, launched on January 12,
2005. It released a 500-kg (1,110-pound) impactor that struck the comet in July
2005 and then studied the resulting crater and ejecta as it ¬‚ew by.

The Deep Impact spacecraft took this
image of Comet Tempel 1 13 seconds
after it had been struck by the
impactor it had released.


deep sky object An astronomical object that does not belong to the solar system.
Deep Space Network A ground-based network of radio dishes used by NASA to
communicate with space missions. The three 70-m (230-foot) antennas are
located at ¤ Goldstone in California, near Madrid in Spain and near Canberra in
Deep Space 1 A NASA spacecraft launched in October 1998 with the main aim of
testing 12 novel technologies. In July 1999 it ¬‚ew close to the asteroid ¤ Braille. On
September 22, 2001, it passed within 2200 km (1400 miles) of the nucleus of
Comet ¤ Borrelly and returned the best images of a comet nucleus up to that time.
degenerate star A term that covers ¤ white dwarfs and ¤ neutron stars, both of
which are made up of degenerate matter. These stars are in an advanced state
of evolution and have suffered extreme gravitational collapse. Normal atoms
cannot exist in them.
In white dwarfs, the electrons and atomic nuclei are squashed together so
there is no space between them. A quantum effect, called degeneracy pressure,
prevents further collapse. However, if the total mass of the star is over 1.4
times the mass of the Sun, even degeneracy pressure is insuf¬cient to hold
back more collapse. If the mass is up to about 3 solar masses, it becomes a
neutron star. In a neutron star, the electrons and nuclei combine together to
create matter consisting of tightly packed neutrons. If the star is more than
about 3 solar masses, a ¤ black hole forms.
Deimos One of the two small moons of Mars, discovered in 1877 by Asaph Hall.
Its shape is irregular and it measures 15 · 12 · 10 km (9 · 7 · 6 miles). Images
from the ¤ Viking 2 Mars mission revealed a heavily cratered surface. The
properties of both moons suggest that they are captured ¤ asteroids.
Delphinus (The Dolphin) A small constellation lying in the ¤ Milky Way just north
of the celestial equator. Though it is quite faint, its shape is very distinctive
and it is one of the ancient constellations listed by ¤ Ptolemy.
Delta Aquarids A double ¤ meteor shower that radiates from the constellation
Aquarius between July 15 and August 20 each year, peaking on July 29 and
August 7.
Delta Cephei A yellow giant star in the constellation Cepheus that varies in
brightness between magnitudes 3.6 and 4.3 over a period of 5.37 days. It is the
prototype of the ¤ Cepheid variable stars and lies at a distance of 300 light years.
Demon Star ¤ Algol.
Deneb (Alpha Cygni) The brightest star in the constellation Cygnus. It is a
supergiant ¤ A star of magnitude 1.3 and is 2600 light years away. Its Arabic
name means ˜˜tail.™™
Denebola (Beta Leonis) An ¤ A star of magnitude 2.1, the third-brightest in the
constellation Leo. It is 36 light years away. The name Denebola comes from
Arabic and means ˜˜the lion™s tail.™™


The diamond ring effect captured at a total solar eclipse in 1983.

Desdemona One of the small moons of Uranus discovered during the ¤ Voyager 2
encounter with the planet in 1986. Its diameter is about 64 km (40 miles).
Despina A moon of Neptune discovered during the ¬‚yby of ¤ Voyager 2 in August
1989. It measures 150 km (93 miles) across.
D galaxy A type of large ¤ elliptical galaxy with a bright nucleus surrounded by an
extensive envelope. D galaxies are often radio galaxies. ¤ cD galaxy.
diagonal (star diagonal) An attachment for a small telescope used to turn the
eyepiece tube through a right angle so the observer can look from the side. It
contains a small plane mirror or prism. When a diagonal is used, the image
is reversed right-to-left.
diamond ring effect A phenomenon observed at the very beginning and end of
totality in a total solar ¤ eclipse when the last or ¬rst glimpse of the Sun shines
through a valley on the limb of the Moon. ¤ Baily™s beads.
dichotomy The time when the Moon, Mercury or Venus is exactly at half phase.
differential rotation The rotation of a gaseous body, such as the Sun or the planet
Jupiter, at a rate that varies with latitude, or the rotation of a non-solid, disk-shaped
structure, such as a galaxy, at a rate that varies with distance from the center.
diffraction The spreading of a beam of light as it passes by the edge of an obstacle.
The effect is to bend some light into the obstacle™s shadow. Interference

disconnection event

Dione imaged by Cassini with Saturn
in the background.

