. 2
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own kind of ¤ Cepheid variable star. Cepheids had been used incorrectly to judge
the distance of the Andromeda Galaxy and Baade showed that it was twice as
far away as previously thought. In the 1950s he worked on identifying radio
sources, including ¤ Cygnus A.
Baade™s Window An area of sky around the globular cluster NGC 6522 in the
constellation Sagittarius, which is particularly rich in stars. The astronomer
Walter ¤ Baade drew attention to it. He realized that very distant stars are
visible in that direction because there is relatively little interstellar material to
hide them.
Baikonur The Russian manned space-¬‚ight center, established by the former
Soviet Union. It is situated north-east of the Aral Sea in Kazakhstan.
Baily™s beads A phenomenon observed just before the Moon completely covers
the Sun at a total solar eclipse and again just as the total phase of the eclipse
ends. The very thin crescent of the Sun breaks up to look like a string of bright
beads because the mountains and valleys on the Moon make its silhouette
uneven rather than a perfect circle. The English astronomer Francis Baily
(1774“1844) drew attention to the phenomenon at the solar eclipse of 1836.
324 Bamberga A large asteroid with a diameter of 252 km (156 miles), discovered
by Johann Palisa in 1892.
Barlow lens An additional lens used in conjunction with a telescope ¤ eyepiece to
produce a higher magni¬cation.
Barnard, Edward Emerson (1857“1923) Barnard was one of the greatest and
most proli¬c observers of his day. He was born in Nashville, Tennessee, and
¬rst developed his skills as an amateur astronomer before getting his ¬rst

Barwell meteorite

An infrared image of Barnard™s Galaxy.

professional job at Vanderbilt University in 1883 where he was also able to
study. He then moved to ¤ Lick Observatory and in 1896 became Professor at
Chicago. There he was able to use the newly built ¤ Yerkes Observatory.
In 1892, he discovered Amalthea, the ¬rst moon of Jupiter to be found
since Galileo discovered the four largest in 1610. His many other discoveries
included 16 comets and ¤ Barnard™s star. He also realized that dark patches
in the Milky Way are not due to the absence of stars but are dark nebulae
hiding the stars behind them and he drew up a catalog of about 200 of them.
Barnard™s Galaxy The galaxy NGC 6822 in Sagittarius, discovered by Edward
¤ Barnard in the 1880s. It is a small ¤ irregular galaxy belonging to the
¤ Local Group and is about 1.5 million light years away.
Barnard™s Loop A faint ring of hot, glowing gas in the constellation Orion. It has
the shape of an ellipse covering 14 by 10 . It is thought to be the result of
the pressure of radiation from the hot stars in the region of Orion™s belt and
sword acting on interstellar material.
Barnard™s Star A ninth-magnitude star in the constellation Ophiuchus that has
the largest known ¤ proper motion of any star. Its rapid motion across the sky
was discovered by Edward ¤ Barnard in 1916. Its position changes by 10.3
arc seconds each year as it moves through space relative to the Sun. It is the
third-nearest star to the Sun at a distance of 5.88 light years. Possible
˜˜wobbles™™ in the motion of Barnard™s star have raised suspicions that it may
have unseen planets but this has never been con¬rmed.
barred spiral galaxy A common type of ¤ spiral galaxy that has a bright central
bar of stars. The spiral arms seem to wind out from the end of the bar.
Barringer Crater ¤ Meteor Crater.
Barwell meteorite A 46-kilogram (101“lb) stony ¤ meteorite that fell near the
village of Barwell, Leicestershire, UK, in 1965. Though it broke up, it is the
largest stony meteorite known to have fallen in the UK.


The barred spiral galaxy NGC 1365, which lies about 60 million light years away in the
Fornax cluster of galaxies.

barycenter The center of mass (balancing point) of a system of bodies moving
under the their mutual gravitational attraction. The barycenter of the
solar system lies about a million km (0.6 million miles) from the center of
the Sun but is constantly moving as the relative positions of the planets
Bayer letters The letters of the Greek alphabet, used in conjunction with
constellation names (Alpha Leonis, for example), as identi¬ers for brighter
stars. Johann Bayer (1572“1625) was responsible for compiling the ¬rst
complete star atlas, called Uranometria, which was published in 1603. In it he
introduced the system of naming the brighter stars in each constellation by
Greek letters, which he allocated approximately according to brightness or,
in some instances, in order of position on the sky. The idea was soon taken up
by others and Bayer™s letters are still in use today.
Becklin“Neugebauer object One of the brightest of all astronomical sources
of infrared radiation. It was discovered by Eric Becklin and Gerry Neugebauer
in 1967 and is located in the ¤ Kleinmann“Low Nebula, within the ¤ Orion Nebula.
It is thought to be a massive ¤ B star, hidden behind so much dust that
very little visible light gets through. There are other infrared sources nearby
in this star-formation region.
Beehive A popular English name for the open star cluster ¤ Praesepe.


