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of Moon landings. Gemini 3 in 1965 was the ¬rst American ¬‚ight with a crew of
more than one astronaut, and Gemini 8 in March 1966 achieved the ¬rst
successful docking in space. The last of the series was Gemini 12 in November
1966. Many of the astronauts who later took part in the Apollo Moon landings
also ¬‚ew in the Gemini program.

Gemini Telescopes

The enclosure of one of the Gemini Telescopes, located in Chile.

Gemini Telescopes Two 8-m telescopes for optical and infrared astronomy.
One is sited in the northern hemisphere, at the ¤ Mauna Kea Observatories in
Hawaii, while the other is in the southern hemisphere, on Cerro Pachon in
Chile, near to the ¤ Cerro Tololo Inter-American Observatory. They are operated
by an international consortium. The sites of the two telescopes were
chosen so that they can see the whole sky between them. The Hawaii
telescope was completed during 1998 and its more southerly twin
in 2000.
Geminids A major annual ¤ meteor shower, the radiant of which lies near the star
Castor in the constellation Gemini. The shower peaks around December 13,
and its normal limits are December 7“16. The meteor stream responsible has
an unusual orbit and in 1983 the ¤ Infrared Astronomical Satellite (IRAS)
discovered in the same orbit a cometary nucleus that is no longer active but
probably generated this stream in the past. It was named 3200 Phaethon.
Gemma An alternative name for the star ¤ Alphekka.
general relativity A theory of gravitation, published in its ¬nal form in 1916. It was
developed by Albert ¤ Einstein from his earlier (1905) ¤ special relativity theory.
Genesis A NASA space mission that collected samples of the ¤ solar wind and
returned them to Earth. It was launched on August 8, 2001 and spent 845 days
orbiting the Sun 1.5 million km (1 million miles) closer to the Sun than Earth.
Capsules containing samples of the solar wind amounting to about 0.4
milligrams were returned to Earth on August 9, 2004. The capsules crashed
when their parachutes failed to open but most of the samples were salvaged.
geomagnetic field The magnetic ¬eld in the vicinity of the Earth. At present,
Earth™s magnetic ¬eld is roughly like that of a bar magnet displaced 451 km


(280 miles) from the center of the Earth towards the Paci¬c Ocean and tilted to
Earth™s axis by an angle of 11 . The strength and shape of the geomagnetic ¬eld
varies gradually over a timescale of years.
geomagnetic storm ¤ magnetic storm.
geospace The region of space around Earth, inluding Earth™s ¤ magnetosphere and
¤ ionosphere.
geostationary orbit ¤ geosynchronous orbit.
geosynchronous orbit An orbit around Earth in which a satellite™s period of
revolution is exactly a ¤ sidereal day (23 hours 56 minutes 4.1 seconds). If a
satellite is in a circular geosynchronous orbit over the equator, it always stays
very close to the same position in the sky and its orbit is described as
geostationary. A geostationary orbit is at an altitude of 35 900 km
(22 300 miles). A satellite in a geosynchronous orbit inclined to Earth™s equator
traces out a ¬gure-of-eight shape over the course of a day.
German mount A type of ¤ equatorial mount for a telescope.
Ghost of Jupiter A popular name for NGC 3242, a ¤ planetary nebula in Hydra.
Giant Magellan Telescope A telescope to be sited in northern Chile consisting of
seven 8.4-m (27-foot) mirrors arranged in a circle. It will be equivalent to a 22-m
(72-foot) telescope. Work on the mirrors began in 2005 and the telescope is
expected to be completed in about 2016.
Giant Metrewave Radio Telescope (GMRT) A ¤ radio telescope located near Poona
in India. It consists of thirty 45-m (146-foot) dishes arranged in an array
extending for 25 km (16 miles) and is the most powerful telescope for ¤ radio
astronomy at meter wavelengths.
giant molecular cloud ¤ molecular cloud.
giant planet Jupiter, Saturn, Uranus or Neptune, or an ¤ extrasolar planet of similar
size or larger.
giant star A star between 10 and 1000 times more luminous than the Sun, and
between 10 and 100 times larger.
Stars that are not giants to begin with become giants when they run out of
hydrogen fuel for nuclear fusion in their cores. Their outer layers expand
greatly and their surface temperature drops, but their overall luminosity rises
because they are so much larger. Massive hot stars, which are much larger than
the Sun even when they ¬rst form, are also referred to as giants.
¤ Hertzsprung“Russell diagram, red giant, stellar evolution.
gibbous A ¤ phase of the Moon, or any other astronomical object, when it is
between half and full.
Giotto A European Space Agency spacecraft that ¬‚ew within 605 km (376 miles) of
the nucleus of Comet ¤ Halley in March 1986. It was the ¬rst spacecraft to
make a close encounter with a comet. Images, including close-ups of the
nucleus, were returned but the camera was subsequently destroyed by


impacts. In 1992, Giotto was reactivated for an encounter with Comet 26P/
ESA named the mission after the artist Giotto di Bondone, who is thought
to have used the 1301 appearance of Halley™s Comet as a model for the star of
Bethlehem in his fresco, The Adoration of the Magi, which he painted in 1303 in
the Scrovegni chapel in Padua.
GLAST ¤ Gamma-ray Large Area Space Telescope.
glitch A sudden change in the rotation rate of a ¤ pulsar. Glitches are thought to be
caused by ¤ starquakes.
globular cluster A roughly spherical cluster of hundreds of thousands “ or even
millions “ of stars. The globular clusters in our Galaxy contain some of its
oldest stars and are distributed within the ¤ galactic halo. These old stars
contain only small amounts of the elements heavier than helium because they
formed from the original material of the Galaxy, before the interstellar
medium had been enriched by heavier elements created inside stars. Globular
clusters have also been identi¬ed in other galaxies.

The globular cluster M80 in the constellation Scorpius as seen by the Hubble Space
Telescope. It is about 28 000 light years away.

Gran Telescopio Canarias

Globules 5900 light years away in the constellation Centaurus imaged by the Hubble Space

globule A small cloud of dark opaque gas. Globules often show up against a bright
background of star clouds or a glowing nebula. Stars form in globules that
become dense enough. The name of the Dutch“American astronomer, Bart Bok
(1906“83), is associated with small globules, known as Bok globules, which
may be only a few thousand astronomical units across.
gnomon A rod or plate mounted vertically to form a shadow-stick, such as on a
sundial. The altitude of the Sun can be calculated from the height of
the rod and the length of its shadow. The direction of the shadow gives the
¤ apparent solar time.
Goddard Space Flight Center (GSFC) A large NASA establishment in Greenbelt,
Maryland, 10 miles north-east of Washington, DC. The work done there
includes astronomical research and the design, development and management
of near-Earth orbiting spacecraft.
Gossamer ring The outermost of Jupiter™s three known rings. ¤ ring systems.
Gould™s Belt A formation of many of the brightest, most conspicuous stars, which
appear to lie in a band around the sky, tilted at 16 to the plane of the ¤ Milky
Way. The belt includes the bright stars of Orion and Taurus in the northern
hemisphere and those of Lupus and Centaurus in the southern hemisphere. It
was ¬rst noted in 1847 by Sir John ¤ Herschel and later studied by the American
astronomer Benjamin A. Gould (1824“96). It is thought to be a spur branching
off the nearest spiral arm (the Orion arm) of the ¤ Galaxy.
Gran Telescopio Canarias A Spanish 10-m telescope at the ¤ Observatorio del
Roque de los Muchachos in the Canary Islands. Its mirror consists of 36 hexagonal
parts (similar to the telescopes of the ¤ Keck Observatory) and it can be used for
both optical and infrared observations. It began operation in 2005.


