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transit (3) The passage of a natural satellite across the disk of its parent planet.
transit circle A telescope mounted so that it is aligned exactly north“south and
can rotate up and down about a ¬xed horizontal axis. Transit circles are used


[353]
transition region




An image of the Sun projected during
the transit of Venus in 2004.

for accurate measurements of the altitudes of stars and for timing their
passage across the ¤ meridian.
transition region The layer in the Sun™s atmosphere above the ¤ chromosphere and
below the ¤ corona. In the transition region, the temperature rises dramatically
from 10 000 K in the chromosphere to over 1 million K in the lower corona.
transneptunian object A small planetary body in the outer solar system farther
away from the Sun than Neptune (beyond about 30 AU). ¤ Kuiper Belt.
¤
Trapezium A popular name for the multiple star system Theta1 Orionis, which lies
at the heart of the ¤ Orion Nebula and illuminates it. Four stars, of magnitudes
5.1, 6.7, 6.7 and 8.0, form the trapezium shape and are visible in a small
telescope. A larger telescope reveals the presence of two other stars of eleventh
magnitude.
Triangulum (The Triangle) A small but distinctive northern constellation between
Andromeda and Aries. Its three brightest stars, of magnitudes 3.0, 3.4 and 4.0,
form a small, elongated triangle. Triangulum includes the large spiral galaxy,
M33, which is a member of the ¤ Local Group.
Triangulum Australe (The Southern Triangle) A small but distinctive southern
constellation introduced in the 1603 star atlas of Johann Bayer. Its three




An infrared image of the Trapezium
star cluster in the Orion Nebula taken
by the Hubble Space Telescope.


[354]
Triton




The Triangulum Galaxy.

brightest stars, magnitudes 1.9, 2.9 and 2.9, form an almost equilateral
triangle.
Triangulum Galaxy (M33; NGC 598) A large, nearby, spiral galaxy in the
constellation Triangulum. It lies at a distance of 2.7 million light years, and is
a member of the ¤ Local Group.
Trifid Nebula (M20; NGC 6514) A large luminous cloud of ¤ ionized hydrogen
around a region of star formation about 8000 light years away. Dust lanes
radiating from the center appear to divide the nebula into three parts. It lies in
the constellation Sagittarius.
Trinculo A small moon of Uranus discovered in 2001. Its diameter is about 10 km
(6 miles).
Triton The largest natural satellite of Neptune. It was discovered in October 1846 by
William Lassell, only 17 days after the discovery of Neptune itself. It circles
Neptune every 5.9 days in a retrograde orbit at a distance of 354 800 km (220 473
miles), which is tilted at 23 to the planet™s equatorial plane. This unusual orbit
has led to speculation that Triton was captured and did not form close to Neptune.
¤ Voyager 2™s passage within 4000 km (2500 miles) of Triton in August 1989
revealed a wealth of detail. Triton™s diameter was found to be 2700 km (1680
miles), slightly less than thought previously. Its gravitational effect on the


[355]
Triton




The central part of the Trifid Nebula.


spacecraft™s trajectory suggests that the bright, icy outer crust and mantle must
overlie a substantial core of rock (perhaps even metal) containing two thirds of
the satellite™s mass. The surface temperature is 38 K, making it the coldest known
object in the solar system.
Triton is surrounded by a tenuous atmosphere (surface pressure 15
microbars) of nitrogen, with a trace of methane. The south polar cap is coated
with a bright frost, possibly of nitrogen ice, which is gradually evaporating.
This region had been in sunlight continuously for nearly 100 years when it was
observed. No impact craters were detected there. In the equatorial region,
Voyager saw a variety of terrains where it looks as if there is complex volcanic
activity, including ¤ plume eruptions. Triton™s surface is certainly relatively
young in astronomical terms.


[356]
T Tauri star




A Voyager 2 mosaic of Triton made
in 1989.


Trojan asteroids Families of ¤ asteroids that share the orbit of a planet “ especially
Jupiter “ clustered around the two ¤ Lagrangian points 60 away from the
planet.
Nearly 2000 Jupiter Trojans are known. They do not stay exactly at the
Lagrangian points but oscillate around them over an arc between about 45
and 80 from Jupiter, taking 150“200 years to complete a cycle. The ¬rst to be
discovered was called ¤ Achilles, and it was decided to use names of warriors in
the Trojan wars for members of the group identi¬ed subsequently.
tropical year The time taken by Earth to travel once round the Sun, measured
from equinox to equinox. It is the time it takes for the cycle of seasons to
repeat exactly and is 365.242 19 days long.
troposphere The lowest layer of Earth™s atmosphere, up to a height of
approximately 20 km (12 miles). It is bounded by the tropopause, which marks
the transition to the more stable conditions of the stratosphere above.
Tsiolkovskii A crater on the lunar farside, 180 km (110 miles) in diameter. The
crater floor is partially flooded by dark lava, through which a central peak
protrudes. Tsiolkovskii is one of the most prominent features on the lunar
farside, where there are no dark maria areas like on the nearside.
T Tauri star A type of very young star in an early phase of evolution and still
contracting. The prototype, T Tauri lies within a dark dust cloud in the
constellation Taurus.
All T Tauri stars vary irregularly and their surface temperatures are in the
range 3500“7000 K. They are found in dense interstellar clouds, usually
alongside young, hot stars. Large numbers of T Tauri stars have been
discovered through the strong infrared radiation they emit. Groups of T Tauri
stars are called T associations. Strong ¤ bipolar out¬‚ows (twin-lobed jets) stream
out from T Tauri stars at speeds of several hundred kilometers per second.
Where these out¬‚ows compress and heat the interstellar gas, they create
luminous nebulae called ¤ Herbig“Haro objects.


[357]
Tucana




The lunar farside crater Tsiolkovskii
photographed by the crew of
Apollo 13.


Tucana (The Toucan) A southern constellation introduced in Johann Bayer™s 1603
star atlas. Its two brightest stars are third magnitude. The Small ¤ Magellanic
Cloud lies within its boundaries, and it also includes the large and bright
¤ globular cluster, known as ¤ 47 Tucanae, which is just visibile to the naked eye.
47 Tucanae (NGC 104) The second-brightest ¤ globular cluster in the sky (after
Omega Centauri). To the naked eye it looks like a fuzzy fifth-magnitude star. It
lies at a distance of 13 000 light years and is a relatively young globular cluster.
Tunguska event The violent explosion in the atmosphere of a comet or meteorite
in the Tunguska region of Siberia on June 30, 1908. Though the event caused
devastation over a large area, no remains of an impacting body or any crater
have been discovered. The height of the explosion has been estimated at
8.5 km (5.3 miles). Observers reported seeing a fireball as bright as the Sun. It
exploded with a deafening sound and caused a shock wave that shook
buildings and caused damage, though there was no loss of human life.
The ¬rst expedition to the remote area of the explosion did not take place
until 1927. It found that a forest of trees had been snapped in half over a region
30“40 km (20“25 miles) in radius. Over a region of radius 15“18 km (9“11 miles)
from the apparent ˜˜impact™™ site, trees had been ¬‚attened in a radial pattern
and had been stripped of their branches.
twenty-one centimeter line Characteristic radio emission or absorption at a
wavelength of 21 centimeters by neutral hydrogen atoms in interstellar space.
Neutral hydrogen is a major component of the ¤ interstellar medium and
observations of the 21-centimeter line are an important means of ¬nding its
distribution, density and velocity in our own Galaxy and thousands of others.
The probability of the small energy change in the hydrogen atom that is
responsible for the 21-centimeter emission actually taking place is very low.
An individual hydrogen atom with the higher energy level typically waits


[358]
Tycho




The globular star cluster 47 Tucanae.


