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the ¤ magnetosphere.
Earth is surrounded by a shallow layer of gas “ the atmosphere. It thins out
gradually with height and most of the gas is concentrated within the 20 km
(12 miles) immediately above the surface. The composition of the atmosphere
has changed since Earth formed about 4.5 billion years ago. In particular,
photosynthesis by plants increased the amount of oxygen signi¬cantly about
2 billion years ago. Molecular oxygen (O2) now accounts for 21% of the
atmosphere. The bulk of the gas (77%) is molecular nitrogen (N2) with 0.9%
argon, about 1% water vapor, and traces of other gases, the most important of
which is carbon dioxide (CO2). A layer of ozone (O3) high in the atmosphere
absorbs ultraviolet light from the Sun, which would be lethal to plants and
animals if it were to reach the surface. Clouds typically cover half of the Earth
at any time. Because of heat trapped by the atmosphere (the ˜˜greenhouse™™
effect) Earth™s average surface temperature of 15  C (59  F) is 30  C (54  F) higher
than it would be if there were no atmosphere.
earthshine Faint light illuminating the dark part of the Moon, seen when the
Moon is at thin crescent phase close to new Moon. The popular name for it is
seeing ˜˜the old Moon in the new Moon™s arms.™™ Earthshine is the result of
sunlight reflected from Earth onto the Moon™s surface.
eccentricity (symbol e) A number used to describe how elongated an ellipse is
compared with a circle. The eccentricity of an ellipse is greater than 0 but less
than 1. The larger its value, the longer and thinner the ellipse. A perfect circle
has an eccentricity of zero, and a parabola, which is an open curve, has an
eccentricity of one. Open curves with values of e greater than one are called
eclipse When a celestial object is not visible for a while, either because another
body passes directly in front of it or because it moves into a shadow in space.
On Earth, there are several eclipses of the Sun or Moon each year. When
the Sun is eclipsed, the Moon passes in front of it. In a lunar eclipse, the Moon
is not actually hidden. It passes through Earth™s shadow where no sunlight falls
directly on its surface to make it shine. Eclipses of the Sun and Moon happen
regularly because the Moon™s orbit around Earth is inclined to Earth™s orbit
around the Sun by only 5 . As a consequence, Sun, Moon and Earth line up in
space frequently, though not every month because their relative movements
are complex. At least two eclipses of the Sun happen every year. Most are
partial, however. The maximum number of eclipses in any one year is seven,
two or three of which are lunar. Theoretically, it is possible for two solar
eclipses to occur a month apart and for there to be a lunar eclipse in between.
However, there can never be lunar eclipses at two successive full Moons. Solar
eclipses take place at (or very close to) new Moon.


An eclipse of the Moon in 2003 photographed from the Kennedy Space Center.

Though the Moon is much nearer than the Sun, both appear very nearly the
same size in the sky. However, the apparent sizes of the Sun and Moon vary by a
small amount because the orbits of the Moon and Earth are elliptical rather than
circular. The ratio between the apparent diameters of the Moon and Sun at a solar
eclipse is called the ˜˜magnitude™™ of the eclipse. If a solar eclipse that would
otherwise have been total occurs when the Moon™s diameter appears less than the
Sun™s, a ring of the Sun™s disk remains visible. Solar eclipses of this kind are
described as ˜˜annular™™ (from the Latin word ˜˜annulus,™™ meaning ˜˜a ring™™).
The Moon™s shadow is only a few hundred kilometers wide at Earth™s
surface. It traces out a curved path from west to east as the Moon and Earth
move relative to each other. Observers in a wider region either side of the path
of totality witness a partial eclipse. Partial eclipses may also occur when no
part of the Earth experiences a total eclipse.
In the few minutes or seconds totality lasts, darkness falls. The faint outer
parts of the Sun “ the ¤ chromosphere and the ¤ corona “ become visible. They
cannot be seen from Earth except during an eclipse because the Sun™s bright
yellow disk is too dazzling. From a ¬xed place on Earth™s surface, the longest a
total solar eclipse can last is 7.5 minutes.
Lunar eclipses occur when the Moon passes through the shadow cast by
Earth. They can take place only close to full Moon and can be seen from any

ecliptic coordinates

location on Earth where the Moon has risen. The Moon does not normally
become completely invisible. Illuminated by sunlight scattered from Earth™s
atmosphere, it often takes on a dark coppery color.
The full shadow cast by Earth “ the umbra “ is surrounded by a region of
partial shadow, called the penumbra. Before and after entering the umbra, the
Moon passes through the penumbra. This is often hardly noticeable to casual
observers. Some lunar eclipses are only penumbral. The length of the Moon™s
path through the umbra, divided by the Moon™s apparent diameter, de¬nes the
˜˜magnitude™™ of a lunar eclipse. Lunar eclipses may last several hours.
The moon of another planet may also be eclipsed when the sunlight
causing it to shine is cut off because it passes into the shadow cast by its parent
planet. When a moon disappears behind its planet, the event is more usually
called an occultation. However, a double star system in which one star
periodically crosses in front of the other is known as an ¤ eclipsing binary.
eclipse year A period of 346.620 03 days, which is the time it takes for the Sun to
move through the sky from one of the points where the Moon™s orbit crosses
its path until it reaches that same intersection point the next time. It is shorter
than a normal year because the orientation of the Moon™s orbit in space is
gradually changing. ¤ saros.
eclipsing binary (eclipsing variable) A system of two stars that periodically hide
each others™ light. As a result, the combined brightness regularly dips.
In a binary system, two stars are held close together by gravity and follow
elliptical orbits around their center of mass. The orbits of some pairs are
oriented so that the two stars alternately cross in front of each other, partially or
totally concealing their partner. If the stars are not identical, the drop in
brightness when the fainter star is in front of its partner is greater than when it
is behind. The greater drop is called the primary minimum and the lesser one
the secondary minimum. The most well-known eclipsing binary is ¤ Algol. Some
eclipsing star pairs belong to multiple systems with more than two members.
ecliptic The plane in space in which Earth™s orbit around the Sun lies, or the circle
around the sky traced out by the Sun in the course of a year. It is called the
ecliptic because eclipses of the Sun or Moon can only take place on occasions
when the Moon crosses it. On the sky, the ecliptic goes through the 12
traditional constellation of the ¤ zodiac. Because of the way astronomers have
formally designated the boundaries of the constellations, it also goes through
¤ Ophiuchus.
The orbits of the other major planets in the solar system are tilted only
slightly to the ecliptic plane. This means that their paths in the sky are all close
to the ecliptic.
ecliptic coordinates A system of latitude and longitude for measuring positions
on the ¤ celestial sphere, based on the ¤ ecliptic. These coordinates are used

Eddington, Arthur Stanley (1882“1944)

particularly for work on the positions and movements of the planets. Ecliptic
latitude is measured in degrees north and south of the ecliptic. Northerly
latitudes are positive and southerly ones negative. Ecliptic longitude is
measured in degrees around the ecliptic from where the Sun crosses the
¤ celestial equator in March, a point called the vernal equinox.
Eddington, Arthur Stanley (1882“1944) Sir Arthur Eddington was one
of the foremost astrophysicists of his time, remembered particularly for his
work on the interiors of stars and in connection with Einstein™s theory of
¤ general relativity. He was born in Kendal, England and from 1913 until his
death was Plumian Professor of Astronomy at the University of Cambridge. He
was knighted in 1930.
In 1919, Eddington traveled with collaborators to the island of Principe off
the west coast of Africa for a total eclipse of the Sun. With observations of stars
made during the eclipse. they showed that the Sun™s gravity de¬‚ected the path
of starlight slightly from a straight line, con¬rming a prediction Einstein had
made three years earlier. Eddington™s ground-breaking theoretical work on
stars was published in1926 in his book, The Internal Constitution of the Stars. His
discoveries include the relationship between the masses of stars and their
luminosity, and the fact that no star more massive than about 120 solar masses
can exist. The ˜˜Eddington limit™™ is the greatest luminosity a star of a particular
mass can attain without blowing itself apart.
Eddington also wrote books for general readers, such as The Expanding
Universe, which became very popular.
Edgeworth“Kuiper Belt ¤ Kuiper Belt.
EGG Abbreviation for ˜˜evaporating gaseous globule.™™ An EGG is essentially a dense
sphere of gas in which a star is forming. It is denser than the cloud of gas in
which it is embedded. The action of ultraviolet light from nearby hot stars
strips away the thinner surrounding gas to expose EGGs so they can be seen.
The name was first coined for globules identified in Hubble Space Telescope
images of the ¤ Eagle Nebula.
Egg Nebula A popular name for the very young planetary nebula more formally
known by its catalog number as CRL2688. Its central star, which was a red
giant until a few hundred years ago, is hidden behind a ring of dust. This star
illuminates a series of shells of gas that it has previously ejected.
Einstein, Albert (1879“1955) Albert Einstein is most famous for his special and
general theories of relativity (¤ special relativity, general relativity), which
revolutionized physics at the beginning of the twentieth century. He received
the Nobel Prize in 1921 for his work on the photoelectric effect.
Einstein was born in Ulm, which is now in Germany. In 1901 he took Swiss
citizenship. As a young man, he did not seem especially brilliant and failed to
get an academic job. While working as a private tutor, and then for the Swiss