between different parts of the diffracted light beam results in a pattern of light
and dark areas called a diffraction pattern.
diffraction grating An optical device used to split light into a spectrum. It
consists of a large number of narrow, closely spaced lines ruled either
on glass to form a transmission grating or on polished metal to form a
re¬‚ection grating. Typically there are several thousand rules per centimeter.
Interference between the beams of light created by ¤ diffraction at each slit
produces the spectrum. Diffraction gratings can produce very high
dispersion spectra of good quality and are used in astronomical
¤ spectrographs.
diffuse nebula A gaseous ¤ nebula. The use of the word ˜˜diffuse™™ dates from a
time when all objects of fuzzy appearance were classed as nebulae, and
observers wanted to distinguish different types even though their true nature
was unknown. Star clusters and galaxies are no longer called nebulae.
Dione A medium-sized moon of Saturn, discovered by ¤ Giovanni D. Cassini in 1684.
Images from the ¤ Voyager 1 mission show several different types of terrain on
Dione: heavily cratered areas, plains with a lower density of craters, and
smooth plains with few craters or other features. The largest craters are over
200 km (125 miles) across, and craters over 100 km are common in the heavily
cratered areas. Another noticeable feature is an irregular network of light
wispy streaks on a dark background, which may be frosty deposits. Dione
is Saturn™s fourth largest moon orbiting at a distance of 337 400 km
(234 500 miles) and measures 1118 km (695 miles) across.
disconnection event A break in the ¤ ion tail of a ¤ comet that happens when
the comet crosses a region where the direction of the magnetic ¬eld in the
¤ solar wind changes.

disk galaxy

disk galaxy A ¤ spiral galaxy that has had most of its interstellar gas stripped away
as it moves through the intergalactic medium in a ¤ cluster of galaxies.
D layer A region of the Earth™s ¤ ionosphere at heights between about 50 and 90 km
(30 and 55 miles). The lower D layer, between altitudes of 50 and 70 km, is also
known as the C layer.
Dobsonian telescope A low-cost, large-aperture, re¬‚ecting telescope on a simple,
¤ altazimuth mounting. Dobsonians are used for visual observing by amateurs and
are easily moved around. The design was pioneered in the 1960s and 1970s by
John Dobson of the San Francisco Sidewalk Astronomers. Dobson also
demonstrated that a large mirror could be made cheaply from ordinary plate glass.
Dog Star A popular name for the star ¤ Sirius.
Dominion Astrophysical Observatory The National Research Council of
Canada™s center for optical astronomy, located near Victoria, British Columbia.
It is part of the Herzberg Institute of Astrophysics. It was founded by John S.
Plaskett and its 1.85-m (72-inch) telescope began operating in 1918.
Dominion Radio Astrophysical Observatory The National Research Council of
Canada™s radio astronomy observatory, located 20 km (12 miles) south-west of
Penticton in British Columbia. It forms part of the Herzberg Institute of
Astrophysics and was founded in 1959. The main instrument is an ¤ aperture
synthesis radio telescope consisting of seven 9-m (30-foot) dishes on an east“west
baseline 600 m (2000 feet) long.
Donati, Comet A brilliant comet discovered by Giovanni B. Donati of Florence in
1858. Contemporary drawings show it with a broad, curved ¤ dust tail and two
narrow, straight ¤ ion tails.
Doppler effect The change in the observed frequency and wavelength of sound
waves or ¤ electromagnetic radiation when the source of waves and the observer
are moving towards or away from each other. ¤ redshift.
Doppler shift The amount by which the ¤ Doppler effect changes the wavelength of
light. A Doppler shift is a ¤ redshift when wavelengths are increased because
the source of light is receding. When wavelengths are decreased because the
source is approaching, the Doppler shift is a ¤ blueshift.
Dorado (The Gold¬sh or Sword¬sh) A southern constellation, probably invented
by sixteenth century navigators and included by Johann Bayer in his 1603 star
atlas ¤ Uranometria. Its stars are inconspicuous but the Large ¤ Magellanic Cloud
lies on its southern border with Mensa.
Double Cluster in Perseus ¤ h and chi Persei.
double star ¤ binary star.
Draco (The Dragon) A large but rather faint constellation stretching halfway
around the north celestial pole and enclosing Ursa Minor on three sides.
Draconids A ¤ meteor shower, associated with Comet Giacobini“Zinner, which is
seen in some years around October 9 or 10. The meteors appear to radiate from