The granulation pattern on the Sun can be seen around these sunspots.

granulation A cell-like pattern observed in high-resolution images of the Sun™s
¤ photosphere. It is caused by gas rising from deeper, hotter layers. Individual
granules measure up to 1000 km (620 miles). ¤ supergranulation.
gravitation The attractive force between masses. According to the theory
formulated by Isaac ¤ Newton, the force between two masses is proportional to
their product divided by the square of the distance between them. In ¤ general
relativity, gravitation is viewed as curvature of ¤ spacetime.
gravitational collapse The sudden collapse of a massive star when the internal
pressure pushing outwards falls so that it cannot balance the weight of
material pressing inward. The gravitational collapse of a massive star
that has reached the end of its life is very sudden and catastrophic, perhaps
taking less than a second. The enormous energy released triggers a ¤ supernova
explosion, and the core of the collapsed star becomes a ¤ neutron star or
¤ black hole.
gravitational lens A massive object, such as a galaxy, that distorts the appearance
of more distant galaxies behind it. A gravitational lens bends the path of light
through space just as a glass lens bends light rays by refraction when they go
through the lens. Gravitational lenses can produce multiple images of
¤ quasars and distort the appearance of a cluster of galaxies so that it looks like
a pattern of arcs. Natural magni¬cation by a gravitational lens sometimes

Great Dark Spot

A cluster of galaxies (Abell 2218) 2 billion light years away acts as a gravitational lens,
magnifying and distorting into arcs another galaxy cluster lying 5“10 times farther away but
in the same line of site. This image was taken with the Hubble Space Telescope.

makes it possible to obtain detailed spectra of remote objects that would
otherwise have been too faint.
On a smaller scale, ¤ microlensing is sometimes seen when a small dark
object, such as a planet, crosses our line of sight to a more distant star.
¤ Einstein ring.
gravitational redshift A ¤ redshift of the light from a massive object caused by
the strong gravitational ¬eld.
gravitational waves Ripples in the structure of ¤ spacetime traveling at the speed of
light. According to general relativity, gravitational radiation is emitted in
certain extreme events, such as when the core of a star collapses and when
matter falls into a black hole. It is very dif¬cult to detect and up to 2005 had not
been detected though astronomer believed they were close to doing so.
gravity ¤ gravitation.
gravity assist Using the gravity of a planet to change the speed and direction of a
spacecraft without consuming any fuel.
grazing occultation An ¤ occultation in which the two objects involved appear
just to touch.
Great Attractor A concentration of galaxies, containing perhaps 5 · 1016 solar
masses of matter, that lies roughly 150“350 million light years from our Galaxy
in the direction of the constellations Hydra and Centaurus. Its gravity is
thought to be causing the motion of galaxies to deviate in its direction.
Great Dark Spot An oval feature on the planet ¤ Neptune, discovered on the
images returned by the ¤ Voyager 2 spacecraft in 1989. It was a storm system in

Great Nebula in Orion

Neptune™s Great Dark Spot as observed
by the Voyager 2 spacecraft.

Neptune™s cloud layers, similar to the ¤ Great Red Spot on Jupiter, but not so
long lived. It had disappeared in 1994 when the Hubble Space Telescope began
to take high-resolution pictures of Neptune. The spot™s largest dimension was
about the same as Earth™s diameter (approximately 12 000 km or 7500 miles),
making it about half the size of the Great Red Spot.
Great Nebula in Orion ¤ Orion Nebula.
Great Red Spot A large, red, oval spot on ¤ Jupiter, 24 000 km (15 000 miles) long
and 11 000 km (7000 miles) wide. It was first reported by Robert Hooke in 1664
and has been observed ever since, through it has varied in size and color. The
reason for its existence remains uncertain but it rotates like a giant anticyclone,
with a westerly wind on its northern edge and an easterly wind to the south.
Great Rift A dark streak in the ¤ Milky Way in the constellations Cygnus and
Aquila. It is due to dust, which hides the light of more distant stars.
Green Bank The location in West Virginia, USA, of a radio astronomy observatory
belonging to the ¤ National Radio Astronomy Observatory. The 92-m (300-foot)
dish built in 1962 collapsed in 1988. Its successor, the 100-m (325-foot)
Robert C. Byrd Green Bank Telescope, was completed in 2000. It is the largest
fully steerable dish in the world. The 43-m (140-foot) dish at Green
Bank, completed in 1965, is the largest telescope in the world on an
¤ equatorial mount. There is also a radio interferometer on the site, consisting of
three 26-m (85-foot) dishes, two of which can be moved along a track 1.6 km
(1 mile) long.
green flash A phenomenon sometimes observed at the moment of sunset over a
clear horizon, especially over the sea. Refraction by the Earth™s atmosphere
makes the last fragment of the Sun to sink below the horizon appear to break
free and flash momentarily green before disappearing.

greenhouse effect

Changes to Jupiter™s Great Red Spot observed by the Hubble Space Telescope between
1992 and 1999.

greenhouse effect Heating in the atmosphere of a planet because infrared
radiation cannot escape. A greenhouse works in a very similar way, with glass
playing the same role as the atmosphere.
The primary source of heat for a planet™s surface and atmosphere is energy
radiated from the Sun in the visible and near-infrared parts of the spectrum.
However, longer-wavelength infrared radiation emitted by the warm planet
cannot travel back out into space. It is trapped in the atmosphere, causing the
planet™s temperature to be higher than it would otherwise be. On Earth, the
increase in temperature amounts to about 33 K. On Venus, a ˜˜runaway™™
greenhouse effect raises the temperature by 500 K. Mars is warmed by a
modest 5 K.
The amount of heating the greenhouse effect causes depends on the
composition of the atmosphere. Carbon dioxide is one of the main greenhouse
gases, but water vapor and rarer gases also play a part.

Greenwich Observatory

The Green Bank Telescope.

Greenwich Observatory ¤ Royal Observatory, Greenwich.
Gregorian calendar The civil calendar now in use in most countries It was
introduced by Pope Gregory XIII in 1582 to replace the ¤ Julian calendar.
A workable civil calendar needs to be organized so that the seasons remain
in step with the months of the year. This is a problem because the time taken
by the Earth to orbit the Sun is not a whole number of days. The introduction
of an extra day every fourth year improves matters, but more adjustments are
necessary if the calendar is to stay synchronized with the seasons over
In the Gregorian system, years exactly divisible by four are leap years,
except that century years must be exactly divisible by 400 to be leap years.
Thus 2000 was a leap year, but 1900 was not. Averaged over 400 years, the rule
gives an average year length of 365.2425, which is close to the true length of
the ¤ year, 365.2422 days.
The Gregorian calendar came into effect in Roman Catholic countries in
October 1582, when the seasons were brought back into step by eliminating 10
days from the calendar. Thursday October 4 was followed by Friday October 15.
Also, on the introduction of the Gregorian system, the new year began on
January 1 for the ¬rst time, instead of March 25. Britain and its colonies did not
introduce the Gregorian calendar until September 1752, by which time an
11-day correction was needed.
Gregorian telescope A type of reflecting telescope proposed by James Gregory
(1638“75) in 1663. The primary mirror is a paraboloid with a central hole and
the secondary mirror is also curved. Gregory was unable to obtain mirrors

Gum Nebula

shaped accurately enough to construct his telescope before Isaac ¤ Newton
made the first working reflector to a simpler design with a flat secondary.
Grimaldi A large lunar crater, 222 km (138 miles) in diameter, situated near the
western limb of the Moon on the border of the Oceanus Procellarum.
Grus (The Crane) A small southern constellation, introduced in the sixteenth
century and included by Johann Bayer in his 1603 atlas ¤ Uranometria. It
contains four stars brighter than fourth magnitude. Delta Gruis is a double star
that can be seen as double by the unaided eye.
G star A star of ¤ spectral type G. G stars have temperatures in the range
4900“6000 K and are yellow in color. Their spectra contain many absorption
lines. The Sun is a typical dwarf G star. ¤ Capella is an example of a giant G star.
Guardians The two stars Beta and Gamma in the constellation ¤ Ursa Minor.
guest star Used by Chinese astronomers in historical times to denote the
appearance of a ¤ nova, ¤ supernova or ¤ comet.
guide star A star on which the manual or automatic guidance system of a
telescope can be locked so that a fainter object being observed is correctly
followed as the Earth rotates.
Gum Nebula A large, circular ¤ emission nebula in the southern constellations Vela
and Puppis. It was discovered by an Australian astronomer, Colin Gum
(1924“60). The nebula stretches across 36 of sky. Its diameter is 800 light years
and it is 1300 light years away. It is thought to be the result of a ¤ supernova
explosion about a million years ago.