Einstein, Albert (1879“1955)

A Hubble Space Telescope image
of EGGs “ evaporating gaseous
globules “ in the Eagle Nebula.

The Egg Nebula imaged by the Hubble Space Telescope.

Patent Of¬ce, he used his spare time to think about a number of fundamental
problems in physics and developed his special theory of relativity, which
was published in 1905. In the same year he announced his famous
equation, E ¼ mc2, in which E represents energy, m mass and c the velocity

Einstein Cross

Albert Einstein (1932).

of light. This followed from his conclusion that a body loses mass when it
radiates energy.
From 1907, Einstein held a series of posts in universities before settling in
Berlin in 1914. His greatest work was all done by the early 1920s. In 1933 he
moved permanently to the United States where he was on the staff of
Princeton University.
Einstein Cross The result of a galaxy called G2237þ0305 acting as a ¤ gravitational
lens, to produce four separate images of a quasar. The four images form a cross
around the bright nucleus of the galaxy and vary in brightness and color on a
timescale of about a day. The quasar happens to lie along the same line of site
as the galaxy, but is much farther away. The quasar™s redshift is 1.7 but that of
the galaxy only 0.04.
Einstein Observatory An orbiting X-ray observatory that was launched on
November 13, 1978 and operated until 1981. It was named to honor the
centenary of the birth of Albert Einstein. It was the first observatory capable of
making X-ray images of extended objects.
Einstein ring A circular image of a distant point-like source of light or radio
waves, such as a quasar, formed when a massive galaxy along exactly the same
line of sight acts as a ¤ gravitational lens. Einstein was the first person to set out
the theory of how such a ring-shaped image would be formed.


The Einstein Cross imaged by the
Hubble Space Telescope.

Einstein rings imaged by the Hubble Space Telescope.

ejecta Material thrown outwards when an impact forms a crater or by a
volcanic eruption. It consists of fragments of rock and solidified droplets of gas
or liquid. Ejecta often blankets a circular area around an impact or eruption, or
it sometimes forms a pattern of rays. Some ejecta created by impacts may
escape into space.


Electromagnetic radiation.

Elara A small moon of Jupiter discovered in 1905 by Charles D. Perrine. It is about
86 km (53 miles) across. With Leda, Himalia and Lysithea it belongs to a family
of four moons with closely spaced orbits. Their average distances from Jupiter
are between 11.1 and 11.7 million km (6.9 and 7.3 million miles).
Electra One of the brighter stars in the ¤ Pleiades. With a magnitude of 3.5 it is
easily visible to the naked eye. Electra is also known as 17 Tauri.
electric propulsion ¤ ion drive.
electromagnetic radiation A form of energy with wave-like properties. It consists
of linked electric and magnetic fields, which vary rapidly. Electromagnetic
radiation travels through a vacuum at a speed of 3 · 108 m/s (represented by the
symbol c), as discrete ˜˜wave packets,™™ called photons. Its properties vary with
wavelength. From the longest wavelengths to the shortest, the spectrum of
electromagnetic radiation covers radio waves, microwaves, infrared radiation,
visible light, ultraviolet radiation, X-rays and gamma rays.
Radio wavelengths cover a wide range from several meters down to
millimeters. The shortest radio waves are usually called microwaves. Infrared
radiation has even shorter wavelengths, ranging down to just under 1 mm.
Visible light is a narrow band of wavelengths between about 700 and 400 nm.
Ultraviolet radiation goes down to about 10 nm, and X-rays to 0.1 nm. The
shortest waves of all are gamma rays. The shorter the wavelength of the
radiation, the more energy carried by each photon.
element (chemical) One of the ˜˜building blocks™™ of matter in the universe. About
90 different elements are found naturally on Earth. Each one has a one- or
two-letter symbol. Elements combine together chemically to make compound
materials, like water (hydrogen and oxygen, H2O) and common salt (sodium
and chlorine, NaCl).


The elliptical galaxy M49 in the Virgo
cluster of galaxies.

Elements consist of atoms. Atoms have a positively charged nucleus, which
is usually surrounded by a cloud of negatively charged electrons. The nucleus
of an atom contains protons, which carry the positive electric charge, and
neutrons with no charge (except for hydrogen which has no neutron). Each
distinct element has atoms with a unique number of protons, called the atomic
number. Hydrogen has one, carbon six and iron 26, for instance. Different
forms of an element may exist, called isotopes, each with a different numbers
of neutrons. For example, most carbon has six neutrons but atoms can exist
with seven or eight. Many isotopes are not stable and their nuclei break down
sooner or later, emitting harmful particles and radiation. Unstable isotopes of
an element are described as ˜˜radioactive.™™
elements (orbital) A set of quantities that define the shape, orientation and timing
of an object™s orbital motion.
elliptical galaxy A galaxy that looks smoothly elliptical in shape and has no
structures such as a disk or spiral arms. In three dimensions elliptical galaxies
are believed to be ellipsoidal “ similar to a regularly shaped potato. The largest,
most luminous galaxies in the universe are giant ellipticals up to 500 000 light
years across. It is likely that many of them harbor massive black holes at their
centers. It is thought that they formed through the merger of smaller galaxies.
Large ellipticals tend to be where there are many galaxies relatively close
together, such as in the center of a rich cluster of galaxies.
Some of the smallest galaxies are also ellipticals. Their typical size is about
1000“10 000 light years. These dwarf elliptical galaxies contain between a few
hundred thousand and a few million stars. Small elliptical galaxies are most
often found as companions to much larger galaxies.
All the evidence suggests that ellipticals are old galaxies. Almost all the
stars in them are more than 10 billion years old “ 5 billion years older than the
Sun. With few exceptions, they contain very little gas and dust and hardly any
newly forming stars or young star clusters.
Elnath (Beta Tauri) The second-brightest star in the constellation Taurus, of
magnitude 1.7. The name Elnath (alternatively spelled Alnath) comes from the


Elysium Planitia. This image from Mars
Global Surveyor shows three
volcanoes: Elysium Mons in the center,
with Hecates Tholus to the north and
Albor Tholus to
the south.