12 million years to make the change spontaneously. However, the radiation is
observed from interstellar hydrogen because of the vast numbers of atoms and
because collisions between atoms trigger the emission.
twilight The period before sunrise and after sunset when the sky is illuminated by
scattered sunlight. Civil twilight is defined as the period when the center of the
Sun™s disk is between 90 500 and 96 below the zenith. Nautical twilight is
when it is between 96 and 102 below the zenith and astronomical twilight
when it is between 102 and 108 below.
Two-Micron All Sky Survey (2MASS) A US collaboration to image the entire sky in
near-infrared radiation. The survey is carried out by two automated 1.3-m
telescopes. One is located on Mount Hopkins in Arizona and began work on the
survey in June 1997. The other is at the ¤ Cerro Tololo Inter-American Observatory
in Chile, and commenced survey operations in March 1998. Each telescope
scans continuously, taking observations in three wavebands simultaneously at
1.25, 1.65 and 2.17 mm.
Tycho A prominent lunar crater in the Moon™s southern uplands. It is surrounded
by the brightest and most extensive ray system on the Moon, which means it is
probably one of the youngest of major lunar features. The terraced walls rise to


[359]
Tycho Brahe


a height of 4.5 km (15 000 feet) and the central peak to 2.3 km (7500 feet) above
the floor of the crater, which is 85 km (53 miles) in diameter.
Tycho Brahe ¤ Brahe, Tycho.
¤
Tycho™s Supernova (Tycho™s Star) A ¤ supernova in the constellation Cassiopeia
observed by ¤ Tycho Brahe in 1572. At its maximum brightness it rivaled Venus
and was visible in daylight. The ¤ supernova remnant is both an X-ray source and
an intense source of radio emission. The expanding shell of gas is faintly visible
with powerful optical telescopes.




[360]
U
UBV photometry A method of measuring the colors of stars introduced in the
1950s. It is based on measuring the ¤ magnitudes of stars in three regions of the
spectrum called U (ultraviolet), B (blue) and V (visual),which are centered on
wavelengths 350, 430 and 550 nm, respectively. More bands were added later
to extend the scheme into the infrared. These are called R, I, J, H, K, L, M and N,
ranging from 0.7 to 10.2 mm.
UFO Abbreviation for ¤ unidenti¬ed ¬‚ying object.
UKIRT Abbreviation for ¤ United Kingdom Infrared Telescope.
UKST Abbreviation for ¤ United Kingdom Schmidt Telescope.
ultra-luminous infrared galaxy (ULIRG) A galaxy emitting exceptionally
strongly in the infrared. ULIRGs were ¬rst detected by the ¤ Infrared
Astronomical Satellite in 1983. They appear to be the results of collisions between
two or more galaxies, which trigger immense bursts of star formation. The
infrared radiation is emitted by dust, which absorbs energy from hot, newborn
stars. Observations suggest that many ULIRGs are the mergers of three or more
galaxies.
ultraviolet astronomy The study of electromagnetic radiation from astronomical
sources in the wavelength band 10“320 nm. Ultraviolet (UV) radiation is
strongly absorbed by Earth™s atmosphere, so all UV observations have to be
carried out from satellites. The earliest observations were made during brief
rocket ¬‚ights in the 1940s and 1950s. The ¬rst satellite to make systematic
ultraviolet observations was the ¬rst Orbiting Solar Observatory (OSO-1) in
1962. The highly successful International Ultraviolet Explorer (IUE) was
launched in 1978 and continued to operate until 1996.
The ultraviolet is often subdivided into the extreme UV (EUV, 10“100 nm),
the far UV (FUV, 100“200 nm) and the near UV (NUV, 200“320 nm). The most
extreme UV, at the transition to X-radiation around 6“60 nm, is also known as
the XUV. At these wavelengths, the techniques of ¤ X-ray astronomy are
required, but the rest of the UV band can be observed and analyzed by methods
similar to those used for visible light. The main dif¬culty is the limited range of
suitable transparent materials and re¬‚ective coatings. Glass, for example,
absorbs UV strongly, so quartz or ¬‚uorite have to be used.
Hotter stars, with surface temperatures in excess of 10 000 K, emit most of
their energy in the UV. Even for cooler stars such as the Sun, UV studies are
important. The ¤ interstellar medium is another important subject for ultraviolet

[361]
Ulugh Beg (1393 or 1394“1149)


astronomy though, at wavelengths below 91.2 nm, almost all the UV radiation
is absorbed by hydrogen, the most widely distributed element in the universe.
This makes the detection of distant sources dif¬cult at such short wavelengths.
Ulugh Beg (1393 or 1394“1149) Ulugh Beg was the most important medieval
Islamic astronomer. He was the grandson of the conquering warlord Timur
(Tamerlane), and was himself a ruler as well as an astronomer. He established
Samarkand as an intellectual center with an enormous observatory. He
measured the positions of almost 1000 stars, making numerous corrections
to the tables made by ¤ Ptolemy, which were still in use at that time.
Ulysses A European Space Agency mission, launched on October 6, 1990, to study
the interplanetary medium and the solar wind at different solar latitudes. It
was the ¬rst mission to observe the poles of the Sun. ¤ Gravity assist was used to
take the orbit of Ulysses out of the plane of the solar system. After an
encounter with Jupiter in February 1992, the spacecraft swung back towards
the Sun to pass over the solar south pole in 1994 and the north pole in 1995.
A second encounter with the Sun took place in September 2000.
umbra (1) An area of total shadow, such as the zone on the surface of the Earth
from which totality is observed during a solar ¤ eclipse.
umbra (2) The dark central region of a ¤ sunspot.
Umbriel A moon of Uranus, discovered by William Lassell in 1851. Its diameter is
1169 km (726 miles). Images from the ¤ Voyager 2 encounter in 1986 show that
Umbriel is much darker than the other four major satellites of Uranus. Its
surface seems to have been covered by dark material relatively recently in
astronomical terms. It is also pitted with many craters. One of them, 110 km
(68 miles) across, is very bright, in marked contrast to the rest of the surface.
unidentified flying object (UFO) Any phenomenon in the sky for which the
observer does not have a ready rational explanation. UFO is often used to mean
a hypothetical unnatural object from space.
United Kingdom Infrared Telescope (UKIRT) A 3.8- m (150-inch) infrared
telescope, located at the ¤ Mauna Kea Observatories in Hawaii. It is the largest
telescope dedicated solely to infrared astronomy and operates in the
wavelength band between 1 and 30 mm.
United Kingdom Schmidt Telescope (UKST) A 1.2- m (48-inch) ¤ Schmidt camera
located at the ¤ Anglo-Australian Observatory and currently administered by the
Anglo-Australian Telescope Board. It was opened in 1973.
United States Naval Observatory A US government observatory in Washington,
DC, the main purpose of which is to provide astronomical data to the
Department of Defense. Its work includes astrometry, the preparation of
almanacs, time measurement and the maintenance of the Master Clock for the
USA. It has telescopes at Anderson Mesa, near Flagstaff, Arizona, and Black
Birch, New Zealand, as well as in Washington.