Hadar (Beta Centauri; Agena) The second-brightest star in the constellation
Centaurus. It is a giant ¤ B star of magnitude 0.6 and is 335 light years away.
Hadley Rille A sinuous channel on the Moon, running across Palus Putredinis. It is
close to the landing site of the Apollo 15 mission and is believed to be a
collapsed lava tube. ¤ Apollo program, rille.
Hale“Bopp, Comet One of the brighter comets of the twentieth century.
Discovered by Alan Hale and Thomas Bopp on July 22, 1995, it reached
perihelion on April 1, 1997 and at its brightest was magnitude “1. Its nucleus
was estimated to be 40 km (25 miles) across, over twice the diameter of Comet
¤ Halley.
Hale, George Ellery (1868“1938) Hale is a key figure in twentieth century
astronomy who foresaw that the development of astronomy required much
larger telescopes. Born in Chicago, the son of a wealthy engineer, Hale took an
early interest in instrument design. While an undergraduate at the
Massachusetts Institute of Technology, he invented the ¤ spectroheliograph.

Hadley Rille photographed from orbit by the crew of Apollo 15.

Halley, Comet

Comet Hale“Bopp imaged in June 1999 by the European Southern Observatory™s
3.5-m New Technology Telescope.

In 1892 he joined the astrophysics staff of the University of Chicago, where he
established the ¤ Yerkes Observatory and its 40-inch refractor. To improve
observational astronomy, he founded the ¤ Mount Wilson Observatory and equipped
it with 60-inch and 100-inch reflectors. He resigned as director in 1923 and devoted
the remainder of his life to raising funds for and planning the construction of
the Palomar 200-inch telescope, now known as the ¤ Hale Telescope.
Hale Telescope The 5-m (200-inch) reflecting telescope at the ¤ Palomar
Observatory. Work to construct the telescope began in the 1930s following the
award of a grant to the California Institute of Technology from the Rockefeller
Foundation. Completion was delayed by World War II. It was officially opened
in 1948 and dedicated to the memory of George Ellery ¤ Hale (1868“1938)
who had been the driving force behind the project.
Hall, Asaph (1829“1907) Hall was a self-taught America astronomer from
Connecticut. He became an assistant at Harvard College Observatory in 1857
and moved to the US Naval Observatory in 1863. At the 1877 ¤ opposition of
Mars he discovered and named ¤ Phobos and ¤ Deimos, the two moons of Mars.
Halley, Comet The most famous of all periodic comets. It travels in an elongated
elliptical orbit around the Sun, returning to the inner solar system every 76 years.
Historical records show that Comet Halley has been observed for over 2200 years.

Halley, Edmond (1656“1742)

The nucleus of Comet Halley as seen by the Giotto spacecraft.

Edmond ¤ Halley, in whose honor the comet is named, did not discover it
but he was the first person to realize the connection between the comet he saw
in 1682 and certain other recorded appearances of comets separated by
intervals of 76 years. He calculated the orbits of a number of comets, using
Isaac Newton™s newly published theory of gravitation. Noticing the similarity
between the orbits of the comets seen in 1531, 1607 and 1682, he deduced that
they were apparitions of one and the same comet and went on to predict that it
would return in 1758“59. The comet duly appeared, as he had predicted.
Comet Halley™s ¤ perihelion is 0.59 AU from the Sun, between the orbits
of Mercury and Venus. At its most distant, the comet travels beyond the orbit
of Neptune. Its orbit is inclined to the main plane of the solar system at an
angle of 162 and it travels around its orbit in the direction opposite to the
motion of the planets.
On its return in 1986, Comet Halley was never located where it could be
observed well from Earth but several countries launched spacecraft to
investigate it, with considerable success. The closest approach was made by the
European craft ¤ Giotto.
The results showed that the comet has a solid nucleus made of ice and
dust. Its shape is irregular and elongated and it measures 16 · 8 km (9 · 5 miles).
It is very dark, reflecting only 4 percent of the sunlight falling upon it. The
nucleus rotates slowly “ once in 7.1 days. On the side facing the Sun,
temperatures as high as 350 K were measured, enough to melt ice, and jets
of escaping material were observed.
Two meteor showers, the ¤ Eta Aquarids and the ¤ Orionids, are associated
with Comet Halley.
Halley, Edmond (1656“1742) Halley made enormous contributions to almost
every branch of physics and astronomy. Born in London, he was the son of a
wealthy merchant. He studied at Oxford, where he demonstrated outstanding

Halley, Edmond (1656“1742)

Edmond Halley

mathematical talent. His scientific career began in 1676 with two years on the
island of St Helena, mapping the southern skies. His catalog of southern stars
earned him election to the Royal Society.
Halley became interested in gravity, and particularly in a mathematical
proof of ¤ Kepler™s laws. When he discussed this with Isaac ¤ Newton in 1684, he
found that Newton had solved the problem but had not published it. Halley
personally financed the publication of Newton™s Principia Mathematica (1687).
His knowledge of geometry and historical astronomy enabled him to deduce
that comets recorded in 1456, 1531, 1607, and 1682 were in fact one and the
same comet. He correctly predicted that it would return in 1758. In 1720 he
succeeded ¤ Flamsteed as ¤ Astronomer Royal and devoted himself to lunar


The double star cluster h and chi Persei.

Hamal (Alpha Arietis) The brightest star in the constellation Aries. It is a giant
¤ K star of magnitude 2.0 and is 66 light years away. Its name comes from the
Arabic for ˜˜sheep.™™
h and chi () Persei (Double Cluster in Perseus; NGC 869 and 884) A pair of open
star clusters in the constellation Perseus. They are visible to the naked eye as
faint hazy patches. Names of this kind were normally allocated to individual
stars, and were given to these clusters before their true nature was known. The
two clusters are very similar in appearance and are less than one degree apart
in the sky. They are 7100 light years away and estimated to be only 50 light
years apart. ¤ open cluster.
Harpalyke A small outer moon of Jupiter discovered in 2000. Its diameter is about
4 km (2.5 miles).
Harvard College Observatory The observatory of Harvard College, established in
1839. In 1847, it was equipped with a 0.38-m (15-inch) refracting telescope,
which is still in its original building, the Sears Tower on Observatory Hill. In
1973, the Harvard“Smithsonian Center for Astrophysics was formed by merging
the ¤ Smithsonian Astrophysical Observatory and Harvard College Observatory.
Harvard“Smithsonian Center for Astrophysics ¤ Harvard College Observatory.
harvest Moon The full Moon nearest the time of the autumnal ¤ equinox. At this
time of the year, the Moon™s path is not inclined very steeply to the horizon and
the Moon rises at approximately the same time each evening for a short period.