Arabic for ˜˜the one butting with horns.™™ In old star maps, this star was shared
with the neighboring constellation Auriga, and was also designated Gamma
Aurigae. Elnath is classed as a ¤ B star.
elongation The angle on the sky between the Sun and an astronomical body, such
as the Moon or a planet. Elongations of 0 , 90 and 180 are called conjunction,
quadrature, and opposition, respectively. Mercury and Venus both have
maximum possible elongations because they are nearer to the Sun than Earth.
The maximum distances they attain from the Sun during each orbit are called
greatest elongation. Mercury™s greatest elongation can be anywhere between
18 and 28 according to circumstances; the range for Venus is 45“47 . Any
elongation is possible for the planets farther from the Sun than Earth.
Elysium Planitia The second largest volcanic region on Mars. The highest
volcano, Elysium Mons, rises to a height of about 12 km.
emersion The reappearance of a body that has been hidden during an ¤ occultation
or an ¤ eclipse.
emission line In a ¤ spectrum, a spike at a specific wavelength. Atoms and
molecules produce emission lines when they lose energy by switching between
two energy states.
emission nebula A cloud of glowing gas in interstellar space. Interstellar gas clouds
give out light when they receive energy in the form of ultraviolet radiation from
hot stars nearby. Hydrogen, the most common gas in space, glows pink.
Enceladus One of the medium-sized moons of Saturn, discovered by William
Herschel in 1789. It measures 512 · 494 · 489 km (318 · 307 · 304 miles) and it
travels in an almost circular orbit 238 020 km (147 905 miles) from Saturn,


An emission nebula around a cluster of young stars (NGC 346) in the Small Magellanic Cloud,
imaged by the Hubble Space Telescope.

which is within Saturn™s ring system. The ¤ Voyager 2 spacecraft returned
images of Enceladus showing details as small as 2 km (just over 1 mile) and
images have also been obtained more recently by the ¤ Cassini spacecraft.
Large areas of the surface have no craters, and the density of craters in areas
that do have them is relatively low. There is also widespread evidence of
tectonic activity, such as troughs, grooves and ridges. The surface of Enceladus
appears to have been completely remodeled by activity of some kind since it
was first formed. In 2005, the Cassini spacecraft observed plumes of icy
droplets jetting from a series of fissures, described as ˜˜tiger stripes,™™ in the
south polar region. These jets are believed to come from a body of liquid water
below the surface. The orbit of Enceladus coincides with Saturn™s faint E ring

Encke, Comet 2/P

A false color image of Enceladus taken by the Cassini spacecraft.

and the eruptions on the satellite are likely to be the source of the ring
Encke, Comet 2/P A short-period comet named in honor of the German
astronomer Johann Franz Encke (1791“1865). Encke did not actually discover
it, but when it appeared in late 1818, he demonstrated that it was the same
object as comets previously seen on three occasions. He also successfully
predicted that it would be visible again in 1822. It was only the second comet
(after Halley™s) proved to return periodically. Pierre Mechain (1744“1804) of
Paris was the first to record it in 1786. It was rediscovered by Caroline Herschel
in England in 1795, by the French observer Jean Louis Pons and others in 1805
and by Pons again in 1818.
Comet Encke is in an unusual orbit for a periodic comet. On its very elliptical
path, it takes only 3.2 years to make one revolution around the Sun. It has been
in a very stable orbit for a long time and is probably one of the most evolved
comets still actively producing a coma and tail. It may well be close to becoming

equatorial coordinates

a defunct core, about 2“3 km across. It has become fainter since ¬rst observed.
In 1829 its magnitude was recorded as 3.5 but during the twentieth century
if never became brighter than ¬fth magnitude. Streams of dust shed from Comet
Encke causes the annual Taurid meteor shower around November 3 each year.
Encke Division (Encke Gap) A narrow gap toward the outer edge of the bright
A ring surrounding ¤ Saturn. A ¤ Voyager 2 image showed a narrow ringlet
within the Encke Division. Saturn™s small moon ¤ Pan orbits within the Encke
Division. ¤ planetary rings.
Eos family A group of asteroids with similar composition orbiting the Sun in the
¤ asteroid belt at a distance of about 3.02 AU. The family was formed when a
single parent asteroid broke up sometime in the past, and takes its name from
its largest member, 221 Eos.
ephemeris (pl. ephemerides) A table of the celestial coordinates, magnitude and
other data for the Moon, the Sun, a planet, a comets or other astronomical
object. An annual handbook containing astronomical tables and data is also
sometimes called an ephemeris.
epicycle A circular path around a center that is itself moving around a larger
circle, which is called the deferent. The astronomer ¤ Ptolemy, who lived in the
second century ad, made his Earth-centered model of the solar system fit the
observations of the planets by having the planets move along epicycles. To
make his predictions more accurate, he said that the center of the epicycle
moved at a uniform angular rate about an off-center point called the equant
and he also set Earth off-center on the side opposite the equant. Ptolemy™s
theory could predict the positions of planets to within a degree with the right
choice of size for each epicycle and deferent.
Epimetheus A small moon orbiting Saturn at a distance of 151 422 km
(94 094 miles), just beyond the far edge of the ring system. It is in the same
orbit as another moon, Janus. The two satellites may be fragments of a larger
object broken apart by an impact. Epimetheus is irregular in shape, measuring
138 · 110 · 110 km (86 · 68 · 68 miles). Little is known about it though an image
returned by the ¤ Voyager 1 spacecraft in 1980 shows numerous craters,
including several 30 km or more across, and ridges and grooves on the surface.
equation of time The difference between mean ¤ solar time, as kept by a clock
running at a steady rate, and the apparent solar time read off a sundial. It
varies during the course of a year up to a maximum of 16.3 minutes.
equator The plane through the center of a rotating object that is perpendicular to
the axis of rotation and, for a solid body such as a planet, the circle where that
plane cuts its surface. In astronomy, equator is sometimes used as an
abbreviation for ¤ celestial equator.
equatorial coordinates A system of celestial latitude and longitude for specifying
positions on the sky, based on the ¤ celestial equator. The coordinate equivalent

equatorial mount

Epimetheus imaged by the Cassini

to latitude is declination (abbreviated to Dec. or “ (delta)). It is measured in
degrees north and south of the celestial equator. Northerly declinations are
positive and southerly ones negative.
The other coordinate, right ascension (RA or ¬ (alpha)), is the equivalent of
longitude but is measured in hours, minutes and seconds of time, because a
telescope pointing at a ¬xed direction in the sky will ˜˜see™™ an angle of one hour
of right ascension sweep by in one hour of ¤ sidereal time. There are 24 hours in a
complete circle of 360 so 1 hour of right ascension is the same as 15 .
The zero point for right ascension is the ¤ equinox that falls in March.
Because of ¤ precession, this point is very slowly moving along the equator.
Equatorial coordinates, therefore, have to be speci¬ed with the date when they
are valid, which is called the epoch.
equatorial mount A mount for a telescope which allows the instrument to track
the east“west rotation of the sky by turning slowly around just one axis, called
the polar axis. The mount is oriented so that the polar axis is parallel to Earth™s
rotation axis. The telescope can also rotate around a second axis, called the
declination axis, which is at right angles to the polar axis. This arrangement
makes it simple to point the telescope to particular ¤ equatorial coordinate (right
ascension and declination).
Equatorial mounts are popular for amateur telescopes because they can
easily follow the motion of the sky automatically if equipped with a small
motor. Equatorial mounts were used for large professional telescopes until the
late twentieth century when computerized controls made cheaper ¤ altazimuth
mounts practicable.
equinox The time when the Sun is at one of the two points where the ¤ celestial
equator and the ¤ ecliptic cross, or the actual position on the sky of either of

433 Eros

these points. The Sun crosses the equator from south to north at the northern
vernal (spring) equinox on March 20 or 21, and from north to south at the
northern autumnal equinox, which falls between September 22 and 24. The
dates vary because of leap years.
The position of the northern vernal equinox was traditionally known as
˜˜the ¬rst point of Aries™™ and is still often represented by the astrological
symbol for Aries (°). However, because of ¤ precession, the equinox points
move around the ecliptic at a rate of about 1.4 per century. The vernal
equinox point now lies in the constellation of Pisces, which neighbors Aries.
Equuleus (The Foal) The second-smallest constellation in the sky. Although it is
small and its stars faint, Equuleus is an ancient constellation. It is located next
to Pegasus near the celestial equator.
Eridanus (The River Eridanus) A large southern constellation, meandering from
the celestial equator to declination “60 . ¤ Ptolemy named it in his list of 48
constellations, but its southern extension was added later. The most southerly
point is marked by the first-magnitude star ¤ Achernar.
Erinome A small outer moon of Jupiter discovered in 2000. Its diameter is about
3 km (2 miles).
Eris A ¤ dwarf planet in the ¤ Kuiper Belt. Before receiving its official name in 2006,
it was known informally as ˜˜Xena™™ and by its temporary designation 2003
UB313. With a diameter of about 2400 km (1490 miles), it is slightly larger than
Pluto and when named in 2006 was the largest known dwarf planet.
Eris was discovered by Mike Brown, Chad Trujillo and David Rabinowitz on
January 8, 2005, at ¤ Palomar Observatory. It had ¬rst been photographed in
October 2003, but its nature was not detected until the subsequent observation
in 2005. The orbit followed by Eris is a very elongated ellipse, which it takes
560 years to complete. Eris™s distance from the Sun ranges between 38 and 97
astronomical units. At discovery, it was nearly as far away as it can ever be, but
at closest it is nearer than Pluto. The spectrum of Eris suggests it is covered by a
yellowish layer of frozen methane.
Eris has one known moon, Dysnomia, which is about 250 km (155 miles)
433 Eros The third largest of the known near-Earth asteroids, and the first asteroid
to be discovered on an orbit that crosses inside the orbit of Mars. It was found
on August 13, 1898 by Gustav Witt in Berlin and independently by Auguste
Charlois at the Nice Observatory in France. Elongated and irregular in shape, it
measures 13 · 13 · 33 km (8 · 8 · 21 miles).
Eros™s distance from the Sun ranges between 1.78 and 1.33 AU. In 1975, it
came within 22 million km (14 million miles) of Earth on one of its nearest
possible encounters. When Eros made a close approach in 1931, astronomers
had never seen any other object in orbit round the Sun come so near to Earth.