[362]
Uranus


The observatory was founded in 1830 and given the title US Naval
Observatory in 1844. For 50 years it was located at the site now occupied by the
Lincoln Memorial. It was moved to its present site, next to the of¬cial residence
of the Vice President, in 1893. The largest telescope at the site is the 66- cm
(26-inch) refractor, dating from 1873, with which Asaph ¤ Hall discovered the
moons of Mars, Phobos and Deimos, in 1877. The largest telescope belonging
to the observatory is a 1.5- m (61-inch) re¬‚ector at Flagstaff. Using this
instrument, James Christy discovered the moon of Pluto, Charon, in 1978.
At the Arizona site, the observatory has also constructed an optical
¤ interferometer, the Navy Prototype Optical Interferometer.
Universal Time (UT) A way of keeping time that relates closely to the Sun™s daily
apparent motion and serves as the basis for civil timekeeping. It is formally
de¬ned by a mathematical formula that links it to ¤ sidereal time, and is thus
ultimately determined from observations of the stars. Time kept precisely by
Earth™s rotation, which is gradually slowing down and subject to small
irregularities, is called UT1. The basis of civil time and broadcast time signals is
called coordinated universal time, or UTC. UTC is time as measured by
smooth-running atomic clocks but kept to within 0.9 seconds of UT1 by
introducing occasional leap seconds.
universe The entirety of all that exists. The size of the observable universe is
limited to the distance light has had time to travel since the ¤ Big Bang.
Uraniborg The observatory of Tycho ¤ Brahe (1546“1601) on the island of Hven,
north of Copenhagen, Denmark. It was completed in 1580 and used by Brahe
to make accurate astronomical observations for 20 years. Only ruins now
remain.
Uranometria A star atlas compiled by Johann Bayer (1572“1625) and published in
1603. In this atlas Bayer introduced the system of labeling stars with Greek
letters, which is still in use.
uranometry A largely obsolete term for positional astronomy or ¤ astrometry.
Uranus The seventh major planet of the solar system in order from the Sun,
discovered by William ¤ Herschel in 1781. Its average distance from the Sun is
19.2 AU and it is just bright enough to be seen by the naked eye under good
observing conditions. From Earth, it appears as an almost featureless greenish
disk. In 1986, the spacecraft ¤ Voyager 2 passed close to Uranus and its
satellites and returned close-up images of them. Ten small satellites were
discovered by Voyager 2; ¬ve larger satellites were already known: Miranda,
Ariel, Umbriel, Titania and Oberon. More recent discoveries have brought the
number of moons up to at least 27.
Uranus is one of the four giant planets of the solar system, with a diameter
four times Earth™s and a mass 15 times greater. Its internal rotation period is 17
hours 14 minutes. It is composed almost entirely of hydrogen and helium.

[363]
Uranus




This infrared view of Uranus, shown in false color, was taken by the Hubble Space
Telescope in August 1998. It reveals the main rings, 10 of Uranus™s moons and about 20
clouds in the planet™s atmosphere.


There is thought to be a small rocky core at the center of the planet,
surrounded by a thick icy mantle of frozen water, methane and ammonia. The
outermost layer is an atmosphere mostly of hydrogen and helium.
A curious feature of Uranus is that its rotation axis lies almost in the plane
of the solar system, rather than being nearly perpendicular to it, as is the case
for the other planets. This means that Uranus has greatly exaggerated seasons
over its ˜˜year™™ of 84 Earth years. One pole faces the Sun for 20 years while the
other side of the planet is in continuous darkness. As spring began in the
northern hemisphere of Uranus in the late 1990s, the Hubble Space Telescope
observed the atmosphere getting more dynamic as it was warmed by the Sun
and many bright clouds appeared.
In 1977, a series of narrow rings was discovered around Uranus. The rings
are each only a few kilometers wide and not visible from Earth. They were
discovered when Uranus occulted an eighth-magnitude star. The rings caused
small dips in the observed brightness of the star just before and just after it was
occultated by the disk of the planet. The ring system was subsequently imaged
by Voyager 2 in 1986, when two additional rings were discovered, and two

[364]
Ursa Minor




+70




8h




º
+70
º
14 h h
10 CAMELOPARDALIS
12h
16




7h
h




URSA MINOR





+6
ρ π1
M82
σ2
+6




π2
0




M81
σ1
º




DRACO
ο
15 h Muscida





8h
LYNX

±
Dubhe
BO–TES
M101

Alcor
µ δ +5
+5
β

ζ Mizar M108
θ
Alioth Megrez Merak
γ
· Talitha
ι
Owl Nebula
κ
M109
Alkaid
14 h Phad



URSA MAJOR h
9
χ
º
+40 ψ +40
º Tania Borealis
ω »
LYNX
Tania Australis µ

CANES
VENATICI

º
+30º
+30
Alula Borealis LEO MINOR
ν
ξ
Alula Australis
LEO
COMA
BERENICES
13h 12h 11h 10h 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 Ursa Major.



more were found by the Hubble Space Telescope in 2003, bringing the
total to 13.
Ursa Major (The Great Bear) One of the most familiar constellations of the
northern sky and the third largest in area. It contains 19 stars brighter than
fourth magnitude. The shape formed by the seven main stars of the
constellation is known as the Big Dipper or the Plough. The two stars Merak
and Dubhe in the Big Dipper are known as the Pointers since the line between
them points to Polaris. Ursa Major contains a group of galaxies belonging to
the ¤ Local Supercluster, including the relatively bright spiral galaxy, M81.
Ursa Minor (The Little Bear) The northern constellation that contains the north
celestial pole. The brightest star in Ursa Minor, second-magnitude Polaris, is
within 1 of the pole. Its main pattern of seven stars is known as the Little
Dipper.


[365]
Ursa Minor


+70º +80º +80º +70º



22 h




8h
Polaris
CEPHEUS
±
North
Celestial
Pole
δ

CAMELOPARDALIS
20




10 h
h




µ



ζ
θ
·
DRACO
Kochab
β
γ




12 h
18 h Pherkad


URSA MINOR
+6 h

13 0º
17 h
+6
RR
h
14
16 h
15h


URSA MAJOR
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 Ursa Minor.




The Viking 2 lander where it landed on Mars™s Utopia Planitia.