heliocentric model

Hayabusa A Japanese mission to explore the asteroid 25143 Itokawa and bring a
sample of material from it back to Earth. Launched in May 2003, the spacecraft
reached its target in November 2005 but a small lander called Minerva that
should have investigated the surface was lost after being released from
the main spacecraft. However, Hayabusa did collect samples despite
some technical difficulties and they are due to be returned to Earth by
June 2010.
6 Hebe The sixth asteroid in order of discovery. Its diameter is 204 km (127 miles)
and it was first seen in 1847 by Karl L. Hencke.
Hegemone A small outer moon of Jupiter discovered in 2003. Its diameter is about
3 km (2 miles).
Heinrich Hertz Submillimeter Telescope A 10-m telescope at the ¤ Mount
Graham International Observatory. It operates in the submillimeter waveband,
between 0.3 and 1 mm. The first observations were made with it in 1994. It is
operated jointly by the University of Arizona and the Max-Planck-Institut fur ¨
Radioastronomie in Bonn, Germany.
624 Hektor The largest ¤ Trojan asteroid in the same orbit as Jupiter. It was
discovered by August Kopff in 1907. It rotates in just under 7 hours and its
brightness varies by a factor of 3 as it does so. The way its light varies indicates
that Hektor is roughly cylindrical in shape, measuring 195 km (121 miles) wide
by 370 km (230 miles) long. It has been suggested that Hektor may in fact be
two asteroids in contact or a close binary.
Helene A small, irregularly shaped satellite of Saturn discovered in 1980. It
measures about 36 · 32 · 30 km (22 · 19 · 19 miles) and orbits at a distance of
377 400 km (234 505 miles).
heliacal rising The rising of a bright star just before sunrise. Since stars rise
about 4 minutes earlier each day, a star™s heliacal rising marks the start of a
period of several months during which it can be seen for at least part of the
night. The heliacal rising of Sirius signaled to the ancient Egyptians the season
when the River Nile flooded.
Helike A small outer moon of Jupiter discovered in 2003. Its diameter is about
4 km (2.5 miles).
heliocentric model The concept of the solar system with the Sun at the center
and the planets in orbit around it. It was suggested as early as c. 200 bc by
¤ Aristarchus of Samos but the idea that Earth is moving was philosophically
unacceptable until centuries later. An Earth-centered system, refined by
¤ Ptolemy (c. ad 100“170), was in general use until the work of ¤ Copernicus
(1473“1543). Over this time, the idea that the Earth was the center of the
created universe became strongly rooted in religious dogma.
In his book De Revolutionibus, Copernicus argued in favor of considering the
solar system as Sun-centered. However, the idea did not gain general


acceptance until ¤ Galileo (1564“1642) and ¤ Kepler (1571“1630) made
observations that could only be explained sensibly by a heliocentric system.
Copernicus presumed that the planetary orbits are circular. Because of
this, he was no more successful than Ptolemy at predicting planetary positions,
though his theory was more elegant and provided a natural explanation
for the ¤ retrograde motion of the planets. Kepler solved this problem for the
heliocentric theory when he discovered that the planetary orbits are elliptical
rather than circular.
helioseismology The study of natural oscillations propagating through the
Sun. These oscillations are detected by the ¤ Doppler shifts they cause in the
¤ absorption line spectrum of the Sun and they reveal information about the
Sun™s interior structure.
heliosphere The spherical region of space around the Sun, extending out to
between 50 and 100 AU. Its outer boundary is where the ¤ solar wind merges
with the ¤ interstellar medium. This boundary region is called the heliopause.
heliostat A movable flat mirror used to reflect sunlight continuously into a fixed
solar telescope. A heliostat follows the motion of the Sun across the sky but
produces an image that slowly rotates during the course of a day.
Helix Galaxy A popular name for the ¤ spiral galaxy NGC 2685 in the
constellation Leo.
Helix Nebula (NGC 7293) A large, ring-shaped ¤ planetary nebula in the
constellation Aquarius. Its apparent diameter is a quarter of a degree (half the
size of the full Moon). At a distance of about 500 light years, it is the nearest
planetary nebula.
Hellas Planitia An almost circular impact basin on the surface of Mars. Hellas
Planitia is 1800 km (1100 miles) in diameter and is conspicuous even in a small
telescope because it is lighter in color than the areas surrounding it. It was
formerly known simply as Hellas.

An ultraviolet image of the Helix
Nebula from the Galex spacecraft.

Henry Draper Catalog

A view near the north-eastern edge of
Hellas Planitia from Mars Express.

Henry Draper Catalog (HD Catalog) A catalog of stellar spectra, compiled at
Harvard College Observatory. Its production was financed by a donation from
the widow of Henry ¤ Draper (1837“82), a pioneering astrophysicist, and the
catalog was named as a memorial to him. Under the direction of Edward C.
Pickering (1846“1919), Annie Jump ¤ Cannon (1863“1941) classified the
¤ spectral type of most of the 225 300 stars in the nine-volume catalog between
1911 and 1915, though the first volume was not ready for publication until
1918 and the ninth did not appear until 1924.

Herbig“Haro object

The Herbig“Haro object HH 46/47,
formed by a low-mass protostar
ejecting a jet and creating a bipolar
out¬‚ow. This infrared image is from
the Spitzer Space Telescope.

Herbig“Haro object (HH object) A type of peculiar nebula associated with newly
forming stars. The first three were discovered on images of the nebula NGC 1999
in Orion in 1946“47 by the American astronomer George Herbig and the
Mexican Guillamero Haro. Many more similar objects have since been identified.
Herbig“Haro objects result when a powerful ¤ bipolar outflow from a newly
forming star heats and compresses the surrounding interstellar gas. They
are typically between 500 and 4000 AU in size but only 0.5“30 Earth masses,
making them among the least massive objects detected outside the solar
system. They move very quickly and, in many cases, their motion can be traced
back to ¤ T Tauri stars or other young stars. ¤ Hubble™s Variable Nebula is an
example of an HH object.
Hercules A large constellation of the northern sky named after the hero of
classical mythology. It contains no first magnitude stars but the brightest
¤ globular cluster in the northern hemisphere, M13, lies in Hercules.
Hercules A The strongest radio source in the constellation Hercules and the third
most powerful in the northern hemisphere of the sky. It is also known as 3C 348
and is associated with a large, elliptical ¤ cD galaxy at the center of a cluster. Two
long jets extend for half a million light years into space from the faint nucleus.
Hercules X-1 An X-ray ¤ pulsar in the constellation Hercules. The pulsar is a
rotating neutron star in a binary star system that is drawing in gas from its
companion. The rotation period of the neutron star is 1.2 seconds and the
orbital period of the system 1.7 days.
Hermes An asteroid discovered by Karl Reinmuth in 1937 when it passed within
800 000 km (500 000 miles) of the Earth, in what was at the time the closest
approach of an asteroid ever recorded. Hermes reached eighth magnitude and
traveled across the sky at a rate of 5 per hour. However, after only five days, it
was lost when its brightness fell to 23rd magnitude because its night side was
facing Earth. It was recovered on October 15, 2003, by Brian Skiff of the Lowell

Herschel, Caroline Lucretia (1750“1848)

19h 18h 17h 16h
+50º +50º


ι •
M92 30
+40º +40º


ζ +30º





19h 18h 17h WIL TIRION

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 Hercules.

Hermippe A small outer moon of Jupiter discovered in 2001. Its diameter is about
4 km (2.5 miles).
Herschel A spectacular impact crater on Saturn™s moon ¤ Mimas. It is named in
honor of William ¤ Herschel, who discovered Mimas. Herschel™s diameter is
130 km (80 miles), one-third the diameter of Mimas. It is 10 km (6 miles) deep
and has at its center a peak that rises 6 km (4 miles) above the crater floor. It
is likely that the impact that created Herschel very nearly shattered Mimas,
evidenced by fractures on the opposite side of the moon.
Herschel, Caroline Lucretia (1750“1848) Caroline was the sister of William
¤ Herschel. She was born in Hanover, but joined her brother in England in 1772.
She became his able observing assistant, and then branched out into her own
research, discovering eight comets and many nebulae. She prepared a catalog