Belinda One of the small satellites of Uranus discovered during the
¤ Voyager 2 encounter with the planet in 1986. Its diameter is about 80 km
(50 miles).
Bellatrix (Gamma Orionis) A giant ¤ B star of magnitude 1.6 in the constellation
Orion. Its name comes from Latin and means ˜˜female warrior.™™ Bellatrix is
240 light years away.
Belt of Orion The line made by the three second-magnitude stars ¤ Mintaka,
¤ Alnilam and ¤ Alnitak in Orion, regarded as the mythological ¬gure™s belt.
Bennett, Comet A spectacular comet discovered by Jack C. Bennett from South
Africa on December 28, 1969. It reached magnitude zero in March 1970 and
had a tail 30 long. Observations made from space revealed a vast hydrogen
cloud surrounding the head and tail measuring 13 million km in the direction
parallel to the tail.
BepiColombo A spacecraft that the European Space Agency proposes to place in
a polar orbit around Mercury in order to map and survey the planet with a
variety of remote sensing instruments. The projected launch date is 2012.
The mission is named in honor of the Italian mathematician and engineer
Giuseppe Colombo (1920“84) who suggested the trajectory ¤ Mariner 10 should
follow past Mercury in 1974“75.
BeppoSAX An Italian/Dutch gamma- and X-ray satellite launched on April 30,
1996. Observations it made in 1997 led to the ¬rst optical identi¬cation of a
¤ gamma-ray burst. It operated until 2003.
Bessel, Friedrich Wilhelm (1784“1846) Bessel was both a mathematician and an
observational astronomer. Born in Minden in what is now Germany, he was
appointed Director of the Konigsberg Observatory at the age of only 26. One of
his major projects was to catalog accurate positions for many thousands of
stars. He is most remembered for being the ¬rst to announce the distance to
a star as a result of measuring its ¤ parallax. He gave the distance of 61 Cygni as
10.3 light years, which is very close to the modern value of 11.4 light years.
Bessell also noted that the positions of ¤ Sirius and ¤ Procyon seemed to deviate
back and forth slightly. He correctly deduced that both stars are orbiting
dim companions that he could not see.
Beta Pictoris A fourth-magnitude ¤ A star in the constellation Pictor, surrounded
by a disk of material that may be a planetary system in the process of forming.
Strong infrared radiation from the disk drew astronomers™ attention to it.
The disk is about 10 times the size of Pluto™s orbit around the Sun.
Betelgeuse (Betelgeux; Alpha Orionis) A massive, red ¤ supergiant star in the
constellation Orion. With a diameter more than 1000 times the Sun™s it is
one of the largest stars known and one of the few to be detectable as a disk
rather than just a point of light. It has bright ˜˜star spots™™ and is surrounded by
a shell of dust and an extended ¤ chromosphere. The brightness of Betelgeuse


The disk around Beta Pictoris imaged by the Hubble Space Telescope. (The black disk
at the center is to cut out the glare of the star itself. The colored effects are an artifact
from the way the observation was made through ¬lters.)

varies irregularly between magnitudes 0.4 and 0.9 with a rough period of
around 5 years. Its distance is approximately 425 light years.
Bianca One of the small moons of Uranus discovered when ¤ Voyager 2 ¬‚ew past
in 1986. It is 51 km (32 miles) across.
Biela, Comet A nineteenth century comet, famous because it split in two before
disappearing completely. The comet was originally discovered in 1772 by
Montaigne of Limoges. When it was recovered by Wilhelm von Biela in 1826, the
orbit was calculated accurately enough to identify the two previous occasions
when its appearance had been recoded. Its period was 6.6 years. At its 1846
return it was double. By 1852, the two components were separated by more than
2 million km though following the same orbit. Neither was ever seen again.
A November meteor shower, the ¤ Andromedids, is associated with Comet
Biela. Brilliant displays have occasionally been seen both before and after the
break-up of the comet.
Big Bang An explanation of the history of the universe that says it began in an
in¬nitely compact state and has been expanding ever since. This theory is
widely accepted because it explains both the ¤ expanding universe and the
existence of ¤ cosmic background radiation. According to current estimates, the
Big Bang took place about 13.7 billion years ago.
Big Bear Solar Observatory A solar observatory located at an altitude of 2000 m
(6600 feet) on an island in Big Bear Lake in California. The site was chosen
because the air is steadier over water than over land. There are four main
telescopes on the same mount. The largest of them is a 65-cm (26-inch) re¬‚ector.
Big Crunch A hypothetical end to the universe in which it totally collapses in
on itself. The universe could only suffer this fate if its present expansion were
to slow down and be reversed. On current evidence, a Big Crunch is very
unlikely because the expansion of the universe appears to be speeding up.
Big Dipper (˜˜Plough™™ in Europe) The pattern formed by the seven stars Alpha,
Beta, Gamma, Delta, Epsilon, Zeta and Eta in the constellation ¤ Ursa Major.
binary star A pair of stars in orbit around each other, held together by the
gravitational attraction between them. About half of all stars have at least one
companion, though many are so close that they cannot be separated even

bipolar outflow

by a powerful telescope. In these cases, the evidence for more than one star
comes from their combined spectrum.
The members of a binary system each move in an elliptical orbit around
their center of mass. The further apart they are, the slower they move. Pairs far
enough apart for the two stars to be distinguished, or ˜˜split,™™ in a telescope often
have orbital periods as long as 50 or 100 years. They are called visual binaries.
If one star is much fainter than the other, its presence may be revealed
only by the obvious orbital motion of its brighter companion. Pairs of this type
are called astrometric binaries.
As the members of a binary system move around their orbits, they move
alternately towards and away from the Earth. Their spectra can reveal details
about both the nature of the stars and their orbits. Binary stars recognized only
by their spectra are called spectroscopic binaries. Their periods are usually
between a day and a few weeks.
Some binaries are so close that the pull of gravity distorts the individual stars
from their normal spherical shape. They are known as ¤ contact binaries. The
energy released results in the emission of X-rays. ¤ Novae are another consequence
of material being transferred between partners in certain binary stars.
In an eclipsing binary, the orbits of the stars are oriented so that one of
them crosses in front of the other as seen from the Earth. Eclipsing binaries are
also variables since one star periodically blots out light from the other.
binoculars An optical instrument consisting of two small telescopes, mounted
side by side, one for each eye. The tubes are kept short and manageable by
using prisms to re¬‚ect the light internally. The prisms also makes the image
upright, rather than inverted as it is in an astronomical telescope. The size and
magnifying power of binoculars is usually given in the form A · B, where A is
the linear magni¬cation and B is the diameter of each objective lens in
millimeters (e.g. 10 · 40).
bipolar nebula A luminous nebula consisting of two lobes pointing in opposite
directions. Any nebula of this shape may be called ˜˜bipolar,™™ but this term is
most often used for a group of nebulae that are intense sources of infrared
radiation. They are thought to harbor a bright star that is completely concealed
by a dense ring of dust and gas around its equator. The dust emits in the
infrared because it is heated to a temperature of a few hundred degrees by
the radiation from the star. The visible starlight is funneled along the star™s
poles and illuminates the more tenuous part of the nebula around the star.
¤ bipolar out¬‚ow.
bipolar outflow Gas streaming outwards in two opposing directions from a newly
formed star. Because the star is surrounded by an ¤ accretion disk, the gas
cannot escape around the equator and is forced to ¬‚ow out from over the
poles. This stellar wind sweeps up interstellar material before it and creates the