[366]
UV


UT Abbreviation for ¤ Universal Time.
Utopia Planitia An extensive, sparsely cratered plain in the northern hemisphere
of Mars. It was the landing site for the ¤ Viking 2 spacecraft. Panoramic images
returned by the Viking lander showed terrain littered with large numbers of
boulders made of open-textured rock.
UV Abbreviation for ultraviolet. ¤ electromagnetic radiation, ultraviolet astronomy.
¤




[367]
V
V The symbol for visual ¤ magnitude. ¤ UBV photometry.
¤
vacuum tower telescope A design of telescope for observations of the Sun. One at
the Sacramento Peak Observatory is typical. In that instrument, sunlight enters
the tower 41 m (135 feet) above the ground. A further 67 m (220 feet) of the
telescope lie below ground. The entire optical path is virtually air-free, to avoid
distortion of the solar image that hot air would cause. In the observing room,
an image of the Sun 51 cm (20 inches) in diameter is produced with a
resolution better than a quarter of an arc second. Sunlight can be directed into
spectrographs or other instruments by tilting the main mirror at the bottom of
the central tube.
Valhalla A large circular feature on ¤ Callisto, surrounded by 15 concentric rings.
The radius of the outermost ring is 1500 km (930 miles). It was caused by an
impact, but there is no vertical relief. The rings are like ripples.




The vacuum tower telescope at the
Teide Observatory on Tenerife in the
Canary Islands.


[368]
Van Allen belts




A perspective view across Coprates Chasma, part of the Valles Marineris, constructed from
images taken by Mars Express.


Valles Marineris A network of canyons on Mars, extending more than 5000 km
(3100 miles) in the east“west direction in the equatorial region. Its western
extremity is Noctis Labyrinthus, a complex area of fault valleys that cut the
surface into polygonal shapes. The central section consists of several parallel
canyons with an average depth of 6 km (3.7 miles). At the center they join
up with Melas Chasma, a depression 160 km (100 miles) across. The eastern
end of the system is Capri Chasma. Erosion of the canyon sides has deposited
debris in the ¬‚at valley bottom and revealed layered structure in the
surrounding plateau. Flow channels leading into the canyon suggest that
erosion by water took place in the remote past. Valles Marineris is thought to
have been created by the uplift of the volcanic ¤ Tharsis Ridge to the west.
Van Allen belts Two ring-shaped regions around Earth where there are
concentrations of high-energy electrons and protons trapped by the Earth™s
magnetic ¬eld. They were discovered by the USA™s ¬rst successful arti¬cial
Earth satellite, Explorer 1, which was launched on January 31, 1958, and
named after James Van Allen, the physicist who led the experiment. The inner
Van Allen belt lies about 0.8 Earth radii above the equator. The main
concentration of the outer belt lies between about two and three Earth radii
above the equator, but a broader region, extending from the inner belt out to
as far as 10 Earth radii, contains protons and electrons of lower energy,
believed to come primarily from the ¤ solar wind. Because the magnetic ¬eld is

[369]
variable star




Simulated Van Allen belts generated in a NASA laboratory.

offset from Earth™s rotation axis, the inner belt dips down towards the surface
in the region of the South Atlantic Ocean, off the coast of Brazil. This ˜˜South
Atlantic Anomaly™™ presents a potential hazard to the operation of arti¬cial
satellites.
variable star A star whose light output varies, whether regularly or irregularly.
A graph showing brightness in relation to time for a variable is known as its
˜˜light curve.™™
Eruptive and cataclysmic variables are unpredictabe. Eruptive variables
include ¤ T Tauri stars, ¤ luminous blue variables, and ¤ ¬‚are stars. Cataclysmic
variables undergo explosive processes and include ¤ novae, ¤ dwarf novae and
¤ supernovae. Pulsating variables oscillate in and out because they are unstable
internally and vary in a regular way. These include ¤ Cepheid variables,
¤ RR Lyrae stars and ¤ Mira stars. Eclipsing binaries vary because one star
periodically passes in front of another. The most well-known example is
¤ Algol. Some stars vary as they rotate because their surface is not uniformly
bright.
The curious method of naming variable stars is mainly a legacy from
Friedrich W. A. Argelander (1799“1875), who used the letters R to Z in
conjunction with the constellation name for the nine brightest variables in
each constellation. After that, pairs of letters, RR to RZ, SS to SZ and so on to ZZ
were used (J being omitted). For further variables, the pairs of letters AA to AZ,
BB to BZ and so on were introduced, bringing the number of available
designations to 334. Since far more than 334 variables are now known in many

[370]
Venera




Part of the Veil
Nebula.

constellations, the ones in excess of 334 are designated as V335, V336,
and so on.
Vega (Alpha Lyrae) The brightest star in the constellation Lyra and the
¬fth-brightest in the sky. It is an ¤ A star of magnitude 0.03 lying 25 light
years away.
Veil Nebula (NGC 6960) Part of the ¤ Cygnus Loop, which is an old ¤ supernova
remnant.
Vela (The Sail) A large southern constellation, which is one of the four parts into
which Nicolas L. de Lacaille divided the ancient constellation Argo Navis. It lies
in part of the Milky Way rich with faint nebulosity and contains 10 stars
brighter than fourth magnitude. The stars Delta and Kappa, together with
Iota and Epsilon Carinae, make an asterism known as the ˜˜false cross™™
because it is sometimes confused with the constellation Crux, the true
Southern Cross.
Vela pulsar A ¤ pulsar in the constellation Vela, associated with a 10 000-year-old
supernova remnant. It is one of the strongest radio pulsars, and the strongest
gamma-ray source in the sky. It was discovered in 1968 and has a short
period of 89 milliseconds, which is steadily increasing at a rate of 10.7
nanoseconds a day as the pulsar loses energy. Since observations of it started,
the pulsar has also undergone several major ¤ glitches.
Venera A series of Soviet spacecraft sent to explore the planet Venus. The ¬rst to
land successfully was Venera 7 in 1970. There were a further nine Venera
probes that returned images of the surface of Venus, and data about the
atmosphere and the composition of the planet™s crust. In 1982, Venera 13
returned what remains the best ever color images from the surface of Venus,
which show a panorama around the landing site.

[371]
Venus




An X-ray image of the Vela pulsar
showing jets ¬ring from the pulsar,
which is the white dot at the center.




Part of a panorama taken by Venera 13 at its landing site on Venus.


Venus The second major planet of the solar system, in order from the Sun. It is one
of the ˜˜terrestrial™™ planets, similar in nature to Earth and only slightly smaller.
Like Earth, it is surrounded by a substantial atmosphere.
Venus comes closer to Earth than any other planet and can be the brightest
object in the sky (apart from the Sun and Moon). Because its orbit lies inside
Earth™s, it can never be more than 47 away from the Sun in the sky. This
means that Venus can be viewed either in the western sky in the evening, or in
the eastern sky in the early morning, but not in the middle of the night. It is
sometimes called the ˜˜morning star™™ or the ˜˜evening star.™™

[372]
Venus




A 3D perspective view of Lavinia Planitia on Venus created from Magellan radar data.