The northern hemisphere of Eros from
a mosaic of images taken by the NEAR
Shoemaker spacecraft orbiting at a
height of 200 km (124 miles).

Observations of its path were used to make the best calculations possible at
that time of the distance between Earth and the Sun. In February 2000, the
spacecraft ¤ NEAR Shoemaker entered orbit around Eros to conduct the ¬rst ever
long-term close-up study of an asteroid. During its year in orbit, it returned a
wealth of images and information about Eros.
Eros is made entirely of primitive, rocky material which is not separated
into layers with different compositions. Its average density is 2.7 g/cm3, about
the same as Earth™s crust. It seems to be a single sold body, possibly porous or
fractured inside, but not a collection of small fragments or ˜˜rubble pile.™™ It was
probably broken off a larger asteroid by an impact. The rough, heavily cratered
surface is covered by a layer of ¤ regolith and scattered with numerous large
Erriapo A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and is 8.6 km (5.3 miles) across.
eruptive variable A star that changes brightness suddenly and unpredictably.
There are many different kinds of eruptive variables, including ¤ flare stars,
¤ luminous blue variables, ¤ R Coronae Borealis stars, ¤ T Tauri stars and
¤ Wolf“Rayet stars.
ESA Abbreviation for ¤ European Space Agency.
escape velocity The least velocity a small body needs in order to escape from the
gravitational attraction of a more massive one. The velocity of escape from
Earth™s surface is about 11.2 km/s.
Eskimo Nebula (NGC 2932) A ¤ planetary nebula in the constellation Gemini. Its
popular name came from photographs of the circular nebula, which suggest
the features of a face, surrounded by a fur-fringed hood. The ˜˜fur hood™™
consists of comet-shaped blobs of gas streaming away from the central star.
The inner part of the nebula is two lobes of expanding gas seen end-on.
ESO Abbreviation for ¤ European Southern Observatory.

Eta Carinae

The Eskimo Nebula imaged by the Hubble Space Telescope.

Eta Aquarids A ¤ meteor shower caused by dust streams in the orbit of Comet
¤ Halley. Eta Aquarids can be seen between about April 24 and May 20 each
year, peaking around May 4 or 5. The radiant of the shower lies in the
constellation Aquarius.
Eta Carinae A ¤ luminous blue variable star inside the ¤ Eta Carinae Nebula. Estimates
put its total mass at about 100 times greater than the Sun™s but it is almost
certainly a binary system rather than a single star. Eta Carinae is the strongest
source of infrared radiation in the sky. Its total energy output, including
infrared, is five million times that of the Sun.
Over the last 300 years, Eta Carinae has varied in brightness unpredictably.
Halley recorded it as a fourth-magnitude star in 1677. By 1843 it was the
second-brightest star in the sky. Subsequently, it faded irregularly and has
been invisible to the naked eye for the last 100 years. Now it appears to be
brightening again.
Eta Carinae is surrounded by the Homunculus Nebula, which is produced
by gas ¬‚owing from the star. It loses gas at a rate of 0.07 solar masses a year,
the fastest known rate for any star. This prodigious loss of material apparently
started when the star was at its brightest but cannot be sustained for long “
possibly only a few hundred years. The twin-lobed shape of the Homunculus
Nebula is thought to be the result of a ring of gas restricting the out¬‚ow. The

Etched Hourglass

Eta Carinae. This image, made from views taken by the Hubble Space Telescope,
shows huge blobs of material ejected by the supermassive star.

ring may have formed from the outer layers of the secondary star in the system
during the 1843 outburst. A star as massive as Eta Carinae is not stable. It is
likely to undergo more dramatic changes in the future and may possibly
become a ¤ supernova.
Etched Hourglass A popular name for a ¤ planetary nebula with the formal catalog
designation MyCn18. It is about 8000 light years away and lies in the southern
constellation Musca. A ring of dust around the equator of its central star may
be responsible for its hour-glass shape.
Euanthe wA small moon of Jupiter discovered in 2001. Its diameter is about 3 km
(2 miles).
Eudoxus of Cnidus (c. 408“c. 355 bc) Eudoxus was one of the greatest
mathematicians of his age and is most famous for the theory of planetary
motion he developed. He studied astronomy and made observations in Egypt
but finally settled in his native Cnidus, in what is now Turkey. All the books he
wrote himself are lost, but his work is known because later writers such as
¤ Hipparchus and ¤ Aristotle referred to it.
To predict the motion of the planets, Eudoxus proposed a complex
system involving the rotation of 27 interlinked spheres centered on Earth. It
was a remarkable achievement in geometry, but he still could not fully

15 Eunomia

The Etched Hourglass. A false color image composed of views taken by the Hubble Space
Telescope through three different ¬lters.

reproduce the observed motion of the planets. Eudoxus probably did not
think of his spheres as real objects in space, but simply as a mathematical
45 Eugenia One of the larger asteroids, discovered from Paris in 1857 by Hermann
Goldschmidt. Its estimated diameter is 215 km (133 miles). In November 1998
astronomers working at the Keck Observatory found that it has a moon about
13 km (8 miles) in diameter, which was named Petit-Prince. It was the first
asteroid moon to be discovered from Earth.
Eukelade A small outer moon of Jupiter discovered in 2003. Its diameter is about
4 km (2.5 miles).
15 Eunomia One of the larger asteroids, with a diameter of 260 km (162 miles),
discovered from Italy in 1851 by Annibale de Gasparis.

31 Euphrosyne

Europa imaged by the Galileo
spacecraft from a range of 677 000 km
(417 900 miles).

31 Euphrosyne One of the larger asteroids, estimated to be 248 km (154 miles)
across, and the first to be discovered from America. It was found 1854 by James
Ferguson working at the US Naval Observatory.
Euporie A small outer moon of Jupiter discovered in 2001. Its diameter is about
2 km (1 mile).
Europa The smallest of Jupiter™s four large Galilean moons, with a diameter of
3130 km (1945 miles). The ¤ Voyager spacecraft returned images showing a
bright icy surface, criss-crossed by numerous dark lines but with relatively few
obvious craters. Close-up images taken by the ¤ Galileo spacecraft revealed a
complex network of grooves and bands, some straight and some curved or
scalloped. These include long features several kilometers wide stretching for
1000 km or more, called triple bands, which are bright in the center with dark
regions either side. Galileo™s images also showed that there are numerous
craters scattered over the surface.
It is not known for certain how Europa is structured inside but the
evidence suggests there could be a core rich in iron, surrounded by rock.
Europa™s icy crust is at least several kilometers thick but underneath there
could be an ocean of liquid water or a mixture of rock and icy slush. Tidal
forces due to Jupiter™s powerful gravity are strong enough to raise the
temperature above the freezing point of water.
The appearance of Europa™s crust shows that it must have changed greatly
since it ¬rst formed. In some places it looks as if great rafts of ice have broken
up and moved about, suggesting there is (or has been) water or slush beneath
them. Some of Europa™s features may have been created by liquid water
welling up from below in a process similar to volcanism on Earth.
52 Europa One of the larger asteroids, with an estimated diameter of 312 km
(194 miles). It was discovered in 1858 by Hermann Goldschmidt.