black body

The bipolar outflow, nicknamed the
Boomerang Nebula, coming from an
old red giant star. Each lobe is nearly
one light year long.

two lobes, which extend for a distance of about a light year. Bipolar out¬‚ows
have been detected by the radio emission from the molecules they contain.
black body An object that absorbs all the radiation falling on it.
black body radiation The characteristic radiation emitted by a ¤ black body. How the
radiation a black body emits varies with wavelength is affected only by its
temperature and can be predicted by quantum theory. The graph of radiation
intensity against wavelength is called the Planck curve after the physicist Max
Planck, who predicted its shape theoretically. Planck curves are hill-shaped and
peak at shorter wavelengths for hotter bodies. The total amount of energy emitted
by a black body goes up steeply with temperature, as the fourth power (T 4).
black drop An effect observed during a ¤ transit of Venus or Mercury across the
Sun, when the small dark disk of the planet is very near the limb of the
Sun. When the limbs of the Sun and planet are not quite in contact, a small
black spot, or drop, appears to join them.
black dwarf A dead star that has stopped shining. A star with less than about 1.4
times the Sun™s mass spends the last stage of its life as a ¤ white dwarf. For a
long time, white dwarfs keep shining because they are very hot to begin with
but, since they cannot generate any new energy, they will gradually cool
and fade into a dark stellar ˜˜corpse™™ or black dwarf. The universe is not yet old
enough for any black dwarfs to have formed.
Black-eye Galaxy (M64; NGC 4826) A popular name for an unusual looking
¤ spiral galaxy in the constellation Coma Berenices that has very smooth spiral
arms and a prominent dust cloud around its nucleus. It is about 65 000 light
years in diameter.
black hole A region of space where the gravitational force is so strong that not
even light can escape from it. Black holes are formed when matter collapses
in on itself catastrophically, concentrating more than a critical quantity of
mass into a particularly small region. Theory suggests that ˜˜mini™™ primordial

blink comparator

The Black-eye Galaxy.

black holes might have formed from large density fluctuations in the
conditions prevailing in the early universe.
Stellar black holes are thought to form when massive stars explode, if the
central relic is more than three solar masses, or is tipped over that mass when
material cascades back onto it. To create a black hole, several solar masses of
material would have to be packed into a diameter of just a few kilometers.
Matter falling into supermassive black holes is how most astronomers explain
the exceptional power of ¤ active galactic nuclei and ¤ quasars. Direct observations of
compact nuclei in galaxies, and motion of gas and stars near the centers of
galaxies, appear to con¬rm that massive black holes do indeed exist at the centers
of many galaxies. Typically, they have a billion times the Sun™s mass.
Black holes cannot be observed directly. Their existence can only be
inferred from their gravitational effects on their surroundings and the
radiation emitted by material falling into them. A number of stellar X-ray
sources, such as ¤ Cygnus X-1, are binary star systems in which one component
appears to be a black hole. The black hole™s orbit and mass can be computed
from observations of its visible companion.
Black Widow pulsar A popular name for the ¤ pulsar PSR 1957 þ 20. It is a
member of a binary system and the action of its intense radiation is gradually
evaporating its small stellar companion.
blazar A ¤ BL Lac object or a ¤ quasar that varies dramatically in brightness and
emits strongly over the whole of the electromagnetic spectrum. Many blazars
are powerful gamma-ray emitters.
Blaze Star A popular name for the recurrent ¤ nova T Coronae Borealis. It is the
brightest recurrent nova ever recorded, having reached second magnitude
in both 1866 and 1946.
blink comparator An instrument for comparing two photographs of a region
of sky, usually a pair taken at different times. It shows up any images that do
not have the same position or brightness on the two photographs. An optical
system makes the two photographs appear to be exactly superimposed but

Blinking Nebula

illuminates them alternately. Any object whose brightness differs appears
to blink on and off; one that is at a different position appears to jump between
the two locations.
Blinking Nebula A popular name for NGC 6826, a ¤ planetary nebula in Cygnus.
Observers using small telescopes say the nebula appears to blink on and
off, leaving only the central star in view, if they switch between looking directly
at it and looking to one side of it.
BL Lac object (BL Lacertae object; Lacertid) A type of ¤ elliptical galaxy with a
bright, highly variable, compact nucleus. The first to be identified was BL
Lacertae, which was thought to be a variable star when it was first spotted
in 1929. Though it is now known to be a galaxy, it has kept its original name
of the kind given to variable stars. BL Lac objects have dramatic short-term
variations and a featureless spectrum. Their brightness can change by as much
as a hundredfold over a period of a month, and day-to-day changes are
sometimes observed. Many are also radio sources; BL Lacertae itself gives out
intense radio bursts.
blue moon The origin of this expression, often used just to mean ˜˜a rare event,™™
is not known. A suggestion that it refers to the second occurrence of a new
Moon in one calendar month appears to be unfounded. An alternative
explanation is that atmospheric effects may occasionally make the Moon
appear blue; a possible cause would be dust in the upper atmosphere from
volcanoes or forest fires.
blueshift A shift of a spectrum towards shorter wavelengths. Blueshifts are
caused by the ¤ Doppler effect when the source of radiation and its observer are
moving towards each other.
Blue Planetary A popular name for NGC 3918, a ¤ planetary nebula in Centaurus,
which to visual observers looks like a blue featureless disk.
Blue Snowball A popular name for the ¤ planetary nebula NGC 7662 in the
constellation Andromeda.
blue straggler A star that appears to belong to a ¤ globular cluster or an old ¤ open
cluster but is excessively blue and bright compared with other cluster members.
The explanation for the anomalous properties of blue stragglers is not known
for certain. They might be ¤ binary stars where mass has been transferred
from one star to the other, or the result of two very close stars merging.
Bode™s law ¤ Titius“Bode law.
bolide A particularly bright ¤ meteor accompanied by an explosive sound or sonic
bolometric magnitude The ¤ magnitude of a celestial object, taking into account
all the energy it radiates at all wavelengths. The bolometric magnitude of
an object that emits strongly in the ultraviolet or infrared, for example, differs
greatly from its visual magnitude.