Herschel, Sir John Frederick William (1792“1871)

of nebulae and star clusters for which she received the Gold Medal of the Royal
Astronomical Society in 1828.
Herschel, Sir John Frederick William (1792“1871) John was the son of William
¤ Herschel. He studied mathematics at Cambridge University, and began to
assist his father with observations from 1816. In 1834 he went to the
observatory at the Cape of Good Hope to survey the southern skies where he
discovered 2000 nebulae and 2000 double stars before returning to England in
1838. He later became interested in photography and pioneered its use in
Herschel, Sir (Frederick) William (1738“1822) William Herschel, the discoverer
of the planet Uranus, was the greatest observational astronomer of the
eighteenth century. He was born in Hanover, then moved to England in 1757,
where he worked initially as a musician in Bath. His interest in astronomy
developed from 1773, when he started telescope making. His early
observations came to the attention of King George III, a strong supporter of
scientific enquiry, who employed him on a salary of £200 a year and financed
the construction of large telescopes.
Herschel took full advantage of his royal pension, which enabled him to
spend very long hours making astronomical observations. He discovered
Uranus in 1781, which brought him worldwide fame, and two moons of
Uranus in 1787. His sky surveys are notable for their precision and
comprehensiveness. He cataloged 2000 nebulae and 800 double stars. Herschel
made about 400 telescopes, culminating in the enormous 40-foot (12-m)
reflector, with which he discovered two moons of Saturn, ¤ Mimas and
¤ Enceladus. His other scientific achievements include discovering infrared
radiation, proving that some apparently double stars are true ¤ binary stars,
demonstrating from ¤ proper motion studies that the solar system is moving
through space. He also invented the term ¤ asteroid.
Herschel Space Observatory A European Space Agency 3.5-m space telescope for
infrared and submillimeter-wave observations. It will detect radiation in the
wavelength band from 100 mm to 1 mm. Launch is scheduled for May 2008
and it is expected to operate for about 3 years located about 1.5 million km
(1 million miles) from Earth in the direction opposite to the Sun. Its mirror
will be the largest ever deployed in space.
Hertzsprung, Ejnar (1873“1967) Hertzsprung discovered the main classes into
which most stars can be grouped according to their overall luminosity: the
bright and relatively scarce ¤ giant stars and ¤ supergiants, and the more
common, fainter, ˜˜dwarfs.™™ Born in Frederiksberg, Denmark, he trained as a
photochemist, and used that experience to devise ways of determining the
luminosity of stars. Unfortunately, he published his principal results,
connecting the colors and luminosities of stars in obscure photographic

Hertzsprung“Russell diagram

A Hertzsprung“Russell diagram made by plotting accurate data on 20 853 stars acquired
by the ¤ Hipparcos satellite.

journals, so credit for the discovery initially went to Henry Norris ¤ Russell,
who independently published essentially the same result in 1913. What later
became known as the ¤ Hertzsprung“Russell diagram has been of immense
importance in the study of stellar evolution.
Hertzsprung also made the first measurement of an extragalactic distance,
using ¤ Cepheid variable stars to find the distance to the Small ¤ Magellanic
Hertzsprung“Russell diagram (HR diagram) A graph of the relationship
between the ¤ spectral types and luminosities of a selection of stars. Spectral
type goes along the horizontal axis, with the hottest stars at the left.
Luminosity is plotted along the vertical axis. Color, temperature or some
other quantity related to spectral type is sometimes plotted along the

Hevelius, Johannes (1611“1687)

horizontal axis instead. Either ¤ magnitude or luminosity relative to the
Sun are frequently used for the vertical scale. The plot may be called a
color“magnitude diagram or color“luminosity diagram, depending on the
actual quantities used.
What is now known as a Hertzsprung“Russell diagram was first plotted by
Henry Norris ¤ Russell in 1913. It was later recognized that Ejnar ¤ Hertzsprung
had independently put forward similar ideas at around the same time.
Any star whose spectral type and luminosity are known may be plotted as
a single point on the HR diagram, but the diagram acquires particular
significance when the data for a related group of stars, such as a star cluster,
are plotted on it. Whatever sample of stars is chosen, the points are not
distributed randomly. The points corresponding to most stars lie on a band
running diagonally from the upper left to the lower right, which is known as
the main sequence. The main sequence is essentially a mass sequence, with the
most massive stars at the upper left and the least massive at the lower right.
Most stars spend 90 percent of their lives on the main sequence.
Main-sequence stars generate their nuclear energy by converting hydrogen to
helium in their cores. The effects of advancing age move stars away from
the main sequence. Evolved stars are found in the giant and supergiant
branches lying above the main sequence. The highly evolved ¤ white dwarfs
form a group well below the main sequence. ¤ stellar evolution.
Hevelius, Johannes (1611“1687) Hevelius is remembered chiefly for his study of
the Moon and the engravings he made of its features, and for the long
unwieldy ˜˜aerial™™ telescopes of enormous focal length he constructed to make
his observations. The son of a wealthy brewer, he was born in Danzig (now
Gdansk) in Poland, where he established his observatory after studying in
Leiden in the Netherlands. For his work in positional astronomy, which led to a
star catalog (1690), he used a ¤ quadrant rather than a telescope. He was the
last astronomer to do major observational work without a telescope.
944 Hidalgo An asteroid discovered in 1920 by Walter ¤ Baade. It follows a highly
elliptical orbit that is inclined at 42 to the plane of the solar system and lies
between 2 and 9.7 AU from the Sun, extending from the main ¤ asteroid belt to
beyond the orbit of Saturn. Its unique orbit among asteroids has led some
astronomers to speculate that Hidalgo may be a ˜˜dead™™ comet nucleus. It is
estimated to be between 40 and 60 km (25 and 38 miles) across.
high-velocity clouds Clouds of hydrogen gas in and around our Galaxy moving at
exceptionally high speeds. Several hundred have been located within the
¤ Local Group of galaxies. Each contains the same amount of mass as a small
galaxy and they are typically 50 000 light years across. They appear to be
composed of primordial material left over from the formation of the Local
Group. Similar clouds have been observed in other galaxy groups.

Hirayama family

high-velocity star A star traveling exceptionally fast (at more than about 65 km/s)
relative to the Sun. High-velocity stars are very old stars that do not share the
motion of the Sun and most stars in the solar neighborhood, which are in
circular orbits around the center of the ¤ Galaxy. Rather, they travel in
elliptical orbits, which often take them well outside the plane of the
Galaxy. Although their orbital velocities in the Galaxy may be no faster than
the Sun™s, their different paths mean that they have high velocities relative
to the Sun.
Hilda asteroids A group of asteroids at the outer edge of the main ¤ asteroid belt,
4.0 AU from the Sun. They are named after 153 Hilda, an asteroid about 180 km
(110 miles) across, which was discovered by Johann Palisa in 1875. The ratio of
their orbital periods to that of Jupiter is 3:2, and they are separated from the
rest of the asteroid belt by a ¤ Kirkwood gap.
Himalia A moon of Jupiter discovered in 1904 by Charles Perrine. Its diameter is
170 km (106 miles). With Elara, Leda, 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).
Hind™s Nebula (NGC 1554/5) A variable ¤ reflection nebula surrounding the star
T Tauri. ¤ T Tauri star.
Hinode A Japanese astronomy satellite, operated in collaboration with the USA
and the UK, to study X-rays and extreme ultraviolet emissions from the Sun. It
was launched in September 2006 and was a follow-up to the earlier ¤ Yohkoh
mission. Hinode is Japanese for sunrise.
Hipparchus (c.170 bc “ c.120 bc) Hipparchus, a Greek geometer and astronomer,
worked in Rhodes and Alexandria. None of his works have survived, but we
know of them through ¤ Ptolemy. After seeing a nova in 134 bc, Hipparchus
constructed a catalog of the positions of 850 stars. By comparing his values
with those in a catalog made 150 years earlier, Hipparchus discovered
the ¤ precession of the equinoxes. He also established the basis of the
¤ magnitude system still in use today to describe the brightness of
astronomical objects.
Hipparcos A European Space Agency orbiting telescope that surveyed the
positions and brightnesses of over a million stars with unprecedented
accuracy. It was launched in 1989 and observing ended on August 15, 1993. The
name ˜˜Hipparcos™™ was an acronym for High Precision Parallax Collecting
Satellite, chosen for its similarity to the name of the Greek astronomer
¤ Hipparchus (also spelt Hipparchos).
Hirayama family A group of ¤ asteroids with similar orbits located near each
other in space. The existence of such groups was first noted by Kiyotsugo
Hirayama in 1918. More than a hundred have been identified. In many cases
the members of the group are asteroids of similar or related types, strongly