Draper, Henry (1837“1882)

near the ˜˜head™™ of Draco. The number of meteors varies from year to year.
Spectacular displays were witnessed in 1933, when meteors brie¬‚y rained
down at a rate of 350 a minute, and again in 1946. Moderate showers occurred
in 1952 and 1985. The Draconids are also known as the Giacobinids.
Drake equation A mathematical expression formulated by Frank Drake in
1961 that gives the number of extraterrestrial civilizations we might detect
as the product of several factors. Drake originally wrote his equation as: N ¼ R
fp ne fl fi fc L, where
N is the number of detectable civilizations
R is the rate of star formation
fp is the fraction of stars forming planetary systems
ne is the number of planets hospitable to life
fl is the fraction of those planets where life actually emerges
fi is the fraction of planets where life evolves into intelligent civilizations
fc is the fraction of planets with intelligent beings capable of interstellar
L is the length of time that such a civilization remains detectable.
Draper, Henry (1837“1882) The American astronomer Henry Draper was a
pioneer in astronomical spectroscopy. He trained as a doctor and became Dean
of the Medical Faculty of New York University but he also established a private
observatory, where he worked on photography and spectroscopy. In 1872 he

Henry Draper.


The Dumbbell Nebula.

took the first photograph of dark lines in the spectrum of a star (Vega) and in
1880 he took the first photograph of the ¤ Orion Nebula. However, he is most
remembered because of the ¤ Henry Draper Catalog of stellar spectra, funded
by his wife after his death as a memorial to him. ˜˜HD™™ numbers from the
catalog are still widely used to identify stars.
Dubhe (Alpha Ursae Majoris) One of the two bright stars of the ¤ Big Dipper in
Ursa Major forming the ¤ Pointers. It is a giant ¤ K star of magnitude 1.8 with a
fifth-magnitude companion that orbits around it in 44 years. They are 124
light years away. ˜˜Dubhe™™ is a short version of an Arabic name meaning
˜˜the back of the greater bear.™™
Dumbbell Nebula (M27; NGC 6853) A large ¤ planetary nebula in the constellation
Vulpecula, discovered by Charles Messier in 1764. It lies 1000 light years away
and is a quarter of a degree across on the sky.
dust grains Small particles of matter, typically around 10“100 nm in diameter,
which co-exist with atoms and molecules of gas in interstellar space.
Interstellar dust grains are thought to be made mainly of silicates and/or
carbon in the form of graphite. They form in the extended atmospheres of
¤ red giant stars. Dark clouds of dust show up when they obscure the light from
stars and luminous gas clouds behind them. Though tenuous, dusty clouds
absorb visible light very easily, but radiation at millimeter and longer

dwarf spheroidal galaxy

wavelengths can pass through them unimpeded. The presence of dust is also
revealed by infrared radiation, which the grains give off after they have
absorbed energy in the form of visible and ultraviolet radiation. The
temperature of interstellar dust is typically in the range 30“500 K.
Dust grains are thought to play an important role in the formation of
¤ interstellar molecules because gas atoms that cannot easily combine when
moving freely in space can combine on the surfaces of grains. The molecules
that form in this way can then leave the grains.
Dust clouds are also an important constituent of star-forming regions. The
dust shields interstellar molecules from the destructive effects of high-energy
radiation and helps ¤ protostars to radiate away surplus energy.
dust lane A dark layer of dust visible against a bright background of stars or
glowing gas. ¤ dust grains.
dust tail (type II tail) One of the two principal types of tail ¤ comets develop as they
approach the Sun. The dust tail is composed of particles about 1 mm in size
that shine by reflecting sunlight. Dust tails can be as much as 10 million km
(6 million miles) long. They curve away from the Sun under the influence
of pressure exerted by the Sun™s radiation.
dust trail A stream of interplanetary dust orbiting the Sun in the wake of a comet.
Dust trails are made up of particles released from the surface of comets when
warmth from the Sun evaporates the icy surface. After several revolution
around the Sun, a dust trail tends to disperse, forming a continuous and more
uniform stream of dust, often called a ¤ meteor stream. Dust streams and trails
are responsible for ¤ meteor showers.
dwarf planet A body in the solar system, traveling in orbit around the Sun and
with enough mass to have a more-or-less spherical shape due its own gravity,
but not massive enough to dominate the vicinity of its orbit to the extent that
its influence has cleared away all or most of the smaller bodies there. The
¤ International Astronomical Union adopted this definition in 2006 following the
discovery in 2003 of ¤ Eris, a ¤ Kuiper Belt object larger than ¤ Pluto. Eris and
Pluto, being located in the Kuiper Belt, fall into this category as does the largest
asteroid, ¤ Ceres.
dwarf nova A type of cataclysmic variable star that suddenly brightens at intervals
ranging from several days to a year. Dwarf novae are binary systems consisting
of an ordinary star and a ¤ white dwarf. Material from its companion streams
towards the white dwarf, building up an ¤ accretion disk around it. Outbursts
happen when hot spots form on the accretion disk. Dwarf novae are also
known as U Geminorum stars.
dwarf spheroidal galaxy (dSph galaxy) A dim, faintly glowing dwarf ¤ elliptical
galaxy. Typically these galaxies are about 1000 light years across, which is less