European Southern Observatory (ESO) A European research organization
founded in 1962 to foster cooperation in astronomy and to provide European
astronomers with a major modern observatory. The eleven member-countries
are Belgium, Denmark, Finland, France, Germany, Italy, the Netherlands,
Portugal, Sweden, Switzerland and the UK. Its headquarters are at Garching bei
Munchen in Germany. Its observatory is split between two sites in Chile: the
¤ La Silla Observatory and the ¤ Paranal Observatory.
European Space Agency (ESA) An organization through which 16 European
countries (Austria, Belgium, Denmark, Finland, France, Germany, Greece,
Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland
and the UK) collaborate on a joint space program. ESA builds and launches
satellites and spacecraft for scientific research and commercial applications.
The headquarters are in Paris. ESA™s largest establishment is its Research and
Technology Center (ESTEC) at Noordwijk in the Netherlands.
Eurydome A small outer moon of Jupiter discovered in 2001. Its diameter is about
3 km (2 miles).
EUV Abbreviation for extreme ultraviolet. There is no exact definition of where
extreme ultraviolet begins and ends but it usually means the range of
wavelengths between about 10 and 100 nm. ¤ electromagnetic radiation,
ultraviolet astronomy, XUV.
evening star Venus or Mercury when it is visible in the western sky after sunset.
event horizon The nearest to a ¤ black hole it is possible to see anything. No light
emitted inside the event horizon can escape because of the strength of the
black hole™s gravitational field. The radius of the event horizon, called the
¤ Schwarzschild radius, depends on the mass of the black hole. For a black hole
with the minimum possible mass of 3 solar masses, it is 9 km (6 miles).
exit pupil The image of a telescope™s field of view, formed by an eyepiece. The
position of the exit pupil in front of an eyepiece is where an observer should
place his or her eye to get the optimum view. If the system is well designed, the
exit pupil should be at a place where it is comfortable to observe and should
match the size of the pupil of the observer™s eye.
exobiology Another word for ¤ astrobiology.
ExoMars A proposed mission to Mars, which would be the first to be led by the
European Space Agency. The mission would deliver a ˜˜rover™™ carrying a
package of experiments to the surface of Mars. The rover would communicate
with Earth via the ¤ Mars Reconnaissance Orbiter. The target launch date is 2011.
exoplanet An ¤ extrasolar planet.
exosphere The outermost layers of a planetary atmosphere, where the gas is very
thin and merges into the interplanetary medium, or the entire layer of gas
around a planet if it is extremely tenuous. In an exosphere, the density is so
low that gas atoms very rarely collide with each other, and atoms moving

expanding universe

rapidly can escape from the gravitational pull of the planet. The Earth™s
exosphere starts at a height of about 400“500 km (250“300 miles).
expanding universe A model for the universe based on the idea that the scale of
space has been getting larger since our universe came into being at the ¤ Big
Bang. The discovery by Edwin Hubble in 1929 that the ¤ redshifts of galaxies
increase with distance and the detection of the ¤ cosmic background radiation in
1964 are the main evidence that the universe is expanding.
extragalactic Outside the limits of our own Galaxy, the Milky Way.
extrasolar planet A planet orbiting a star other than the Sun.
The ¬rst extrasolar planets were discovered in 1991 when Alexander
Wolszczan detected three planets orbiting the ¤ pulsar PSR1257þ12 in the
constellation Virgo. Later, evidence was found for a fourth. The planets are not
directly visible but their presence affects the regularity of the radio pulses
received from the pulsar.
The ¬rst evidence for extrasolar planets around ordinary stars was made
public in 1995 by Michel Mayor and Didier Queloz of the Geneva Observatory
who found a planet with at least half the mass of Jupiter orbiting the star 51
Pegasi. Since 1996, the count of extrasolar planets orbiting ordinary stars has
risen steadily, passing the 200 mark in 2006. Several systems are known to
contain two or more planets. Nearly all these planets have been found by the
tiny regular changes they cause to the motion of their parent stars. This
technique is not sensitive enough to detect planets as small as Earth. Other
search methods include looking for the slight dimming of a star when a planet
crosses in front of it, and ¤ microlensing. Several space missions are under
consideration that would be capable of ¬nding Earth-sized planets.
Extreme Ultraviolet Explorer (EUVE) An orbiting observatory launched by NASA
in June 1992 to detect short-wavelength ultraviolet radiation. It was the first
observatory dedicated to observing ultraviolet radiation in the wavelength
range 7“76 nm. It operated until January 31, 2001. Results from EUVE included
an all-sky catalog of 801 objects. ¤ ultraviolet astronomy.
eyepiece A combination of small lenses mounted in a tube, used to view the image
formed by a telescope or other optical instrument. A telescope for visual use
has a small tube where an eyepiece can be slotted. The field of view of an
eyepiece depends on the combination of lenses used in its design. The
magnification a particular eyepiece gives is the focal length of the telescope
divided by the focal length of the eyepiece.
eye relief The distance from an ¤ eyepiece an observer™s eye has to be in order to
see clearly the whole ¤ field of view. Eyepieces of different designs and
magnification have different eye relief. In general, the higher the power of the
eyepiece, the lower the eye relief. Observers who wear spectacles need more
eye relief than those who do not.

facula (pl. faculae) A bright region on the Sun™s ¤ photosphere. Faculae are linked
with ¤ solar activity and sunspots often appear in the same area of the Sun™s
fall A ¤ meteorite recovered after it has been observed to fall.
falling star A popular North American term for a ¤ meteor.
false color image A colored image in which the colors are not as they would be
seen by a normal human eye in natural conditions. In astronomy, false color is
used to enhance the contrast of an image and so bring out details that would
otherwise be dif¬cult to see. False color is also used to picture observations
made in wavelength regions other than visible light.
False Cross The four stars Epsilon and Iota Carinae, and Delta and Kappa Velorum,
in the southern constellations ¤ Carina and ¤ Vela. They are called the ˜˜False
Cross™™ because they look similar to the nearby constellation ¤ Crux.
farside The hemisphere of the Moon that is permanently turned away from the
Earth. ¤ libration.
Far Ultraviolet Spectroscopic Explorer (FUSE) A NASA astronomy satellite
launched in June 1999 on a three-year mission for spectroscopy in the far
ultraviolet region of the spectrum.
Ferdinand A small outer moon of Uranus, discovered in 2001. It is about 12 km
(7 miles) across.
Fermi“Hart paradox The argument that extraterrestrial life does not exist
because we see no evidence of it and have received no signals from
fibril A streak-like feature in an active region of the Sun. They are typically
725“2200 km (450“1400 miles) wide and 11 000 km (7000 miles) long.
Individual ¬brils have a lifetime of 10“20 minutes, though the overall ¬bril
pattern of a region shows little change over several hours.
In the central parts of active regions, ¬brils are arranged in patterns
connecting spots and ¤ plages of opposite magnetic polarity. Ordinary sunspots
are surrounded by a radial pattern of ¬brils called the superpenumbra. This is
material ¬‚owing down into the spot.
field galaxy A galaxy that appears in the same ¬eld of view as a cluster of galaxies
but is not itself a member of the cluster. The alignment is purely coincidental
and the ¬eld galaxy is nearer or farther away than the cluster.