Bradley, James (1693“1762)

16h 15h 14h 13h
κ +50º


• ν1 +40º
β Nekkar

µ Seginus




ξ Muphrid

π …



15h 14h WIL TIRION

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

Open clusters Globular clusters Planetary nebulae Bright nebulae Galaxies

The constellation Bootes.

Bootes (The Herdsman) A constellation of the northern sky, dominated by the
bright orange star ¤ Arcturus. It is usually said to represent a herdsman driving
the bear, which is the neighboring constellation Ursa Major.
Borrelly, Comet A periodic comet that was visited by the ¤ Deep Space 1 spacecraft
in September 2001. Comet Borrelly was discovered by Alphonse L. N. Borrelly
from Marseilles, France on December 28, 1904. Deep Space 1 discovered
that its nucleus is about 8 km (5 miles) long by 4 km (2.5 miles) wide and very
dark. Its orbital period is 6.8 years.
Bradley, James (1693“1762) The British astronomer James Bradley made the
important discovery of the ¤ aberration of starlight, which he published
in 1729. It was the first direct evidence that Earth is in motion around the Sun

Brahe, Tycho (1546“1601)

Tycho Brahe.

and Bradley used his measurements to calculate the speed of light to within 2%
of its correct value.
Bradley was introduced to astronomy by his uncle and studied at Oxford
University. He became Professor of Astronomy at Oxford in 1721 and was
appointed ¤ Astronomer Royal in 1742. He discovered the phenomenon of
aberration while trying to measure stellar ¤ parallax. As a result of his accurate
observations over many years he also discovered the ¤ nutation of Earth™s axis,
which he announced in 1748.
Brahe, Tycho (1546“1601) Tycho Brahe made the most accurate observations
of the positions of stars and planets before the invention of the telescope. He

brown dwarf

was born into a noble Danish family and started to study astronomy after seeing
a partial eclipse of the Sun in 1560. He realized the importance of making
careful systematic observations of celestial bodies. When a ˜˜new star™™ blazed
out in the constellation Cassiopeia in 1572 “ what we now know to have been a
¤ supernova “ Tycho recognized that it was something astronomical and not a
phenomenon in the atmosphere and the short book he wrote about it made his
name known.
The King of Denmark and Norway was impressed by Tycho and funded
magni¬cently equipped observatories for him on the island of Hven. He
worked there from 1577 but following a disagreement with a new king, left in
1596. By 1599, he had settled in Prague, where he found a new royal sponsor.
In Prague, ¤ Kepler became his assistant and eventually inherited Tycho™s
observations, which he used to formulate his laws of planetary motion. Tycho
himself was never persuaded that the Sun was the center of the planetary
system. He thought the Earth could not be moving because he could not detect
¤ parallax in the positions of the stars he measured. (His instruments were
not accurate enough to do so.) He proposed that the Sun and Moon revolved
around Earth, while all the other planets orbited the Sun.
9969 Braille An asteroid that was imaged from a distance of 26 km (16 miles) by
the NASA spacecraft ¤ Deep Space 1 in 1999. It is an irregular, elongated body
about 2.2 · 1.0 km (1.4 · 0.6 miles). The infrared spectrum of Braille proved
to be nearly identical to that of ¤ Vesta, prompting speculation that Braille
is a fragment broken off Vesta by an impact.
brown dwarf A gaseous body intermediate between a cool star and a giant planet.
The mass of a brown dwarf is between 13 and 80 times the mass of Jupiter,
which is too little for hydrogen-burning nuclear reactions to be sustained in its
core. As it forms, a brown dwarf heats up because gravitational energy is

The size of brown dwarfs compared with Jupiter (right), the Sun (left) and a cool red star
(to the right of the Sun).

B star

released. Some nuclear processes may take place for a short time but, after
peaking, the temperature steadily falls. Hundreds of brown dwarfs are now
known to exist in the Sun™s neighborhood of the Galaxy.
The system for classifying very cool stars and brown dwarfs builds on the
long-standing system of ¤ spectral types. The new types introduced following
the discovery of brown dwarfs are L and T.
Cool dwarf stars and the younger, warmer brown dwarfs look similar
although their masses are different, and it is dif¬cult to distinguish between
them on the basis of their spectra alone. They both fall into types M and L. The
M-type objects, with surface temperatures ranging down to 2100 K, have water
and strong oxide features in their spectra.
The next cooler group, with temperatures of roughly 1500“2100 K are the L
dwarfs (L0 to L8). Though the coolest, least massive stars fall into this range,
their temperatures cannot be lower than 1800 K. Objects with temperatures
between 1500 and 1800 K must be brown dwarfs.
The coolest objects so far detected are the methane brown dwarfs, so-
called because their spectra show strong absorption by methane as well as
water. These brown dwarfs, with surface temperatures ranging down from
about 1000 K to 800 K, are allocated to type T.
B star A star of ¤ spectral type B. The surface temperatures of B stars are in the
range 11 000“25 000 K and they look bluish white. The most prominent
features in their spectra are ¤ absorption lines of neutral helium. Hydrogen lines
are also present and are stronger the cooler the star. Examples of B stars are
¤ Rigel and ¤ Spica.