Hoba meteorite

suggesting that they were formed when a single parent body broke up. About
half of all asteroids are thought to belong to Hirayama families.
Hoba meteorite The largest known ¤ meteorite in the world. It is of the iron type
and weighs about 55 000 kilograms (60 tons). It remains at the place in
Namibia, where it was discovered in 1928. A layer of rusty weathered material
surrounds the meteorite. Taking this into account, the original meteorite must
have weighed more than 73 000 kilograms (80 tons).
Hobby“Eberly Telescope (HET) A large telescope at the ¤ McDonald Observatory in
Texas, designed specifically for ¤ spectroscopy. It became fully operational in
1999. Its 11-meter segmented mirror is permanently tipped at a 35 angle to
the zenith and is mounted on a structure that can turn to point in any
direction. The telescope tracks its targets with a movable secondary mirror.
Though the tilt of the main mirror is fixed, the telescope is able to observe
objects in about 70 percent of the sky at the site. ¤ Southern African Large
Homunculus Nebula A small nebula surrounding the star ¤ Eta Carinae.
Hooker telescope The 2.54-m (100-inch) reflecting telescope at the ¤ Mount Wilson
Observatory, near Pasadena in California. It was completed in 1917, having been
financed by a gift from John D. Hooker. Until the opening of the 5-m (200-inch)
¤ Hale Telescope in 1948, it was the largest telescope in the world. It was
temporarily closed in 1985 but subsequently renovated and brought back into
use in the early 1990s.
horizon The boundary between the visible and hidden halves of the celestial
sphere from the point of view of a ground-based observer. Horizon is also used
more generally for any boundary between events that can in principle be
observed and those that cannot. ¤ event horizon.
horizontal branch On a ¤ Hertzsprung“Russell diagram, the part of the diagram
where low-mass stars that have lost material during the giant phase of their
evolution are located.
horizontal coordinates A coordinate system in which points on the celestial
sphere are identified by their altitude and azimuth. Altitude is angular distance
above the horizon, and azimuth is angular distance around the horizon,
measured eastwards from north. The altitude and azimuth of a celestial object
vary according to the latitude and longitude of the observer and the time of
Horologium (The Clock) A faint and inconspicuous constellation of the southern
sky introduced by Nicolas L. de Lacaille in the mid-eighteenth century.
Horsehead Nebula (NGC 2024) A dark dust nebula in the constellation Orion,
shaped similar to a horse™s head. It is conspicuous because it protrudes into the
bright emission nebula IC 434. It is about 1500 light years away.
Horseshoe Nebula An alternative name for the ¤ Omega Nebula.

Horseshoe Nebula

The Horsehead Nebula, imaged by the 8.2-m KUEYEN Telescope of the European Southern

Fred Hoyle.

hour angle

hour angle (HA) For a celestial object, the ¤ sidereal time that has elapsed since it
last crossed the meridian.
Hourglass Nebula A bright, luminous nebula within M8, the ¤ Lagoon Nebula. It
was first noted by John ¤ Herschel, and its name describes its shape.
Hoyle, Sir Fred (1915“2001) Hoyle was one of the greatest theoretical
astronomers of the twenteith century. He was born in West Yorkshire, to
parents who strongly encouraged his interest in science. At Cambridge
University, he graduated as the top applied mathematician. His postgraduate
research was in nuclear physics but, in 1939, his interest changed to
astrophysics, at which point his career was interrupted by five years of war
service as a theorist working on radar. This secret research took him to the USA
in 1944 where he visited Henry Norris ¤ Russell and Walter ¤ Baade, both of
whom stimulated his interest in stellar evolution.
On his return to Cambridge in 1945, he applied his knowledge of nuclear
physics to the origin of the chemical elements, showing how nuclear reactions
in giant stars could create the elements from carbon to iron. In 1957, together
with E. Margaret Burbidge, Geoffrey Burbidge, and William Fowler, he
accounted for almost all the chemical elements and their isotopes by
¤ nucleosynthesis in stars. This was his most important and lasting contribution
to astrophysics.
Famously, he proposed a ¤ steady-state theory as an alternative to ¤ Big Bang
cosmology in 1948. The discovery of the ¤ cosmic background radiation in 1963
confirmed the Big Bang, much to his dismay.
HR diagram (H“R diagram) Abbreviation for ¤ Hertzsprung“Russell diagram.
HST Abbreviation for ¤ Hubble Space Telescope.
Hubble, Edwin Powell (1889“1953) Hubble was an American observational
astronomer who discovered the expansion of the universe. He was born in
Missouri and studied at the University of Chicago, where he was influenced by
George Ellery ¤ Hale. Later, in 1919, he joined the staff of the Mount Wilson
Observatory, where Hale was the Director. His early work demonstrated that
spiral ˜˜nebulae™™ (what would now be called galaxies), such as the ¤ Andromeda
Galaxy, lay far beyond our ¤ Galaxy (the Milky Way).
Using the 100-inch telescope at Mount Wilson, he determined the
distances to 18 galaxies, an enormous achievement at that time. He found that
the galaxies were all receding and that recession velocities increased in
proportion with their distance. This work confirmed that the idea of an
expanding universe, already proposed by theorists, was indeed correct.
Hubble also devised a scheme for classifying galaxies according to their
shape, known as the ¤ Hubble classification, which is still used today.
Hubble classification A method devised in 1926 by ¤ Edwin Hubble for classifying
¤ galaxies according to their shape. The scheme sorts elliptical galaxies on a

Hubble parameter

The Hubble classification scheme for galaxies.

scale from E0 for a circular disk, through E1, E2, and so on, to E7 in order of
increasing elongation. Spirals are designated as Sa, Sb or Sc in order of
increasing openness of the arms and decreasing size of the nuclear bulge in
relation to the overall size of the galaxy. There is a parallel sequence for
¤ barred spiral galaxies, which are designated SBa, SBb or SBc. Galaxies that are
neither elliptical nor spiral in form are designated Ir for ˜˜irregular.™™ Hubble
suggested in 1936 that ¤ lenticular galaxies, designated S0, provide a ˜˜missing
link™™ in an evolutionary chain from E0 through to the open spirals Sc and SBc.
This progression is no longer accepted as an evolutionary sequence, but
Hubble™s classification continues to be widely used as a simple way of
describing the shapes of galaxies.
Hubble constant ¤ Hubble parameter.
Hubble diagram A graph on which the redshifts or recession velocities of galaxies
are plotted against distance. The original diagram, plotted in 1929, gave the
first strong evidence that the universe is expanding. It continues to be a crucial
tool for cosmologists because any deviation from a straight line reveals
information about the geometry of the universe and its rate of expansion in
the past. ¤ Hubble™s law, Hubble parameter, expanding universe.
Hubble parameter (symbol H) The ratio of a galaxy™s velocity to its distance. It is
related to the rate of expansion of the universe. Most theories of cosmology
assume that its value changes over time as the universe evolves. The symbol H0
is used for its present value, which is often called the Hubble ˜˜constant.™™ Its

Hubble™s law

The Hubble Space Telescope after the release from Columbia™s robot arm at the close
of a successful servicing mission in March of 2002.

value was difficult to determine, because the distances of galaxies could not be
measured accurately enough. However, two independent methods have given
similar values with relatively small margins of error. A long-term project
carried out using the ¤ Hubble Space Telescope, known as the Hubble Key Project,
came up with the value 72 ± 8 km/s per megaparsec in 2001. The ¤ Wilkinson
Microwave Anisotropy Probe (WMAP) results yielded 71 Æ 4 km/s per megaparsec
in 2003.
Hubble™s law The observation that distant galaxies are receding at speeds that are
proportional to their distance from us. This relationship was discovered by
¤ Edwin Hubble and was first announced by him in 1929. It is a consequence
of the expansion of the universe. ¤ Hubble parameter.
Hubble Space Telescope (HST) An orbiting observatory built and operated jointly
by NASA and ESA, and named in honor of Edwin ¤ Hubble. It has been used to
make observations of virtually every kind of celestial object, from planets in
the solar system to the most remote galaxies detectable. The science operations
are conducted from the Space Telescope Science Institute (STScI) in Baltimore,
The idea behind the HST was to put a telescope above the atmosphere,
which degrades the quality of the images in ground-based telescopes. It was
designed with a 15-year lifetime as an observatory that could be maintained
and upgraded in orbit using the ¤ Space Shuttle. It can make observations in the
ultraviolet, in visible light and in the near-infrared.