field of view

field of view The angular size of the image formed by an optical instrument, such
as a telescope. The ¬eld of view of a telescope becomes smaller the greater the
magni¬cation being used.
field star A star that appears in the same ¬eld of view as a ¤ star cluster but is not
itself a member of the cluster. The alignment is purely coincidental and the
¬eld star is nearer or farther away than the cluster.
filament A solar ¤ prominence viewed against the bright disk of the Sun “ that is,
from above. Prominences can be seen in this way when the Sun is observed
through a ¬lter to isolate the light at the wavelength of particular ¤ spectral
lines such as ¤ hydrogen alpha. Filaments then show up as dark streaks.
find A ¤ meteorite discovered accidentally or as the result of a search on the
ground, as opposed to one actually seen to fall.
finder A small telescope attached to the tube of a larger instrument to help with
pointing the main telescope correctly. A ¬nder usually has a cross-wire in its
¬eld of view and is aligned so that an object located on the cross-wire appears
centrally in the smaller ¬eld of view of the main telescope.
fireball A particularly bright ¤ meteor. There is no precise de¬nition: magnitudes
À3, À4 or À5 are variously quoted as the minimum for a meteor to be
described as a ¬reball. Exceptional ¬reballs are sometimes a sign that a
¤ meteorite may have fallen, though the object more often burns up in the
first contact In an ¤ eclipse of the Sun, the point when the Moon ¬rst begins to
move across the Sun™s disk. In a lunar eclipse, ¬rst contact occurs when the
Moon ¬rst enters the full shadow (umbra) of the Earth. The term also describes
a similar stage in the progress of a ¤ transit or ¤ occultation.
First Point of Aries (°) One of the two points on the celestial sphere where
the ¤ ecliptic and the ¤ celestial equator cross. It is the point on the celestial
equator where ¤ right ascension is zero. Because of the effects of ¤ precession,
the celestial equator is slowly sliding around the ecliptic. It takes about
25 800 years to make one revolution. Though their crossing point was in the
constellation Aries several thousand years ago, it is now in Pisces. However, its
name has not been changed. The Sun is at the First Point of Aries on the
northern vernal ¤ equinox, and ˜˜vernal equinox™™ is also used to mean this
place on the celestial sphere.
first quarter The ¤ phase of the Moon when half its visible disk is illuminated
and the Moon is waxing towards full. First quarter occurs when the celestial
¤ longitude of the Moon is 90 greater than the Sun™s.
Flaming Star Nebula A popular name for IC 405, a star cluster surrounded by
nebulosity in the constellation Auriga.
Flamsteed, John (1646“1719) The British astronomer Flamsteed was a skilled
observer, who was appointed the ¬rst ¤ Astronomer Royal in 1675. He was born


in Derby, where he attended school until aged 14. Thereafter he was
self-taught. He studied astronomy from 1662 and began observations in 1671.
His career developed under the patronage of the president of the Royal Society,
which promoted the establishment of the Royal Observatory. The King
appointed Flamsteed as the astronomer responsible for improving tables for
determining longitude at sea, and built the Royal Observatory for him.
He designed the most accurate instruments of his day, to make a new star
catalog and a star atlas, containing positions of about 3000 stars. Ever a
perfectionist, he refused to release his data, then the most accurate in the
world, to Isaac Newton and Edmond Halley, which led to bitter clashes. Halley
published the catalog in 1712, against Flamsteed™s wishes. Flamsteed later
seized 300 of them and burnt them. His revised catalog was published in 1725.
Flamsteed numbers Identi¬cation numbers assigned to stars listed in Historia
coelestis Britannica by John ¤ Flamsteed. In the of¬cial version of the catalog,
which was published posthumously in 1725, the numbers are not included
explicitly. However, they do appear in a preliminary version published by
Edmond Halley and Isaac Newton in 1712 without Flamsteed™s approval. Few
copies of this version survive because Flamsteed was greatly angered at the
action of Halley and Newton and destroyed many of them.
The numbers were allocated by constellation and in order of right ascension.
They were subsequently used by other catalogers, including John Bevis (1750) and
Joseph-Je ˆ me de Lalande (1783). The Flamsteed numbers are still commonly
used for stars that do not also have a Greek ¤ Bayer letter designation.
flare A phenomenon in the solar ¤ chromosphere and ¤ corona, caused by a sudden
release of energy that heats and accelerates gas in the Sun™s atmosphere. Flares
are explosions lasting typically for a few minutes. In a ¬‚are, the temperature
reaches tens or hundreds of millions of degrees and particles are accelerated

A powerful solar flare was recorded in
this ultraviolet image by SOHO on
November 4, 2003.

flare star

almost to the speed of light. Most of the radiation is emitted as X-rays but ¬‚ares
are also observed in visible light or radio waves. They are associated with active
regions of the Sun. Charged particles ejected from the Sun by ¬‚ares are
potentially hazardous to humans and sensitive electronic equipment in space.
flare star A dwarf ¤ M star that unpredictably brightens by a magnitude or more
for a few minutes. All ¬‚are stars have emission lines in their spectra and the
¬‚ares are thought to be outbursts of energy in the star™s ¤ chromosphere. Stellar
¬‚ares are similar to solar ¤ ¬‚ares but far more powerful. The nearest star to the
Sun, Proxima Centauri, is a ¬‚are star. Flare stars are also known as UV Ceti
flocculi A mottled pattern on the Sun visible when an image of the solar
¤ chromosphere is made that isolates the light emitted by calcium or hydrogen
Flora group A complex grouping of ¤ asteroids near the inner edge of the asteroid
belt at a distance of 2.2 AU from the Sun. The group is separated from the
main belt by one of the ¤ Kirkwood gaps and is not a true family with a common
flyby A space mission in which a spacecraft passes close to a target without landing
or going into orbit.
Fomalhaut (Alpha Piscis Austrini) The brightest star in the constellation Piscis
Austrinus. The name comes from Arabic and means ˜˜the ¬sh™s mouth.™™
Fomalhaut is an ¤ A star of magnitude 1.2 and is 25 light years away. Infrared
observations show it to be surrounded by a vast ring of dust, 370 astronomical
units in diameter, where planets may be forming.
Footprint Nebula ¤ Minkowski™s Footprint.

The dusty disk around the star
Fomalhaut imaged by the Spitzer
Space Telescope.

Fred Lawrence Whipple Observatory

forbidden lines ¤ Spectral lines, not normally observed in a laboratory situation.
Under astrophysical conditions, however, spectral lines ˜˜forbidden™™ in a
laboratory on Earth can sometimes be very strong.
fork mount A particular form of ¤ equatorial mount.
Fornax (The Furnace) A small, inconspicuous constellation of the southern sky. It
was introduced in the mid-eighteenth century by Nicolas L. de Lacaille with the
longer name, Fornax Chemica “ the chemical furnace. None of its stars are
brighter than fourth magnitude.
Fornax A A strong radio source in the constellation Fornax. It is associated with
the spiral galaxy NGC 1316.
fourth contact In an ¤ eclipse of the Sun, the point when the Moon ¬nally moves
clear of the Sun™s disk. In a lunar eclipse, fourth contact occurs when the
Moon leaves the full shadow (umbra) of the Earth. The term also describes the
similar stage in the progress of a ¤ transit or ¤ occultation.
Fra Mauro A lunar crater, 95 km (60 miles) in diameter, close to the Apollo 14
landing site. The geological formation sampled by the Apollo 14 astronauts was
named after it. That formation is part of the ejecta or debris excavated when an
impact formed the ¤ Imbrium Basin. ¤ Apollo program.
Francisco A small outer moon of Uranus, discovered in 2001. It is about 12 km
(7 miles) across.
Fraunhofer, Joseph von (1787“1826) Fraunhofer was an instrument-maker who
specialized in astronomical optics and investigated the solar spectrum. He was
born in Straubing, Germany. After serving an apprenticeship as a glazier in
Munich, he joined an optical institute specializing in optics of the highest
quality. He made the world™s ¬nest optical glass for telescopes. In 1813, while
researching the refractive properties of glass, he accidentally rediscovered the
dark lines in the solar spectrum, ¬rst noted by William Wollaston in 1802. He
measured the wavelengths of 574 ¤ Fraunhofer lines, introducing alphabetical
names, such as sodium D lines, for the most intense. He also invented the
spectroscope and the diffraction grating.
Fraunhofer lines The dark ¤ absorption lines in the spectrum of the Sun. Many of
the stronger ones were ¬rst recorded by Joseph von ¤ Fraunhofer, who also
labeled some of the most prominent ones with letters of the alphabet. Some of
these identifying letters are still commonly used in physics and astronomy,
notably the sodium D lines and the calcium H and K lines.
Fred Lawrence Whipple Observatory An observatory on Mount Hopkins in
Arizona, operated by the ¤ Harvard“Smithsonian Center for Astrophysics. The
instruments located there, at an altitude of 2600 m (8500 feet), include a
10-m (32-foot) optical re¬‚ector for gamma-ray astronomy and the recon¬gured
¤ Multiple Mirror Telescope (MMT), which is operated jointly with the University
of Arizona for optical and infrared observations. The observatory is

F star

named in honor of a former director of the ¤ Smithsonian Astrophysical
Observatory who was particularly well known and distinguished for his work on
F star A star of ¤ spectral type F. F stars have surface temperatures in the range
6000“7400 K. Their spectra feature strong absorption lines of ionized calcium.
There are also many medium-strength absorption lines due to iron and other
heavier elements. ¤ Procyon and ¤ Polaris are examples of F stars.
full Moon The ¤ phase of the Moon when its celestial ¤ longitude is 180 greater
than the Sun™s and its disk is fully illuminated.