A Hubble Space Telescope image of the Bubble Nebula.

butterfly diagram

Bubble Nebula A popular name for a faint glowing nebula (NGC 7635) in the
constellation Cassiopeia. The massive central star of the nebula is blowing off a
fast ¤ stellar wind. The ˜˜bubble™™ marks where the stellar wind encounters
the surrounding interstellar medium.
338 Budrosa An asteroid of the rare metallic type discovered in 1892 by Auguste
Charlois. It is 63 km (39 miles) across and is the prototype of the Budrosa
family of unusual asteroids, with six known members at 2.9 AU from the Sun.
Bug Nebula A name given to the ¤ bipolar nebula NGC 6302 in the constellation
Scorpius. The star assumed to be at the center is not visible, but gas can be seen
streaming out of the hot central region with speeds up to 400 km/s (over
2000 mph).
butterfly diagram The popular name for a diagram illustrating how the latitudes
on the Sun at which ¤ sunspots appear vary through the ¤ solar cycle. It was first
plotted in 1922 by E. Walter Maunder and its more formal name is the
Maunder diagram. The diagram is a graph with solar latitude as the vertical
axis and time (in years) as the horizontal axis. A short vertical line covering one
degree in latitude is plotted for each sunspot group centered at that latitude
within one rotation of the Sun. The result is a pattern reminiscent of pairs of
butterfly wings.

A Hubble Space Telescope image of the Bug Nebula.

butterfly diagram

A butterfly diagram of the 11-year solar cycle.

Caelum (The Chisel) A small constellation with no star brighter than fourth
magnitude. It was introduced into the southern sky in the mid-eighteenth
century by Nicolas L. de Lacaille, who called it ˜˜Caela Sculptoris.™™
caldera A large volcanic crater.
Caliban One of two small moons of Uranus discovered in 1997 by Brett Gladman
and others, using the ¤ Hale Telescope. It is reddish in color and thought to be a
captured ¤ Kuiper belt object. Its diameter is estimated to be 98 km (61 miles).
California Nebula (NGC 1499) A bright ¤ emission nebula in the constellation
Perseus, named for it resemblance to the shape of the US state. It forms the rim
of a dark nebula of gas and dust illuminated by the star Xi Persei.
Callirrhoe A small outer moon of Jupiter discovered in 1999. Its diameter is about
9 km (6 miles).
Callisto The second largest moon of Jupiter and one of the four discovered in
1610 by Galileo. With a diameter of 4821 km (2996 miles), it is also the third
largest moon in the solar system. Callisto is the darkest of the Galilean
satellites and also the least dense, which suggests that it contains a high
proportion of water, though detailed images returned by the ¤ Galileo
spacecraft indicate that the surface has more rock and dust than previously
supposed. Under its thick icy crust, Callisto may have a liquid ocean several
kilometers deep. ¤ Voyager and Galileo images show a heavily cratered
surface with no high hills or mountains. A multi-ringed impact basin, called
Valhalla, is the most prominent feature. It has a bright central zone 600 km
(375 miles) across, surrounded by numerous rings spaced between 20 and
100 km (12 and 60 miles) apart. There are at least seven other multi-ring
features on Callisto.
Caloris Basin (Caloris Planitia) A large, multi-ringed, impact basin on Mercury. It is
1300 km (800 miles) in diameter and the most conspicuous feature on the planet.
Calypso A small satellite of Saturn discovered in 1980. It measures 30 · 16 · 16 km
(19 · 10 · 10 miles) across and it shares the same orbit as ¤ Tethys and ¤ Telesto
at a distance of 294 660 km (183 093 miles) from Saturn.
Camelopardalis (alternatively Camelopardus; The Giraffe) A large but not very
conspicuous constellation near the north celestial pole. It was ¬rst mentioned
in 1624 by the German mathematician Jakob Bartsch, who was a son-in-law of
Johannes ¤ Kepler.

107 Camilla

A close-up of Callisto showing ancient multi-ring impact basins.

A Cassini image of Saturn™s small moon

107 Camilla An asteroid estimated to be 240 km (149 miles) across, discovered
from Madras, India, by Norman R. Pogson in 1868. In 2001, astronomers
observing Camilla with the Hubble Space Telescope reported that it has a small

Cannon, Annie Jump (1863“1941)