Huygens, Christiaan (1629“1695)

The HST was placed in orbit on April 25, 1990, by the Space Shuttle.
However, a fault in the optical figuring of the 2.4-m (94.5-inch) main mirror
meant that sharp focusing was impossible until the fault could be corrected.
During the first servicing mission in December 1993, a Space Shuttle crew
successfully installed a unit to correct the faulty optics. They also replaced the
solar arrays. Since 1993, there have been two further servicing missions to
replace instruments and worn-out parts. The second was in February 1997 and
the third was partially carried out in December 1999 but the mission was cut
short and the remainder of the work carried out in 2001.
Following an accident in 2003 in which a Space Shuttle was destroyed,
Space Shuttle missions were suspended until 2005. NASA took the view that
further maintenance of the HST could be too risky to undertake and the future
of the HST beyond 2008 became uncertain. However, in 2006, NASA
announced plans for a servicing mission in 2008. If successful, the HST™s
lifetime would be extended to 2013.
Hubble™s Variable Nebula (NGC 2261) A luminous triangular-shaped nebula in
the constellation Monoceros. From photographs taken between 1900 and 1916,
Edwin ¤ Hubble discovered that the nebula varies in shape and brightness.
An irregularly variable star, R Monocerotis, is embedded in the nebula. R Mon
is a strong source of infrared radiation and is probably a very young star,
surrounded by a circumstellar disk and ejecting a ¤ bipolar outflow. The nebula
is now thought to be an example of a ¤ Herbig“Haro object.
Huggins, Sir William (1824“1910) Huggins was a British astrophysicist who was
one of the principal founders of astronomical spectroscopy. He was the first
person to make intensive investigations of stellar spectra. In 1863 he showed
that the stars are composed of the same chemical elements as the Sun. His next
great discovery was to show that ¤ emission nebulae are glowing clouds of gas.
He was the first, in 1868, to measure a ¤ Doppler shift, which he did in the
spectrum of the star Sirius. He subsequently determined the velocities of many
stars from their spectra. Huggins™ wife Margaret was his partner in research
and she made significant contributions to their joint work.
Hungaria group A group of asteroids at the inner edge of the asteroid belt,
1.95 AU from the Sun, with orbits inclined at 24 to the plane of the solar
system. The group is separated from the main belt by a ¤ Kirkwood gap, and is
not a true family with a common origin. The group takes its name from 434
Hungaria, a small asteroid discovered by Max Wolf in 1898.
Huygens, Christiaan (1629“1695) Huygens was a Dutch physicist and
astronomer, whose many contributions to science include the wave theory of
light and the discovery of Saturn™s largest moon, ¤ Titan. The importance of his
work in dynamics and optics was second only to that of Isaac ¤ Newton in the
seventeenth century.

Huygens probe

Born in the Hague, and educated at the University of Leiden, he eventually
settle back in the Hague. Working with his brother Constantin, he made
significant improvements in the design of telescope optics. He used the
telescopes he built to observe Saturn and found Titan in 1655. He was the first
to give the correct description of Saturn™s rings. In 1657 he invented the
pendulum clock.
Huygens probe ¤ Cassini“Huygens mission.
Hyades An open star cluster in the constellation Taurus. Its members appear to be
scattered over an area 8 in diameter around the star Aldebaran. However,
Aldebaran is closer than the Hyades and does not belong to the cluster. The
Hyades is the nearest star cluster, lying at a distance of about 150 light years.
¤ open cluster.
Hyakutake, Comet A bright comet that reached magnitude zero in March
1996 and developed a tail stretching at least 7 across the sky. It was bright
mainly because it happened to come relatively close to Earth, passing within
15 million km (10 million miles).
Hydra (1) (Sea Monster) The largest constellation in the sky by area, but a difficult
one to identify since it contains only one moderately bright star, the
second-magnitude Alphard.

Hyperion imaged by the Cassini spacecraft.


Hydra (2) A small moon of Pluto discovered in 2005. It is estimated to be
about 45“60 km (30“37 miles) across.
Hydra A The brightest radio source in the constellation Hydra. It is associated with
a large elliptical galaxy at the center of a small cluster of galaxies about one
billion light years away.
Hydrus (The Water Snake) An inconspicuous southern constellation, introduced
by Johann Bayer in his 1603 star atlas. Its three brightest stars are third
10 Hygeia The fourth largest asteroid, discovered by Anatole de Gasparis in 1849.
Its diameter is 430 km (267 miles).
Hyperion A satellite of Saturn, discovered in 1848 by William C. Bond William
Lassell. It is an elongated irregular body measuring approximately
360 · 280 · 225 km (223 · 174 · 140 miles). Its orbit is 1481 100 km (920 312
miles) from Saturn. There are large craters and a curving scarp-like feature,
apparently 300 km (200 miles) long. The evidence suggests that it may be a
remnant of a larger body shattered by an impact.
hypernova A cosmic explosion that releases a hundred times more energy than
a ¤ supernova. A hypernova may be the result of the implosion of a massive,
rapidly rotating stellar core, resulting in the formation of a ¤ black hole. The
small number of hypernova remnants identified are exceptionally strong
sources of X-rays. It is thought that hypernovae may be linked to ¤ gamma-ray

Iapetus The third largest moon of Saturn, discovered by ¤ Giovanni Cassini in 1671.
It orbits Saturn at a distance of 3561 300 km (2212 888 miles) and measures
1436 km (892 miles) across. With a density only 1.1 times that of water, Iapetus
must be mostly ice. The ¤ Voyager spacecraft con¬rmed a hypothesis, proposed
by Cassini after he noticed variations in Iapetus™s brightness, that one
hemisphere is very much darker than the other. The satellite always keeps the
same face towards Saturn and its dark and light hemispheres face Earth
alternately as it travels around its orbit. The surface is cratered and the
¤ Cassini spacecraft showed a remarkable ridge of mountains up to 20 km
(12 miles) high running almost parallel to the equator for about 1300 km (800
miles). The dark area is blanketed with a material as black as tar. Its nature and
origin are unknown but Cassini images give the impression that the dark
coating has fallen onto the surface.
IAU Abbreviation for ¤ International Astronomical Union.
IC Abbreviation for ¤ Index Catalogue.

A Cassini spacecraft image of Iapetus.

Imbrium Basin

A Galileo spacecraft image of asteroid
Ida with its moon Dactyl.

1566 Icarus A small asteroid, diameter 1.4 km (0.9 mile), discovered in 1949 by
Walter ¤ Baade. It is a member of the ¤ Apollo group and its highly elliptical
orbit takes it closer to the Sun than Mercury.
IceCube An experiment to detect cosmic neutrinos. built into the ice at the South
Pole. It consists of 4200 light detectors, arranged in strings, buried in 70 holes,
2.4 km (1.5 miles) deep. The detectors record faint ¬‚ashes of light emitted
when neutrinos interact with particles in the ice. In all, the detectors occupy a
cubic kilometer of ice. IceCube began operation in 2006 and was built around a
smaller experiment called ¤ AMANDA. It is an international project led by the
United States. ¤ neutrino astronomy.
ice dwarf A small planetary body composed of a mixture of ices and rock.
Examples include ¤ Pluto, planetary moons such as ¤ Triton, and objects
populating the ¤ Kuiper Belt.
243 Ida A member of the ¤ Koronis family of asteroids, imaged from close-up on
August 28, 1993, by the ¤ Galileo spacecraft on its way to Jupiter. It measures
58 · 23 km (36 · 14 miles). The spacecraft observations revealed that Ida has a
small satellite, subsequently named Dactyl, which measures about 1.6 · 1.2 km
(1.0 · 0.75 mile). Observations of Dactyl™s orbital motion put Ida™s density in the
range 2.2“2.9 g/cm3. Ida and Dactyl do not have identical compositions,
suggesting that the pair were created when larger asteroids collided and broke
up to create the Koronis family. The surfaces of both Ida and Dactyl are heavily
Ijiraq A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and is about 10 km (6 miles) across.
Ikeya“Seki, Comet A particularly brilliant comet, discovered on September 18,
1965 by two Japanese amateur astronomers. It was especially conspicuous in
the southern hemisphere after it had passed perihelion. It belonged to the
group of comets known as ¤ sungrazers.
IKI The Space Research Institute of the Russian Academy of Sciences, which
undertakes projects for the Russian Space Agency.
Imbrium Basin The largest and youngest of the very large circular impact basins
on the Moon. The large crater created by the impact was subsequently flooded