Gaia hypothesis The idea that life on Earth regulates the composition of the
lower atmosphere. Gaia was a Greek Earth goddess.
galactic center The central region of our Galaxy, which is hidden from view by
dense concentrations of dust. A compact radio source called Sagittarius A*
appears to mark the very center of the Galaxy and is the zero point for the
system of ¤ galactic coordinates. It is believed to be a ¤ black hole of about three
million solar masses. The ¤ Chandra X-ray Observatory has observed frequent
X-ray ¬‚ares from Sagittarius A*. Around 10 light years from the center there is a
ring of gas and dust, rotating at about 110 km/s.
galactic coordinates A latitude and longitude coordinate system that takes the
¤ galactic plane as its equator and the ¤ galactic center (RA 17h 42.4m, Dec. “28
550 ) as the zero point of longitude measurement.

An infrared image of the galactic center from the Spitzer Space Telescope. The brightest white
spot in the middle is the very center of the galaxy, which also marks the site of a supermassive
black hole.

galactic halo

galactic halo A spherical region around a spiral galaxy. The halo around our own
Galaxy extends to about 50 000 light years from the center. The stars outside
the disk of the galaxy but within the halo are the oldest and their distribution
indicates the size and shape of the galaxy before much of it collapsed to a disk.
Galactic halos contains very hot gas that emits X-rays.
galactic plane A circle around the sky, dividing it into two equal hemispheres,
that passes through the ¤ galactic center and the densest parts of the ¤ Milky
Way. It is inclined to the celestial equator by about 63 .
galactic poles The points on the sky at latitudes 90 north and south in ¤ galactic
coordinates. The north galactic pole is in the constellation Coma Berenices and
the south pole is in Sculptor.
galactic wind A ¬‚ow of tenuous hot gas out of a galaxy. The temperature of the
gas is millions of degrees and it comes from regions where there is a high rate
of star formation.
galactic year (cosmic year) The time taken for the Sun to complete one orbit
around the galactic center, roughly 220 million years.
Galatea A satellite of Neptune discovered during the ¬‚yby of ¤ Voyager 2 in August
1989. Its diameter is 176 km (109 miles).
galaxy A large family of stars, often accompanied by clouds of gas and dust, kept
together by gravity.
Galaxies cover a huge range of size and mass. The smallest known are
relatively nearby dwarf galaxies containing only 100 000 stars. At the other end
of the scale, the most massive galaxy known, the giant elliptical M87, contains
3000 billion solar masses, and is about 15 times more massive than our own
¤ Galaxy.
The shapes of most galaxies can be described as ˜˜spiral,™™ ˜˜elliptical™™ or
˜˜irregular™™. Spiral galaxies are disk-shaped, with a central bulge, from which

An artist™s impression of our Galaxy
seen face on.


spiral arms appear to wind outwards. In barred spirals, a bar of stars
extends out from the bulge and the arms come from the ends of the bar. Spiral
galaxies contain very luminous young stars and have signi¬cant amounts of
interstellar material in their arms.
Most of the conspicuous galaxies in the sky are spirals, but ellipticals are
the most numerous. Both the smallest and largest galaxies are of this kind.
Most consist entirely of old stars with relatively little interstellar material. In
three dimensions they are spheroidal or sometimes nearly spherical.
Irregular galaxies form the third main group. These are neither spiral nor
elliptical and account for up to a quarter of all known galaxies. At visible
wavelengths, irregular galaxies show no particular circular symmetry and look
A very small number of galaxies have very unusual structure, often
because they have interacted with another galaxy. Active galaxies emit
exceptionally large amounts of energy and show evidence of violent processes
at work. Active galaxies include ¤ Seyfert galaxies and ¤ radio galaxies. ¤ Hubble
Galaxy When written with a capital initial G, the galaxy to which the Sun and
solar system belong, which is visible in the night sky as the ¤ Milky Way.
Our Galaxy is a barred ¤ spiral galaxy, containing of order 200 billion stars as
well as much interstellar matter, both dark and luminous. It is disk-shaped with
an almost spherical bulge in the middle, surrounding the ¤ galactic center. At the
very heart of the bulge is a supermassive black hole. The disk is 100 000 light
years across but much of its content is concentrated in a thin layer that is only
2000 light years thick towards its outer edges, though stars are distributed
through a somewhat thicker disk The central bulge has a radius of about 15 000
light years. The movements of the stars and interstellar material we can see
suggest that they account for as little as 10 percent of the total mass of the
Galaxy. The rest is ¤ dark matter, in a form not yet identi¬ed. This is distributed
in a sphere around the Galaxy extending out as far as 300 000 light years.
The spiral arms are concentrations of stars and interstellar matter winding
outwards from the edge of the bulge. Regions of star formation and ¤ ionized
hydrogen are located in the arms. In the space between the arms, the average
density of matter is a factor of 2 or 3 lower than within the arms. The Sun is
about 28 000 light years from the galactic center, within the disk, near the
inner edge of a spiral arm. The whole Galaxy is rotating, but not like a rigid
object. The Sun takes about 220 million years to complete a circuit, but stars
nearer the center take shorter times.
The disk of the Galaxy is surrounded by a sparsely populated, roughly
spherical region, known as the galactic halo. Its radius is at least 50 000 light
years. The halo contains the oldest stars in the Galaxy, including

galaxy cluster

The Galilean moons of Jupiter imaged by the Galileo spacecraft. From left to right, the moons
shown are Ganymede, Callisto, Io, and Europa.

¤ globular clusters. There is very little luminous matter in the halo compared
with the disk and central bulge.
galaxy cluster ¤ cluster of galaxies.
Galilean moons (Galilean satellites) The four largest moons of Jupiter, ¤ Io,
¤ Europa, ¤ Callisto and ¤ Ganymede, which were discovered by ¤ Galileo in 1610.
Galilean telescope A simple telescope of the kind used by ¤ Galileo in the ¬rst
astronomical telescopes. It consists of two lenses. A long-focus convex
(converging) lens acts as the objective lens and a single concave lens is used as
the eyepiece. The resulting image is the right way up.
Galileo (spacecraft) A NASA spacecraft sent to explore ¤ Jupiter, its rings and
satellites. It was launched in October 1989 from the ¤ Space Shuttle and arrived
at Jupiter in December 1995. It went into orbit around Jupiter and returned
high-resolution images and data for more than seven years. On the way to
Jupiter, it made close ¬‚ybys of the asteroids ¤ Gaspra and ¤ Ida.
A major disappointment was the failure of the craft™s high-gain
communications antenna to open properly. Relying only on a low-gain
antenna limited the amount of data that could be transmitted back to Earth.
Despite this, the mission was a great success overall. In 2003 it was deliberately
crashed into Jupiter.
A probe carried by Galileo separated from the craft and entered Jupiter™s
atmosphere on December 7, 1995. It parachuted down, returning data on the
composition and physical state of the atmosphere for 57 minutes.
Galileo Galilei (1564“1642) Galileo was an Italian physicist and astronomer who is
regarded as the principal founder of modern scienti¬c methods. Born in Pisa,
Tuscany, the oldest of seven children, he received private tuition. From 1581 he
studied for a medical career at the University of Pisa. His interest changed from
medicine to mathematics and he left the university in 1585 without a degree.
Nevertheless, four years later, his mathematical ability resulted in Pisa appointing
him professor of mathematics. In 1592 he moved to a professorship in Padua.

Galileo Galilei (1564“1642)

Galileo Galilei.