Because it is nearer to the Sun than Earth, Venus goes through a cycle of
phases similar to the Moon™s. At its brightest and nearest, even a small
telescope will show that Venus is actually a crescent.
The surface of Venus is always hidden from us by dense, highly re¬‚ecting
clouds, which are practically featureless in visible light, though ultraviolet
images reveal bands, including a characteristic Y-shape. These clouds consist of
droplets of dilute sulfuric acid, created by the action of sunlight on the carbon
dioxide, sulfur compounds and water vapor present in the atmosphere.
The atmosphere is almost entirely carbon dioxide, and the surface
pressure is more than 90 times the pressure at Earth™s surface. The
exceptionally high surface temperature of 730 K (450  C) is a result of the
¤ greenhouse effect.
Venus was the target of a large number of Soviet and American probes in
the 1970s and 1980s, notably the Soviet ¤ Venera series and the American
¤ Pioneer Venus. In the extremely high temperature and pressure, many of the
probes were destroyed either before returning data or after a relatively short
period of operation. Nevertheless, some managed to analyze the chemical
composition of some surface rocks and to return limited panoramic views of
the surface terrain, showing a rocky desert landscapes.
The ¬rst radar maps produced by spacecraft orbiting the planet showed
that most of the surface consists of vast plains, above which several large
plateaux rise to heights of several kilometers. The two main highland areas are
Ishtar Terra in the northern hemisphere and Aphrodite Terra in the equatorial

[373]
Venus Express


region. The Maxwell Montes are the highest feature, rising to 11 km above the
average surface level.
In 1990, the US ¤ Magellan spacecraft arrived in orbit around Venus and
began mapping the surface by radar in much greater detail than had been
achieved previously. The radar maps show much evidence of both impact
craters and volcanic activity in the relatively recent past. By solar system
standards, the surface of Venus is young: the oldest craters appear to date from
800 million years ago. However, no de¬nite evidence has yet been found of
current volcanism.
The thick atmosphere and high surface temperature mean that impact
craters are rather different from those on other planets and satellites. Smaller
meteorites easily burn up in the atmosphere so there is an absence of smaller
craters. The material thrown out in the powerful impacts of larger meteorites
did not travel far and tended to melt and spread round the craters.
Large numbers of volcanic features have been identi¬ed: lava ¬‚ows, small
domes 2“3 km across, larger volcanic cones hundreds of kilometers across,
˜˜coronae™™ and so-called ˜˜arachnoids.™™ The coronae of Venus are circular or
oval volcanic structures surrounded by ridges, grooves and radial lines. They
appear to be collapsed volcanic domes and are different from any features seen
on other planets or satellites. The ˜˜arachnoids,™™ which get their informal
name from their spider-like appearance, are similar in form to coronae, but
generally smaller. The bright lines extending outwards for many kilometers
indicate formations that may have been created when magma welled up from
the planet™s interior, causing the surface to crack.
Venus Express A European Space Agency spacecraft to study the atmosphere of
Venus, launched on November 9, 2005. After a journey lasting 153 days it was
captured into orbit around Venus and maneuvered into an operating orbit
ranging between 250 km (150 miles) and 66 000 km (41 000 miles) from the




Venus Express. An artist™s impression
of the spacecraft at orbit insertion.


[374]
very-long-baseline interferometry




The Very Large Array (VLA) of the National Radio Astronomy Observatory.


planet. It carried seven instruments and was based closely on the ¤ Mars
Express spacecraft. It was scheduled to carry out a mission lasting 500 days.
Very Large Array (VLA) A radio telescope consisting of 27 dishes, each 25 m
(82 feet) in diameter. Located near Socorro, New Mexico, it is the world™s
largest ¤ aperture synthesis telescope. The dishes are arranged in a Y-shape with
three arms each 21 km (13 miles) long.
Very Large Telescope (VLT) The ¤ European Southern Observatory™s set of four,
linked 8-m (300-inch) telescopes located at ¤ Paranal Observatory in Chile. The
light-gathering power of the four telescopes together is equivalent to that of a
mirror 16 m (52 feet) in diameter. They were completed in 2001. The four
individual telescopes were given names in the local Mapuche language: ANTU
(the Sun), KUEYEN (the Moon), MELIPAL (the Southern Cross) and YEPUN
(Sirius).
Very Long Baseline Array (VLBA) A network of radio telescopes in North America
for ¤ very-long-baseline interferometry. It consists of 10 dishes, distributed from
Hawaii to St Croix in north-east Canada. The effective diameter is 8000 km
(5000 miles) and the resolution that can be achieved is 0.2 milliseconds of arc.
very-long-baseline interferometry (VLBI) A technique in radio astronomy that
creates a ¤ radio interferometer in which the component antennas are separated
by very large distances, typically thousands of kilometers. The antennas are
not connected but signals are recorded, together with very accurate timings, at
each observing station. The data from each station are brought together later.
This technique gives extremely accurate positions for radio sources,
resolutions (but not maps) down to a few milliseconds of arc and directly
detects continental drift.

[375]
very-long-baseline interferometry




The European Southern Observatory™s Very Large Telescope (VLT) at Paranal Observatory in
Chile. Each enclosure houses an 8.2-m telescope.




The locations of the radio antennas forming the Very Long Baseline Array (VLBA).




[376]
Viking


Baselines can be made even greater than Earth™s diameter by placing radio
telescopes in orbit and using them in conjunction with ground-based
telescopes. This was done with the Japanese satellite, HALCA launched in
February 1997. HALCA was placed in an elliptical orbit, providing a baseline up
to three times larger than Earth, and deployed an umbrella-shaped antenna
8 m (26 feet) in diameter. HALCA developed problems in 2003 and its mission
was formally terminated in 2005.
4 Vesta An asteroid discovered by Heinrich W. M. Olbers in 1802. It is the
third-largest asteroid known, with a diameter 576 km (358 miles), and the
brightest of all. It sometimes reaches magnitude 6, making it just detectable by
the naked eye under optimum observing conditions. Vesta™s brightness is due
to its bright surface, which re¬‚ects 25 percent of the light falling on it. As it
rotates every 5.43 hours, regular changes in the color and spectrum are
observed, re¬‚ecting the fact that the surface is not uniform.
Vesta appears to be a true mini-planet which has survived largely intact
since the solar system formed, rather than being a fragment from a larger
body. Hubble Space Telescope images reveal details down to 80 km (50 miles)
across, including impact craters. One large crater seems to have torn away part
of the crust completely, exposing the mantle below. There is evidence for
ancient lava ¬‚ows dating from four billion years ago when the interior was hot
and molten. It is one of the two targets of NASA™s ¤ Dawn spacecraft, to be
launched in 2006. It is scheduled to go into orbit around Vesta in 2011 for eight
months before going on to ¤ Ceres.
Viking Two identical American spacecraft sent to the planet Mars in 1975. Vikings
1 and 2 both consisted of an orbiter, which remained circling the planet, and a
soft lander.
Viking 1 was launched on September 9, 1975 and reached Mars orbit on
June 19, 1976. The lander touched down on Chryse Planitia on July 20, 1976.
The orbiter™s path was adjusted several times to obtain close-up images of
Mars™s moons, Deimos and Phobos, and to observe different aspects of the
martian surface.
Viking 2 was launched on August 20, 1975 and reached Mars orbit on
August 7, 1976. The landing was on Utopia Planitia on September 3, 1976.
The orbiters were equipped with two television cameras and instruments
to map water vapor and temperature. The landers sampled the upper
atmosphere during their descent, made meteorological measurements and
carried out experiments on samples of martian soil. One of the mission™s prime
objectives was to test for the presence of organic material, which might
indicate the existence of life, but nothing incontrovertible was found. The
orbiters and landers returned thousands of images. The whole of the martian
surface was mapped with a resolution of 150“300 m.