Canada“France“Hawaii Telescope (CFHT) A 3.6-m (140-inch) telescope at the
¤ Mauna Kea Observatories in Hawaii. It was commissioned in 1979 and is used
for both optical and infrared observations.
canals Imagined linear features ¤ Percival Lowell and others claimed to see on Mars.
In the nineteenth century, the Italian astronomers Angelo Secchi and
Giovanni Schiaparelli used the word canale, meaning ˜˜channel,™™ to describe
linear features they thought they could see on Mars. The word was translated
into English as ˜˜canal.™™ Percival Lowell was excited by these reports and built
an observatory at Flagstaff, Arizona, with the main purpose of observing Mars.
His drawings of the planet showed extensive networks of straight ˜˜canals,™™
and he proposed that a civilization of intelligent beings on Mars was
responsible for constructing them. Later observers found little evidence for
Lowell™s canals, and spacecraft images of Mars show no trace of them. The
linear features that Lowell and others genuinely believed they had observed
can probably be explained as optical effects.
Canary Islands Large Telescope ¤ Gran Telescopio Canarias.
Cancer (The Crab) A constellation in the traditional ¤ zodiac. It is said to represent
the crab Hercules crushed under his foot when he was ¬ghting the Hydra.
None of its stars is brighter than fourth magnitude. The star cluster ¤ Praesepe
at the center of Cancer can be seen with the naked eye.
Canes Venatici (The Hunting Dogs) A small constellation in the northern sky,
lying between Bootes and Ursa Major. It was introduced by ¤ Johannes Hevelius
in the late seventeenth century and is supposed to represent the dogs Asterion
and Chara held on a leash by Bootes. Though small, Canes Venatici contains
several interesting objects including the bright star ¤ Cor Caroli, the ¬ne
¤ globular cluster M3 and the ¤ Whirlpool Galaxy.
Canis Major (The Greater Dog) A small constellation, just south of the celestial
equator and next to Orion, containing the brightest star in the sky, ¤ Sirius. It is
said to represent one of the dogs following the hunter, Orion.
Canis Minor (The Lesser Dog) A small constellation near to ¤ Orion. It is supposed
to represent the smaller of two dogs following the hunter Orion. Canis Minor
contains only two stars brighter than third magnitude. The brightest is
¤ Procyon.
Cannon, Annie Jump (1863“1941) The American astronomer Annie Jump
Cannon is remembered for her phenomenal skill at classifying the spectra of
stars. While working at ¤ Harvard College Observatory from 1896, she catalogued
the ¤ spectral types of about 400 000 stars. She was born in Delaware, the
daughter of a state senator, and studied physics and astronomy at Wellesley
College and Radcliffe College. As a young woman, she became almost totally
deaf after contracting scarlet fever. Cannon was responsible for reorganizing
the way stellar spectra are classi¬ed into the system still used today. Her


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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 Canis Major.

classi¬cations of over 225 000 stars were published in several volumes, called
the ¤ Henry Draper Catalog. She received a number of honors, including a medal
from the National Academy of Sciences in the US and an honorary doctorate
from Oxford University in the UK.
Canopus (Alpha Carinae) The brightest star in the constellation Carina and the
second brightest star in the sky. Canopus is a supergiant ¤ F star of magnitude
“0.7. It is 313 light years away. In Greek mythology, Canopus was the pilot of
the ¬‚eet of King Menelaos.
Cape Canaveral The location in Florida, USA, of the Kennedy Space Center and of
a US Air Force base from where many of NASA™s space missions are launched.
Capella (Alpha Aurigae) The brightest star in the constellation Auriga, magnitude
0.1. It is a ¤ spectroscopic binary 42 lights away. Both members of the binary
system are giant ¤ G stars. Capella means ˜˜little she-goat™™ in Latin.
Cape York Meteorite A large iron ¤ meteorite broken into three pieces that were
found by Robert Peary in Greenland in 1894. Three years later he transported
them to New York and they are now at the American Museum of Natural
History in New York City. The heaviest piece, weighing 31 tonnes, was
nick-named ˜˜Ahnighito,™™ meaning ˜˜the tent,™™ by the local people in
Greenland. It is the largest meteorite on view in any museum.

carbon star

Capricornus (The Sea Goat) One of the traditional constellations of the ¤ zodiac.
Its brightest stars are third magnitude.
carbonaceous chondrite A rare type of stony ¤ meteorite. Because their average
chemical composition (except for hydrogen and helium) is very similar to
the Sun™s, carbonaceous chondrites are thought to consist of primitive,
unprocessed material dating from the time when the solar system formed.
They are made up of carbon-rich minerals and have ¤ chondrules embedded in
them. Their water content can be as high as 20 percent. The largest known
example is the ¤ Allende meteorite.
carbon cycle (carbon“nitrogen (CN) cycle; carbon“nitrogen“oxygen (CNO) cycle;
Bethe“Weizsacker cycle) A series of nuclear reactions that takes place inside
stars. The outcome of the carbon cycle is that hydrogen is converted to helium
and large amounts of energy are released.
carbon star A peculiar, red giant star with unusually strong features in its
spectrum caused by C2, CN, CH or other carbon compounds. Carbon stars have
temperatures similar to the more common ¤ K stars and ¤ M stars but
contain more carbon and oxygen. Though carbon stars are rare in our own
Galaxy, many thousands have been discovered in the Large and Small
¤ Magellanic Clouds.
The spectra of some carbon stars reveal the presence of the radioactive
element technetium. Its longest-lived isotope has a half-life of only 210 000

10h 9h 8h 7h

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Eta Carinae
0º Southern
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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 Carina.


years, a short period on astronomical timescales. A few of the coolest carbon
stars have in their spectra an extremely strong line of lithium, a chemical
element that is easily destroyed by the nuclear processes in stars. Technetium
and lithium in the outer layers of carbon stars show that material is being
dredged up from deep inside them by the process of ¤ convection. All the
evidence is consistent with carbon stars being in an advanced state of evolution.
Carina (The Keel) A large constellation in the southern Milky Way, formerly part
of ¤ Argo Navis. It contains the second-brightest star in the sky, ¤ Canopus.
Carina Nebula (NGC 3372) A large cloud of glowing hydrogen gas in the southern
Milky Way in the constellation Carina. The star ¤ Eta Carinae is embedded
in it, near its center. The nebula is 3 across on the sky. It actual diameter is

The Carina

Carrington rotation number

400 light years and it lies at a distance of 8000 light years. It is a region of star
formation containing a number of young star clusters. ¤ Keyhole Nebula.
Carme A small outer moon of Jupiter, discovered in 1938 by Seth B. Nicholson. Its
diameter is 46 km (29 miles).
Carpo A small outer moon of Jupiter discovered in 2003. Its diameter is about 3 km
(2 miles).
Carrington rotation number A number that uniquely identifies each rotation
of the Sun. The sequence began with rotation number one on November 9,
1853. The system was started by Richard C. Carrington, based on the average
rotation rate of sunspots, which he had determined. In reality, the Sun does

An image of the Cartwheel Galaxy formed by combining X-ray data from the Chandra X-ray
Observatory, ultraviolet light from the Galaxy Evolution Explorer satellite, a visible image by
the Hubble Space Telescope and an infrared view from the Spitzer Space Telescope.