by lava to form the dark area known as the Mare Imbrium, which is 1300 km
(800 miles) in diameter. The Imbrium Basin is surrounded by three concentric
rings of mountains. The outer one is the most conspicuous and includes the
Carpathian, Apennine and Caucasus mountains. The lunar Alps form part of
the second ring.
immersion The disappearance of a star, planet, moon or other body at the
beginning of an ¤ occultation or ¤ eclipse.
inclination (symbol i) The angle between the plane of an orbit and a reference
plane. The reference plane for the orbits of planets and comets around the Sun
is usually the ¤ ecliptic. For satellite orbits, the reference plane is normally the
plane of the equator of the parent planet.
Index Catalogue (IC) Two supplements to the ¤ New General Catalogue (NGC) of
nebulae and star clusters, compiled by John L. E. ¤ Dreyer and published in
1895 and 1908.
Indus (The Indian) An inconspicuous southern constellation, representing a native
American. It was introduced in the 1603 star atlas of Johann Bayer and contains
no stars brighter than the third magnitude.
inferior conjunction The position of Mercury or Venus when either of these
planets lies directly between Earth and the Sun. Because the orbits of the
planets are tilted to each other, ¤ transits of Mercury and Venus across the face
of the Sun are rare. Normally they pass just north or south of the Sun at
inferior conjunctions.
inferior planets Mercury or Venus, the two planets that have orbits closer to the
Sun than Earth.
inflation A period in the very early history of the universe, soon after the ¤ Big
Bang, when the universe expanded extremely rapidly. Inflation can account for
the immense size of the universe today, and its uniformity. According to the
theory, inflation converted tiny quantum-scale energy fluctuations into the
uneven density of matter that seeded stars and galaxies. Observations
made by the ¤ Wilkinson Microwave Anisotropy Probe (WMAP) support the theory of
Infrared Astronomical Satellite (IRAS) An orbiting infrared telescope with a
primary mirror 57 cm (22.5 inches) across, which was launched on January 25,
1983, and operated until November 23, 1983. It was a collaborative mission
between NASA, the Netherlands Aerospace Agency, and the UK. During its
10-month mission, it scanned 96 percent of the sky twice and detected a
quarter of a million individual sources. It also discovered five comets. The first
and brightest, Comet IRAS“Araki“Alcock discovered in May 1983, passed
within five million km (3 million miles) of the Earth “ the closest approach of
any comet for 200 years.

infrared astronomy

Infrared astronomy. An image of infrared radiation from the whole sky shows the plane of the
Galaxy and the ˜˜bulge™™ at the galactic center.

infrared astronomy The study of infrared radiation from astronomical sources.
Infrared is ¤ electromagnetic radiation in the wavelength range between visible
red light and radio waves, which extends from about 0.1 to about 100 mm. It is
invisible to the human eye and is absorbed almost completely in the lower
layers of the Earth™s atmosphere, primarily by water vapor. For this reason,
infrared astronomy observations have to be conducted from the highest
mountain sites, or from aircraft or satellites.
The ¬rst infrared observation was made accidentally by William ¤ Herschel
in 1800 when a thermometer he placed just to one side of the red end of a
visible solar spectrum recorded a rise in temperature. Infrared images
predominantly show the distribution of heat. Since all warm objects
radiate infrared, infrared telescopes must be cooled to a few degrees above
absolute zero so they are not blinded by the radiation they are emitting
Systematic infrared astronomy began in the 1960s, when suitable
detectors became available. The ¬rst infrared survey of the sky was carried out
by Gerry Neugebauer and Robert Leighton of the California Institute of
Astronomy (Caltech). They published a list of 5612 sources in 1969. Infrared
astronomy has made important advances with the development since the
1980s of two-dimensional arrays of infrared detectors, capable of making a
complete image in a single exposure.
Infrared astronomy was boosted by the successful operation of IRAS, the
¤ Infrared Astronomical Satellite, in 1983. Its successor, the ¤ Infrared Space
Observatory (ISO), was launched in November 1995. The largest, most capable

infrared galaxy

infrared telescope ever placed in orbit was NASA™s ¤ Spitzer Space Telescope,
launched in 2003. NASA™s proposed James Webb Space Telescope and the
European Space Agency™s orbiting ¤ Herschel telescope will also operate in the
The best ground-based site for infrared astronomy is the ¤ Mauna Kea
Observatories in Hawaii. Three infrared telescopes started operation there in
1979: the ¤ United Kingdom Infrared Telescope (UKIRT), NASA™s ¤ Infrared Telescope
Facility (IRTF) and the ¤ Canada“France“Hawaii Telescope (CFHT), which also
functions as an optical telescope. The telescopes of the ¤ Keck Observatory and
many other recently constructed large telescopes can detect the near-infrared
as well as visible light.
Infrared radiation is detected from stars and galaxies, and from dust clouds
within the solar system and in the interstellar medium. Strong infrared
emission is particularly characteristic of dust that has been heated by visible
and ultraviolet radiation from stars. Protostars in the process of formation and
evolved red giant stars are surrounded by shells of dust that emit infrared.
Unlike visible light, infrared radiation passes relatively easily through dust
clouds. So, for example, the ¤ galactic center, which is largely obscured by dust
as far as visible light is concerned, can be explored by means of infrared and
radio astronomy. The way in which infrared radiation is scattered from the
surfaces of objects in the solar system provides important clues to their
composition. Infrared observations are also important for remote objects with
large ¤ redshifts.
infrared galaxy A galaxy that emits most of its energy (typically more than 90
percent) as infrared radiation. Such galaxies are thought to have unusually
high rates of star formation and so are also called ¤ starburst galaxies.
Infrared Space Observatory (ISO) An orbiting infrared telescope launched by the
¤ European Space Agency on November 17, 1995. It operated until May 1998 and
made observations in the waveband between 2.5 and 200 mm with a sensitivity
much greater than that of its predecessor, the ¤ Infrared Astronomical Satellite, IRAS.
Infrared Telescope Facility (IRTF) A NASA infrared telescope located at the
¤ Mauna Kea Observatories in Hawaii, where it has been in operation since 1979 as
a national facility for the USA. The main mirror is 3 m (120 inches) in diameter.
inner planets The planets Mercury, Venus, Earth and Mars.
Institut de Radio Astronomie Millimetrique (IRAM) A collaborative project
between France, Germany and Spain for studies in ¤ millimeter-wave astronomy.
The institute operates a 30-m dish in the Sierra Nevada, Spain, and a four-dish
interferometer located in France, south of Grenoble.
INTEGRAL Abbreviation for ¤ International Gamma-Ray Astrophysics Laboratory.
interacting galaxies Galaxies so close to each other that they are distorted by the
effects the gravity of one has on another. Most galaxies are in clusters, and


Interacting galaxies. The large barred spiral galaxy, NGC 6872, is interacting with a
smaller lenticular galaxy, IC 4970 (just above the centre). The bright object to the lower
right of the galaxies is a star in the Milky Way.

interactions between pairs are not uncommon. They often result in long wisps
or filaments of stars and gas that form bridges between them.


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