[377]
Virgo




The jet streaming from the supermassive black hole at the center of the galaxy M87,
which is identi¬ed with the radio source Virgo A. The jet consists of electrons and other
subatomic particles traveling at close to the speed of light.


The Viking 1 orbiter operated until August 7, 1980, and the Viking 2
orbiter until July 25, 1978. The landers ceased operating in November 1982 and
February 1980, respectively. The mission was regarded as very successful, and
it had greatly exceeded its expected lifetime.
Virgo (The Virgin) The second-largest constellation in the sky and one of the 12
traditional constellations of the ¤ zodiac. The brightest star in Virgo is the ¬rst-
magnitude ¤ Spica, and there are seven others brighter than fourth magnitude.
The constellation contains the rich and relatively nearby ¤ Virgo Cluster of galaxies.
Virgo A The strongest radio source in the constellation Virgo, identi¬ed with the
giant elliptical galaxy M87, which dominates the ¤ Virgo Cluster of galaxies. The
radio emission is associated with a jet 4000 light years long, which is ¬red out
as a consequence of matter falling onto a supermassive black hole in the
nucleus of M87.
Virgo Cluster The nearest rich cluster of galaxies at a distance of about 50“60
million light years and the center of the ¤ Local Supercluster. It covers 120
square degrees of sky and contains several thousand galaxies. It is an irregular
cluster with no central condensation. The giant elliptical galaxy M87 is the
most massive member of the cluster.
VISTA Abbreviation for ˜˜Visible and Infrared Survey Telescope for Astronomy,™™ a
4- m survey telescope at the European Southern Observatory™s ¤ Paranal
Observatory in Chile. It is due to start operation in 2007.

[378]
Voyager 1




An infrared mosaic of the central part of the Virgo cluster of galaxies.


visual binary A ¤ binary star in which the two components can be resolved as
separate images by a telescope.
visual magnitude The ¤ magnitude of a celestial object measured over a
wavelength band corresponding to the sensitivity of the human eye. ¤ V. ¤
Volans (The Flying Fish) A small and faint southern constellation introduced in the
1603 atlas of Johann Bayer with the longer name Piscis Volans, which was later
shortened. Its six main stars are third and fourth magnitude.
Voyager 1 One of a pair of almost identical interplanetary spacecraft launched by
the USA in 1977. The other was ¤ Voyager 2.
The Voyager missions were possible only because of a chance favorable
alignment of the outer planets, Jupiter, Saturn, Uranus and Neptune, that
happens only once in more than a hundred years. Between them, the two
spacecraft were able to explore all these four planets and many of their moons.
¤ Gravity assist was used to accelerate the craft from one encounter to the next.
The missions were immensely successful, making numerous discoveries and
returning huge quantities of data as well as images.
The instruments on the Voyagers consisted of two groups. One set was
designed to sample the craft™s environment and these remained in operation
constantly, even between planetary encounters. The other instruments, to
study the target moons and planets, included a wide-angle camera and a


[379]
Voyager 2




An artist™s concept of one of the two identical Voyager spacecraft.

close-up camera. The main communication dish was 3.7 m in diameter and a
radioactive power source was used.
Voyager 1 was launched on September 5, 1977. Its closest encounter with
Jupiter was on March 5, 1979 at 350 000 km (217 500 miles) and that with
Saturn was on November 12, 1980 at 124 000 km (77 000 miles). At Jupiter, it
passed close to the moons Io and Callisto and at Saturn got closest to Titan,
Rhea and Mimas. After its encounter with Saturn, Voyager 1 left the plane of
the solar system and traveled out into interstellar space. ¤ Voyager Interstellar
¤
Mission.
Voyager 2 One of a pair of interplanetary spacecraft launched by the USA in 1977.
It was virtually identical to ¤ Voyager 1, except that its power source was
designed to last for much longer to survive its longer journey to Uranus and
Neptune.
Voyager 2 was launched on August 20, 1977. Its ¬rst encounter was with
Jupiter on July 9, 1979, and it went within 71 400 km (44 000 miles). It passed
close to Europa and Ganymede, complementing the coverage of the ¤ Galilean
moons obtained by Voyager 1. Voyager 2 arrived at Saturn in August 1981.
Closest approach was on August 25, at a distance of 101 000 km (63 000 miles).
The spacecraft™s trajectory took it near the moons Tethys and Enceladus.
On January 24, 1986 Voyager 2 reached Uranus, which it passed at a
distance of 107 000 km (67 000 miles). The mission was completed with the
spacecraft™s encounter with Neptune and its moon Triton on August 24, 1989,
when it passed within 48 000 km (30 000 miles) of the surface of Neptune.
¤ Voyager Interstellar Mission.
¤


[380]
Vulpecula


Vulcan A hypothetical planet traveling around the Sun within the orbit of
Mercury. Searches were made for such a planet during the late nineteenth
century but it is now known that none exists.
Vulpecula (The Fox) A faint constellation, next to Cygnus, introduced by Johann
Hevelius in 1690 with the name Vulpecula et Anser “ the fox and goose “ which
was later shortened. It contains no stars brighter than fourth magnitude,
but does include the well-known planetary nebula known as the
¤ Dumbbell Nebula.




[381]
W
walled plain A large, ¬‚at-¬‚oored lunar ¤ crater, particularly one that has been
¬‚ooded by lava.
waning The part of the cycle of the Moon™s phases when the illuminated portion
of the visible disk is decreasing. The opposite is waxing.
Water Jar The group of stars Gamma, Eta, Zeta and Pi in the constellation
Aquarius, normally shown as the Water Carrier™s jar in representations of the
mythological ¬gure associated with the constellation.
waxing The part of the cycle of the Moon™s phases when the illuminated portion of
the visible disk is increasing. The opposite is ˜˜waning.™™
Westerbork Observatory A Dutch national radio astronomy observatory that is
part of the Netherlands Foundation for Research in Astronomy. Its
administrative headquarters are at ¤ Dwingeloo Observatory. The instrument at
Westerbork Observatory is called the Westerbork Synthesis Radio Telescope
(WSRT). It is a 14-dish ¤ aperture synthesis telescope, and came into operation
in 1970.
Whipple Observatory ¤ Fred Lawrence Whipple Observatory.
Whirlpool Galaxy (M51; NGC 5194) A ¤ spiral galaxy in the constellation Canes
Venatici, which we see face-on. It is 13 million light years away. This galaxy
was the ¬rst to be recognized as having spiral structure. The discovery was




Part of the Westerbork Observatory™s Synthesis Radio Telescope.