Carte du Ciel

not rotate as if it were a solid body, its rotation rate varying with latitude. For the
purpose of counting the rotation numbers, the period is taken as 25.38 days
relative to the stars, which is equivalent to 27.28 days as viewed from Earth.
Carte du Ciel An ambitious project, begun in 1887, to make photographic charts
of the entire sky and compile a star catalog. In the event, the charts were never
completed because the standard methods laid down for the project were
overtaken by technical advances in astrophotography. However, the catalog
was finally completed and published in 1964.
Cartwheel Galaxy A ¤ peculiar galaxy, 500 million light years away, more formally
known as A0035. It consists of a circular rim, 170 000 light years in diameter,
inside which are a hub and spokes made up of old red stars. It is believed that
the galaxy was once a large, ordinary ¤ spiral galaxy. A few hundred million
years ago, a smaller galaxy passed right through it. That intruder is still nearby.
The shock of the collision caused large numbers of massive stars to form in the
˜˜rim.™™ When these stars reach the end of their relatively short lives, they
explode as ¤ supernovae. As a result, the rate of supernova explosions in the
Cartwheel Galaxy is now about a hundred times greater than in a normal
Cassegrain telescope A reflecting telescope in which the image is focused just
behind a central hole in the primary mirror. The design was proposed in about
1672 by Jacques Cassegrain (1652“1712), professor of physics at Chartres in
France, some four years after Isaac ¤ Newton constructed the first reflecting
telescope. Its secondary mirror is convex rather than flat as in Newton™s design.
Cassegrain did not build a telescope himself and it was some years before his
idea was put into practice. Today, the Cassegrain focus is popular and widely
used in both modest amateur instruments and large professional telescopes.
Cassini The orbiting spacecraft of the ¤ Cassini“Huygens mission to Saturn.
Cassini Division A conspicuous dark gap between the A and B rings of ¤ Saturn. It
is 2600 km (1616 miles) wide.
Cassini, Giovanni Domenico (Jean-Dominique) (1625“1712) Cassini was born
near Genoa in Italy and became a professor of astronomy in Bologna but in
1669 King Louis IV of France persuaded him to move to Paris to take charge of
the newly established ¤ Paris Observatory. He became a French citizen in 1673
and adopted the French form of his first names. His astronomical work was
concerned with the solar system and from his observations calculated that the
distance from Earth to the Sun was about 140 million km (87 million miles).
Though still 7 percent too small, his figure was much closer to the true value
than previous estimates. He discovered four of Saturn™s moons “ Iapetus, Rhea,
Dione and Tethys “ and the gap in Saturn™s rings called the Cassini division.
Both Cassini™s son, Jacques, and his grandson, Cesar Franc ¸ois,
subsequently became directors of the Paris Observatory.

Cassiopeia A

The Cassini“Huygens mission. An artist™s impression of Cassini releasing the Huygens probe.

Cassini“Huygens mission A joint NASA/ESA mission to explore Saturn, its rings,
its magnetosphere and several of its moons. The spacecraft was launched in
October 1997 and used ¤ gravity assist flybys of Venus (in April 1998 and June
1999), of Earth (in August 1999) and of Jupiter (in December 2000). It finally
arrived at its destination in July 2004 and was placed in orbit around Saturn.
The nominal length of its mission is four years.
One of the major objectives of Cassini“Huygens was a study of Saturn™s
moon ¤ Titan. Cassini carried the Huygens probe, an instrument package that
successfully parachuted down through Titan™s atmosphere in January 2005 and
landed on the surface. The Huygens probe was the principal ESA contribution
to the mission.
Cassiopeia A conspicuous W-shaped constellation near the north celestial pole. It
is said to represent the seated figure of Queen Cassiopeia, a character from
Greek mythology. Cassiopeia was the wife of ¤ Cepheus and the mother of
¤ Andromeda, both represented by their own constellations. Tycho ¤ Brahe
observed a ¤ supernova in Cassiopeia in 1572. This constellation also contains
the strongest radio source in the sky, known as ¤ Cassiopeia A.
Cassiopeia A The strongest radio source in the sky (other than the Sun). It is the
remnant of a ¤ supernova that must have occurred around ad 1667 though no
records exist of anyone seeing a supernova around that time. The light from

Cassiopeia A




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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 Cassiopeia.

An image of the supernova remnant
Cassiopeia A formed from infrared
data from the Spitzer Space Telescope
(colored red), optical data from the
Hubble Space Telescope (yellow) and
X-ray data from the Chandra X-ray
Observatory (green and blue).

the explosion of the star, which took place 10 000 light years away, was
obscured by the large quantities of dust that lie in the line of sight from Earth.
The radio emission is concentrated in a ring 4 arc minutes across and there is
X-ray emission from the ring too. Optical photographs of that region of the sky
detect a faint nebula.