In 1610 he constructed a simple refracting ¤ telescope, and became the ¬rst
person to use such an instrument for astronomical observations. He was
astonished by the sheer numbers of stars, by the rough surface of the Moon,
and by ¤ sunspots. In January 1610 he discovered the ¤ Galilean moons of Jupiter.
A few months later he observed the phases of Venus. All of these discoveries
were at odds with the philosophy of ¤ Aristotle but were consistent with the
¤ Copernican system, which Galileo openly promoted in his writings and
High of¬cers of the Church supported Aristotelian philosophy as a matter
of faith. Pope Paul V summoned Galileo to Rome in 1616 to inform him that his
public promotion of the Copernican system must cease. In 1632, after the
election of a new Pope, Galileo wrote about his ideas in the form of a
conversation between three men in his book, Dialogue Concerning the Two Chief
World Systems. However, he presented the arguments in favor of a Sun-centered
solar system so persuasively that he was tried before the Inquisition in
Rome in 1633. Pope Urban VIII, a former friend of Galileo, threatened torture.
Galileo was forced to recant his ˜˜heresies™™ and lived in enforced isolation for
the rest of his life. In 1992 Pope John Paul II expressed regret for the unjust
treatment of Galileo.

Galileo National Telescope

Galileo National Telescope ¤ Telescopio Nazionale Galileo.
gamma-ray astronomy The study of gamma rays from astronomical sources.
Gamma rays are the most powerful form of ¤ electromagnetic radiation, with
wavelengths shorter than X-rays (less than about 0.1 nanometers). Though the
most powerful gamma rays can be detected at ground level, Earth™s
atmosphere absorbs the rest and nearly all astronomical gamma-ray
observations are made from satellites.
Many spacecraft have carried detectors for ¤ gamma-ray bursts since 1969.
Early sky surveys were carried out by the satellites SAS-2 and COS-B. SAS-2 was
launched in 1972 and operated for seven months. COS-B was launched in 1975
and operated for over six years. In 1997, observations by the Italian/Dutch
satellite BeppoSAX led to the ¬rst optical identi¬cation of an object that had
emitted a gamma-ray burst.
A great advance in gamma-ray astronomy was achieved with the launch by
NASA in April 1991 of the ¤ Compton Gamma Ray Observatory, which operated
until June 2000. Many new sources were identi¬ed with greater positional
accuracy than was possible previously. Its successor is the Gamma-Ray Large
Area Space Telescope (GLAST), to be launched by NASA in August 2007. The
European Space Agency™s ¤ International Gamma-Ray Astrophysics Laboratory
(INTEGRAL), with an expected lifetime of six years, started observations in 2002.
Astronomical sources of gamma rays include solar ¤ ¬‚ares, ¤ pulsars, ¤ X-ray
binary stars and ¤ quasars, but the nature of the most powerful sources, which
are responsible for gamma-ray bursts, remains uncertain. Strong identi¬able
sources of gamma-rays include the ¤ Vela pulsar, the ¤ Crab pulsar, ¤ SS433 and
¤ Geminga. There is also diffuse gamma radiation from the Milky Way.
gamma-ray burst A temporary intense burst of gamma rays and X-rays from a
cosmic source. All bursts are short and exceedingly powerful but there two
distinct varieties. Short bursts last a few tenths of a second while long burst go
on for between 2 and 20 seconds. Gamma-ray bursts were ¬rst discovered by
chance in the late 1960s by military satellites monitoring nuclear weapons
tests and have since been observed by a variety of spacecraft. The ¤ Compton
Gamma-Ray Observatory (CGRO) showed that bursts occur about twice a day, at
random positions all over the sky.
Though the CGRO could determine the positions of the bursts with greater
accuracy than previously possible, the positions were still not accurate enough
to allow optical identi¬cation. From 1997, however, the ¤ BeppoSAX satellite
was able to pinpoint the positions of gamma-ray bursts precisely enough for
them to be identi¬ed optically, and for radio emission to be detected. The ¬rst
one to have a spectrum recorded turned out to be about half way to the edge
of the observable universe. This showed that the energy given out by a gamma-ray
burst is immense “ over a million times more energy than a whole galaxy.


The visible and X-ray afterglows of the longer gamma-ray bursts have
similarities to ¤ supernovae. This has led astronomers to favor the idea that
longer bursts result from the collapse of very massive stellar cores, leading to
the formation of ¤ black holes. These events are also called ˜˜hypernovae.™™ Short
bursts are believed to come from the merger of two ¤ neutron stars, or a
neutron star and a black hole.
Gamma-ray Large Area Space Telescope (GLAST) A NASA orbiting gamma-ray
observatory, planned for launch in August 2007. It will provide a larger ¬eld of
view and better resolution than the ¤ Compton Gamma-Ray Observatory and is
expected to operate for 10 years.
gamma-ray pulsar A ¤ pulsar that emits gamma rays.
Ganymede One of the four large moons of Jupiter. Measuring 5262 km
(3270 miles) across, it is the largest natural satellite in the solar system.
The ¬rst high-resolution images of Ganymede were returned by
¤ Voyagers 1 and 2. Images showing even ¬ner detail were obtained by the
¤ Galileo spacecraft. It has several different types of terrain. There are dark,
heavily cratered areas and lighter grooved terrain covering around 60 percent
of the surface area that has been seen. Galileo images of the dark areas suggest
they have been changed by various episodes of shearing and furrowing. Galileo
also revealed many small craters on the ¬nely grooved areas.
One of the most signi¬cant discoveries made by the Galileo spacecraft was
that Ganymede has a signi¬cant magnetic ¬eld, which is stronger at its surface
than the ¬elds of Mercury, Venus or Mars. Data from Galileo™s trajectory,
combined with the magnetic, ¬eld, suggest that Ganymede must have a
molten iron-rich core. Overall, Ganymede™s density is about twice that of

A close-up of the surface of Ganymede
taken by Voyager 2.

Garnet Star

Asteroid Gaspra imaged by the Galileo spacecraft. The colors are exaggerated in this view.

water. Its core is probably surrounded by a rocky mantle, overlain by a thick
layer of ice.
Garnet Star The name William ¤ Herschel gave to the strikingly red star Mu
Cephei. It is a red ¤ supergiant and a ¤ semiregular variable ranging in magnitude
between 3.6 and 5.1.
gaseous nebula A glowing cloud of gas in interstellar space, which may be either
an ¤ emission nebula or a ¤ re¬‚ection nebula.
951 Gaspra An asteroid in the main asteroid belt imaged by the ¤ Galileo spacecraft,
which passed Gaspra at a distance of about 16 000 km (10 000 miles) on October
29, 1991. It was the ¬rst asteroid to be imaged from close by. It measures
about 20 · 12 · 11 km (12 · 7 · 7 miles) and the largest of many craters on its
surface is 1.5 km (1 mile) across. Gaspra was discovered on July 30, 1916, by the
Russian astronomer Grigory N. Neujmin and is a member of the ¤ Flora group.
Galileo detected a magnetic ¬eld, suggesting that it has a metallic composition.
Gassendi A lunar crater, 100 km (60 miles) in diameter, on the northern border of
the Mare Humorum. Clefts cross the crater ¬‚oor and it has multiple peaks.
Gassendi has been linked with ¤ lunar transient phenomena.
gegenschein ¤ zodiacal light.
Geminga A powerful gamma ray ¤ pulsar in the constellation Gemini. It is one of
the most intense sources of gamma rays in the sky. It was discovered in 1972 by
the orbiting observatory SAS 2 (¤ Small Astronomy Satellite). Weak X-rays from


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Eskimo Nebula
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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 Gemini.

Geminga were detected by the ¤ Einstein Observatory and its optical counterpart
looks like a 25th-magnitude star. Geminga emits almost all its energy as gamma
rays. Its X-ray emission is a thousand times weaker and its luminosity in visible
light a thousand times weaker again. Observations made by ¤ ROSAT con¬rmed
the X-ray emission and found that its radiation is pulsed, with a period of about a
quarter of a second. Geminga is 350 000 years old and lies 500 light years away.
Gemini (1) (The Twins) One of the twelve constellations of the ¤ zodiac. The two
brightest stars in Gemini, both ¬rst magnitude, have the names of the twins
Castor and Pollux of classical mythology. Pollux, though the brighter of the
pair, was given the designation Beta. Castor is Alpha.
Gemini (2) A series of manned, orbiting spacecraft, launched by the USA in the
1960s. They were an important part of the preparation for the ¤ Apollo program


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