[382]
white dwarf




The central part of the Whirlpool galaxy. The pink regions reveal where new bright stars
are forming. This picture combines images from the Hubble Space Telescope and the
ground-based National Optical Astronomy Observatories.


made by Lord Rosse in 1845. It is accompanied by a much smaller irregular
galaxy, NGC 5195, which is in orbit around it.
white dwarf The remains of a star in an advanced state of ¤ stellar evolution,
composed of ¤ degenerate matter, in which atomic nuclei and electrons are
packed in close together. A white dwarf is created when a star ¬nally runs out
of fuel for nuclear fusion. Its outer layers blow off and form a ¤ planetary nebula
and its core collapses under its own gravity. The process stops when the
electrons in the core cannot be compacted further and instead resist the
collapse. Subramanyan ¤ Chandrasekhar demonstrated theoretically
that the upper mass limit for white dwarfs is 1.4 times the mass of the Sun.
If a more massive stellar core collapses, it must become a ¤ neutron star or
¤ black hole.


[383]
Wild 2, Comet




The size of the white dwarf Sirius B, which has a mass similar to the Sun™s, compared with
the size of Earth.


The ¬rst white dwarf was recognized in 1910. It was the star 40 Eridani B,
which was shown to have a surface temperature of 17 000 K but a total
luminosity so low that it must be smaller than Earth. Other well-known white
dwarfs include van Maanen™s star and Sirius B, a faint companion to the
brightest star in the sky. Sirius B, ¬rst seen in 1862, has a mass about the same
as the Sun™s concentrated in a ball with ¬ve times Earth™s diameter. It is 10 000
times fainter than Sirius A, which is a normal ¤ A star.
Though called ˜˜white™™ dwarfs as a group, these degenerate stars actually
cover a range of temperatures and colors from the hottest, which are white and
have surface temperatures as high as 100 000 K, to cool red objects at only
4000 K. Since they have no internal source of energy, white dwarfs are in a long
process of gradually cooling off, during which their temperature declines.
Their ultimate fate is to become a black dwarf “ a non-luminous dead star.
The spectra of white dwarfs are bewilderingly complex, re¬‚ecting a range
of temperature and composition. Typically, their spectra contain very broad
absorption lines, though some show no lines at all. The line-forming region is
only a few hundred meters thick. Some white dwarfs show evidence only
for hydrogen, presumably because the helium and heavier elements have
sunk in the strong gravity. In other cases, helium and heavier elements are
seen but no hydrogen.
A new classi¬cation scheme for white dwarfs was adopted from 1983. Each
star™s designation consists of three capital letters, the ¬rst being D for
degenerate. The other two letters depend on features seen in the spectrum.
Wild 2, Comet A periodic comet discovered in 1978 by the Swiss astronomer Paul
Wild, observing near Berne. Its orbital period is 6.4 years. On January 2, 2004,
the spacecraft ¤ Stardust collected a sample of material from the ¤ coma of
Comet Wild 2 and returned images of its nucleus, which measures about 5 km
(3 miles) across.

[384]
Wilkinson Microwave Anisotropy Probe




This image of Comet Wild 2 was taken
during the Stardust spacecraft™s close
approach in January 2004. It is a distant
side view of the roughly spherical
comet nucleus. One hemisphere is in
sunlight and the other is in shadow.

Wild Duck Cluster (M11; NGC 6705) An ¤ open cluster of about 200 stars in the
constellation Scutum. Its shape as seen in a small telescope is similar to a ¬‚ight
of wild ducks.
Wilkinson Microwave Anisotropy Probe (WMAP) A NASA space mission
launched in late 2000 to measure the properties of the ¤ cosmic background
radiation at microwave wavelengths over the whole sky. It was placed in an
orbit around the Sun, in a halo orbit around the L2 ¤ Lagrangian point. Its ability
to resolve detail was much greater than that of its predecessor, the ¤ Cosmic
Background Explorer (COBE). The results from WMAP strongly support the ¤ Big
Bang theory of the universe and give an age for the universe of 13.7 billion




The dome of the William Herschel Telescope.


[385]
William Herschel Telescope


years. They also show that the geometry of the universe is ¬‚at (rather than
curved), which supports the idea of ˜˜in¬‚ation™™ “ that the universe expanded
very rapidly soon after it began.
William Herschel Telescope A 4.2-m (160-inch) re¬‚ecting telescope in the Isaac
Newton Group at the ¤ Observatorio del Roque de los Muchachos, La Palma, Canary
Islands. Observing time is shared between the collaborating countries “ the UK,
Spain and the Netherlands. It is a general-purpose telescope, equipped with a
large range of instruments, and came into operation in 1987.
Wilson effect A change in the appearance of a ¤ sunspot as the Sun™s rotation
carries it close to the edge of the Sun™s visible disk (the ˜˜limb™™). The penumbra
of the spot nearest the limb appears wider than that on the other side of the
spot. This is because the sunspot is a depression. The phenomenon was ¬rst
observed by the Scottish astronomer Alexander Wilson (1714“86) in 1769.
WIYN Telescope A 3.5-m telescope at ¤ Kitt Peak, opened in 1994. It is operated
jointly by the University of Wisconsin, Indiana University, Yale University, and
the National Optical Astronomy Observatories.
WMAP Abbreviation for ¤ Wilkinson Microwave Anisotropy Probe.
Wolf, Johann Rudolf (1816“1893) The Swiss astronomer Wolf is remembered for
his comprehensive pioneering work on ¤ sunspots and the ¤ solar cycle. While
director of the Zurich Observatory, he used historical data to discover that
the length of the solar cycle is 11.1 years on average. Under his direction,
Zurich became a world center for information on sunspots and he developed a
formula, based on the number of sunspots visible and their size, to indicate the
level of sunspot activity at any time.
Wolf“Rayet star A rare type of exceptionally hot star with ssurface temperatures
between 20 000 K and 50 000 K. The spectra of Wolf“Rayet stars contain strong,
broad emission lines. The emission lines are thought to come from an
expanding envelope of gas ¬‚owing off the star. Some are the central stars of
¤ planetary nebulae. Their name comes from two nineteenth-century French
astronomers, Charles Wolf and Georges Rayet.




[386]
X
Xena A temporary nickname given by its discoverers to the ¤ dwarf planet, now
formally named ¤ Eris.
XMM-Newton Observatory An X-ray astronomy observatory launched by the
¤ European Space Agency in January 2000 into a 48-hour elliptical orbit around
Earth. The nominal mission was two years, though it was designed to operate
for up to 10 years. The satellite carries three identical X-ray telescopes, each
consisting of 58 nested precision re¬‚ectors, together with a 30- cm optical/
ultraviolet telescope. There are a total of nine instruments for imaging and
spectroscopy. Originally known only as XMM, it was renamed after launch in
honor of ¤ Isaac Newton.
X-ray astronomy The study of X-radiation from astronomical sources. The X-ray
waveband is usually considered to be from about 10 to 0.01 nm, between the
extreme ultraviolet (XUV) and gamma rays.
No X-rays from space can penetrate the atmosphere to the ground, so all
X-ray astronomy is carried out with instruments on rockets or satellites. X-rays




An artist™s impression of the XMM-Newton spacecraft.