Castor (Alpha Geminorum) The second-brightest star in the constellation Gemini,
after ¤ Pollux. Its magnitude as seen by the naked eye is 1.6, but this is the
combined brightness of a multiple system with at least six components. There
are two ¤ A stars of magnitudes 2.0 and 2.9 forming a close visual pair, each of
which is a ¤ spectroscopic binary. There is also a more distant ninth-magnitude
red star, which is an ¤ eclipsing binary. Castor lies at a distance of 52 light years.
cataclysmic variable A star that brightens dramatically and suddenly when some
kind of explosive event takes place. The term is applied particularly to ¤ novae,
¤ supernovae and ¤ dwarf novae.
Cat™s Eye Nebula The ¤ planetary nebula NGC 6543. It is about 3000 lights years
away and lies in the constellation Draco. Estimated to be about 1000 years old,
its intricate structure suggests that there is a binary star at its center.
CCD Abbreviation for charge-coupled device, an electronic imaging device widely
used in astronomy.
A CCD is made of semiconducting silicon. Light falling on this material
releases electrons. The number of electrons increases in proportion to the
brightness of the light. To create an image, a CCD collects light over a matrix of
small picture elements (pixels). The electric charge from each pixel is

This Hubble Space Telescope image of the Cat™s Eye Nebula shows at least eleven shells of
gas around the central star, each blown off at a different time.

cD galaxy

registered and then converted into a form that allows the whole image to be
stored in a computer or displayed on a screen.
cD galaxy A type of giant elliptical galaxy that is very bright at the center and has
an extended diffuse halo of stars. cD galaxies are found at the centers of rich
galaxy clusters. Many are radio emitters and they are five to ten times more
luminous than ordinary elliptical galaxies. Their masses are in the range
1013“1014 solar masses and their diameters 1“3 million light years. A
significant amount of their mass is in the form of ¤ dark matter.
CD galaxies probably result from galaxy mergers. NGC 6616 is a typical
example. It appears to have multiple nuclei and to be swallowing smaller
nearby galaxies.
celestial equator The circle on the ¤ celestial sphere between the northern and
southern celestial hemispheres. It is the projection into space of the Earth™s
celestial mechanics The branches of astronomy dealing with the movements and
positions of astronomical objects.
celestial poles The two points on the celestial sphere about which the sky appears
to rotate daily. Earth™s rotation axis points towards them. The north celestial
pole currently lies close to the star Polaris and the south pole is in the
constellation Octans, unmarked by any bright star. Because of the effects of
¤ precession, the positions of the poles are not stationary but sweep out circles
with radii of about 23 over a period of 25 800 years.
celestial sphere The sky regarded as the inside of a distant hollow sphere. Though
astronomical objects such as stars and planets really move through three-
dimensional space, the idea of the celestial sphere is very useful for describing
their positions and motion as seen from a particular location, such as Earth™s
surface. Every observer is located at the center of their own celestial sphere. Half
the sky is always hidden from an observer on Earth™s surface; the visible half
varies according to the observer™s latitude and with the date and time.
Measurements on the celestial sphere are angles given in degrees, or minutes
and seconds of arc, and take no account of how far away the objects actually are.
Centaurs A class of asteroids with orbits in the outer part of the solar system.
Their orbits are within Neptune™s but they approach no closer to the Sun than
Jupiter. They are easily perturbed when they pass close to one of the giant
planets. Two well-known examples are ¤ Chiron and ¤ Pholus.
Centaurus (The Centaur) A large southern constellation. The Milky Way runs
through it so it is very rich in stars. Centaurus contains the nearest star to the
solar system, ¤ Proxima Centauri, and the finest and brightest of all globular star
clusters, ¤ Omega Centauri.
Centaurus A A ¤ radio galaxy, identified as the elliptical galaxy NGC 5128. At a
distance of 15 million light years, it is the nearest radio galaxy and

Cepheid variable

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Open clusters


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Planetary nebulae

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A map of the constellation Centaurus.

consequently one of the most studied. The visible galaxy is crossed by a thick
dark lane of dust. The radio-emitting lobes are at right angles to the dust lane.
They extend across 7 of sky, equivalent to almost two million light years. The
galaxy is also a strong source of X-rays.
central meridian (CM) The imaginary north“south line bisecting the disk of an
astronomical body as it is viewed by an observer.
Cepheid variable A type of pulsating ¤ variable star, named after the group™s
prototype, Delta Cephei, which varies between magnitudes 3.6 and 4.3 in a
period of 5.37 days. Cepheid variables pulse in and out because their structure
is unstable. Their size may change by as much as 10 percent and their
temperature varies too. Pressure builds up inside the star and it expands until
the pressure is released, rather like the action of a safety valve. The star then
contracts and the cycle starts again.
Cepheids are luminous yellow giant stars. They radiate ten thousand times
as much energy as the Sun so they can be seen at very great distances. In 1912,
Henrietta ¤ Leavitt, who worked at Harvard College Observatory, noted a number
of Cepheids in the Small Magellanic Cloud and plotted their light curves. She
realized that there was a relationship between the periods over which the stars


The radio galaxy Centaurus A, imaged by the 8.2-m KUEYEN unit of the
European Southern Observatory™s Very Large Telescope.

varied (typically between 3 and 50 days) and their average apparent brightness.
The brighter the star, the longer it took to complete a cycle. This is called the
period“luminosity relation. This important discovery meant that Cepheids can be
used to determine the distances of nearby galaxies. Once the distance of a single
Cepheid variable had been found by an independent method, the distances
of all others could be deduced simply by measuring their periods.
There are two distinct varieties of Cepheid variables: classical Cepheids and
¤ Population II Cepheids, also known as W Virginis stars. The period“luminosity
relations of the two types are not the same. Classical Cepheids are about
two magnitudes brighter than W Virginis stars with the same period because
their masses and chemical composition are different.
Cepheus A constellation close to the north celestial pole, taking its name from the
legendary King of Ethiopia, who was the husband of Cassiopeia and the father
of Andromeda. It is one of the ancient constellations but is not conspicuous,
having no star as bright as second magnitude.
1 Ceres The first ¤ asteroid to be discovered. It was found by Giuseppi Piazzi
from Palermo, Sicily, on January 1, 1801. It is by far the largest asteroid,
measuring 940 km (585 miles) across and according to criteria adopted by the

Chandrasekhar limit

¤ International Astronomical Union in 2006, is also regarded as a ¤ dwarf planet.
Ceres orbits in the main ¤ asteroid belt at a distance of 2.77 AU from the Sun.
With a mass of 1.17 · 1021 kg, it accounts for about one-third the entire mass of


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