dwarf star

than one percent of the diameter of the Milky Way Galaxy. Several have been
discovered in the ¤ Local Group.
dwarf star A star that is not especially large. All but the hottest, most massive stars
on the main sequence of the ¤ Hertzsprung“Russell diagram may be described as
dwarf stars. ¤ giant star, stellar evolution, white dwarf.
Dwingeloo Observatory A radio astronomy observatory in the Netherlands,
established in 1956. It is also the headquarters of the Netherlands Foundation
for Research in Astronomy (NFRA). The 25-m (82-foot) Dwingeloo radio
telescope is owned and operated by the NFRA. ¤ Westerbork Observatory.
Dysnomia The moon of ¤ Eris.

Eagle Nebula (M16; NGC6611) A glowing nebula in the constellation Serpens
shaped rather like a ¬‚ying bird with broad wings. The nebula is about 7000
light years away and surrounds a small cluster of brilliant stars that are only
about 2 million years old. Near the center, new stars are still being formed
where columns of cool gas and dust, up to a light year long and known as
˜˜elephant trunks,™™ protrude from a dark molecular cloud.
Earth The third planet from the Sun. Like Mercury, Venus and Mars, Earth is one
of the rocky, smaller planets in the inner solar system known as the terrestrial
planets. It is the only one of the four with a large natural satellite “ the ¤ Moon.
Earth™s diameter is 12 756 km (7927 miles). It revolves around the Sun
in a slightly elliptical orbit at an average distance of 149.6 million km
(92.96 million miles), taking 365.256 days to complete one circuit. Earth is

The Eagle Nebula.


Earth imaged by the Galileo spacecraft
in December 1990 from a distance of
2.1 million km (1.3 million miles).

closest to the Sun on January 3rd or 4th, when the Sun is 147.0 million km
(91.35 million miles) away. Six months later Earth is at its maximum distance
of 152.0 million km (94.45 million miles). Relative to the stars, Earth spins in 23
hours 56 minutes but, because it is simultaneously traveling around the Sun, it
takes 4 minutes longer to turn once relative to the Sun giving us a day of 24
hours. Earth™s rotation axis is tilted so that the equator makes an angle of
23 270 to Earth™s orbit. The yearly cycle of ¤ seasons arises because of this tilt
(and is not linked to the variation in Earth™s distance from the Sun.)
Liquid water covers 71 percent of Earth™s surface. Where there are
continents, Earth™s rocky crust is typically 30 km (20 miles) thick but under the
oceans the crust is considerably thinner À 10 km (6 miles) or less. The crust lies
over the mantle, a thick layer of silicate rock nearly 3000 km (1900 miles) deep.
Many of the major features on Earth™s surface result from the motion of crustal
plates, which slide over the softer mantle below. Earthquakes and volcanoes
occur particularly in areas along the boundaries between plates. Folded
mountains rise up where plates collide. All this geological activity means that
the surface of Earth is constantly changing and much of it is young,
geologically speaking.
The ocean ¬‚oors are some of the most recent rocks to have formed. On the
oldest parts of Earth™s surface, some traces remain of impact craters formed in
the early days of the solar system but they have mostly been worn away by
The core of Earth is predominantly iron and nickel. The very center is a
solid ball about 2800 km across but the outer part of the core is molten. The
temperature in the core rises to over 5000  C (9000  F). Electric currents ¬‚owing
in the molten metal generate Earth™s magnetic ¬eld. The region of space


around Earth where its own ¬eld is the strongest magnetic in¬‚uence is called


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