[387]
X-ray binary


from the Sun were detected during rocket ¬‚ights in the 1950s. The ¬rst X-ray
source beyond the solar system to be discovered was ¤ Scorpius X-1, found in
1962 by a group led by Ricardo Giacconi. By 1970, there were more than 40
known X-ray sources detected during rocket-borne experiments. However,
satellites were needed to conduct more extensive surveys.
The ¬rst satellite dedicated to X-ray astronomy was Uhuru (1970), the ¬rst
of NASA™s ¤ Small Astronomy Satellite series. In 1973, a telescope capable of
producing X-ray images was used successfully to image the Sun during the
Skylab mission. This X-ray telescope used an array of concentric, cylindrical
mirrors to re¬‚ect the X-rays at grazing incidence and bring them to a focus, and
detectors capable of recording the positions of arrival of the photons over a
¬eld of view. Such an imaging X-ray telescope was used for objects other than
the Sun for the ¬rst time by the ¤ Einstein Observatory. In 1985, a different type
of X-ray telescope, using the ˜˜coded mask™™ technique, was deployed in orbit on
Spacelab 2. This incorporates a diaphragm with a complex pattern of holes.
Other important X-ray astronomy satellites include ¤ ROSAT (1990),
¤ BeppoSAX (1996), the ¤ Chandra X-ray Observatory (1999), and XMM-Newton
(2000).
¤ Black body radiation in the X-ray band comes from sources at temperatures
in excess of one million degrees. However, much of the X-ray emission
detected from astronomical sources is generated in other ways, such as nuclear
reactions in interacting binary star systems.
Most bright X-ray sources are ¤ X-ray binaries, which are interacting binary
stars. The other main sources of astronomical X-rays are the hot diffuse gas
surrounding galaxies and between the galaxies in clusters, ¤ supernova
remnants, and ¤ active galactic nuclei. In 1996, X-rays were for the ¬rst time
detected from several ¤ comets. ¤ Yohkoh.
¤
X-ray binary An interacting binary star system in which one component is a
degenerate star “ a ¤ white dwarf, a ¤ neutron star or a ¤ black hole. There are two
kinds. In high-mass X-ray binaries (HMXB), the degenerate star™s companion is
a star of 10 or 20 solar masses and matter from its extended envelope ¬‚ows
directly onto the degenerate star. In low-mass binaries (LMXB) the two
components are of similar mass and material is transferred to the degenerate
star via an ¤ accretion disk. As it gains gravitational energy, the material
¬‚owing between the stars reaches temperatures high enough for it to emit
X-rays. X-ray binaries often vary. The timescales for the variations may re¬‚ect
the orbital period of the stars around each other, the rotation period of the
degenerate star or a ˜˜wobble™™ of the accretion disk. Their X-ray luminosity
ranges from 100 to 100 000 times the total luminosity of the Sun. Some
systems, called ¤ X-ray bursters, show much more dramatic and random
variations.

[388]
XUV




This artist™s concept of an X-ray binary shows a double star system with a normal Sun-like
star in orbit around a black hole. As gas is pulled from the normal star, it forms a disk
around the black hole and is heated to temperatures of millions of degrees. Intense
electromagnetic forces in the disk can expel jets of high-energy particles.


X-ray burster A stellar X-ray source that has violent and random changes to its
emission.
X-ray bursters were discovered by a Dutch satellite in 1976. The bursts may
last for several days and may recur, but are not regular. A rapid burster repeats
at intervals no longer than 10 seconds. The generally accepted explanation
is that X-ray bursters are interacting binary systems, similar to a ¤ nova, except
that material falls onto a ¤ neutron star rather than a ¤ white dwarf, and the gas
transferred is predominantly helium rather than hydrogen. The X-ray burst
occurs when the accumulation of transferred material reaches the critical
temperature and density to detonate a nuclear explosion. ¤ X-ray astronomy.
¤
X-ray nova An ¤ X-ray binary system that suddenly becomes a temporary very
intense source of X-rays.
X-ray pulsar A ¤ pulsar that emits X-rays.
XUV A term sometimes applied to the short-wavelength end of the ultraviolet
region of the ¤ electromagnetic spectrum in the range 6“60 nm, where it merges
with the X-ray band. It overlaps the region also known as the extreme
ultraviolet (EUV). ¤ ultraviolet astronomy.
¤




[389]
Y
year The period of time taken for the Earth to orbit the Sun. The exact length of
the year depends on the reference point taken.

Types of years

Year type How defined Length in days

Tropical From equinox to equinox 365.242 19
Sidereal Relative to the stars 365.256 36
Anomalistic Between successive perihelion 365.259 64
passages of Earth
Eclipse Time for intersection of Moon 346.620 05
and Earth™s orbits to return to
same alignment relative to
Sun
Applying the third of ¤ Keplers
Gaussian 365.256 90
laws to Earth™s orbit



Yerkes Observatory An observatory in Williams Bay, Wisconsin. The observatory
has the largest refracting telescope ever built, with an objective lens 1 m
(40 inches) in diameter. It was constructed between 1895 and 1897. The project
was largely the brainchild of ¤ George Ellery Hale, who persuaded the Chicago
millionaire Charles Yerkes to ¬nance it.
Ymir A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and is 16 km (10 miles) across.
Yohkoh A Japanese astronomy satellite launched in August 1991 to study X-rays
and gamma rays from the Sun. It operated until December 2001. Yohkoh
means ˜˜sunbeam™™ in Japanese.




[390]
Z
z The symbol normally used for ¤ redshift.
zenith The point directly overhead.
zenithal hourly rate (ZHR) The hypothetical rate at which meteors belonging to a
particular ¤ meteor shower would be observed by an experienced observer,
watching a clear sky with limiting magnitude 6.5, if the radiant were located in
the zenith. In practice, observed rates are always lower, because fewer meteors
are detected when the radiant is lower and skies are rarely so ideally clear.
zodiac A belt of 12 constellations through which the Sun™s path in the sky “ the
¤ ecliptic “ passes. They are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra,
Scorpius, Sagittarius, Capricornus, Aquarius and Pisces. Formerly, the ecliptic
went through only these 12 constellations, but the effects of ¤ precession
and the precise de¬nitions of constellation boundaries mean that it now
also goes through a thirteenth, Ophiuchus. Since the orbits of all the planets,
apart from Pluto, lie very nearly in a plane, the apparent paths of the planets
remain in or close to the zodiacal constellations.
In traditional astrology, the zodiac is divided into 12 equal 30  portions,
each of which is allocated to a ˜˜sign,™™ but these do not correspond exactly to
the astronomical constellations, which are of varying sizes. The effect of
precession has also contributed to increasing disparity between the
astrological signs and the astronomical constellations.
zodiacal light A faint cone of light in the sky extending along the ecliptic. It is
visible on clear moonless nights in the west following sunset, and in the east
just before sunrise. It is caused by sunlight scattered from micrometer-sized
dust particles between the planets. The zodiacal light is dimly present all
round the ecliptic. There is a brighter patch directly opposite the Sun. This is
known as the ˜˜gegenschein,™™ or ˜˜counterglow.™™

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