. 10
( 12)


The Space Shuttle Discovery approaching the International Space Station for docking in
July 2006. A module for the Space Station can be seen in the Shuttle™s cargo bay.

and fourth Shuttles, first flew in 1984 and 1985, respectively. Columbia was
destroyed with the loss of its crew on re-entry into the atmosphere on February
1, 2003. Following this disaster, Shuttle launches were suspended for safety
improvements. Discovery flew again (to the ¤ International Space Station) in July

spectral line

2005 but the next flight was again delayed until July 2006 because of further
safety concerns about the remaining craft in the ageing fleet. NASA hopes that
Shuttles will be able to continue long enough to see the completion of the
International Space Station in 2010 before they are grounded permanently.
space telescope A telescope placed in orbit around the Earth so as to be above the
atmosphere. ¤ Hubble Space Telescope.
Space Telescope Science Institute A research institute in Baltimore, Maryland,
that manages the science program of the ¤ Hubble Space Telescope, processes
data from the telescope and coordinate its operations with the Space Telescope
Operations Control Center.
spacetime A unified framework for specifying the place and time of events and
describing the relationships between events in a mathematical way. Spacetime
takes account of the fact that the speed of light is always the same even when
the emitter or the observer are moving relative to each other. In ¤ general
relativity, gravity is curvature of spacetime.
Spacewatch A project to search for and study movements of comets and asteroids
particularly any that might be a hazard to Earth. It began in the 1980s and carries
out full-time surveys with 1.8-meter and 0.9-meter telescopes at ¤ Kitt Peak.
space weather The physical conditions in space immediately around Earth and
between Earth and the Sun. These conditions vary because of the ¤ solar wind,
¤ coronal mass ejections and other phenomena related to ¤ solar activity.
Special Astrophysical Observatory (SAO) The principal observatory of the
Russian Academy of Sciences, for both optical and radio astronomy. It is
located in the Caucasus region between the Black Sea and the Caspian Sea.
The optical observatory near Zelenchukskaya is the location of the Bolshoi
Teleskop Azimutalnyi (Large Altazimuth Telescope), which has a 6-m (236-inch)
primary mirror. Completed in 1975, it was the ¬rst very large telescope to be
built on an ¤ altazimuth mount. The ¤ RATAN-600 radio telescope is located on
the outskirts of Zelenchukskaya.
special relativity A theory published by Albert ¤ Einstein in 1905, which describes
how relative motion at a constant speed affects the way physical phenomena
are perceived. The special theory of relativity was a direct consequence of the
fact that the speed of light in a vacuum is always the same regardless of
whether the source of light or the observer is moving. The other underlying
principle of the theory is the ˜˜principle of relativity.™™ This states that no
physical experiment can detect that the ˜˜laboratory™™ is in uniform motion.
¤ general relativity.
spectral line A sharp peak or dip in a ¤ spectrum. Spectral lines result when an
atom or ion gives out or absorbs a characteristic amount of energy “ a
˜˜quantum.™™ Emitting a quantum of energy gives an ¤ emission line. Absorbing a
quantum of energy makes an ¤ absorption line in a continuous spectrum.

spectral type

A set of spectra of stars illustrating the range of main spectral types.

spectral type A classification given to a star according to the appearance of its
spectrum. A star™s spectral type mainly reflects its surface temperature but a
luminosity class (whether the star is a dwarf or giant) is often included as well.
The strange order of letters for the classes is a legacy from the ¬rst
comprehensive attempt at classi¬cation, which was undertaken at Harvard
College Observatory and published in 1890. The classes were originally
designated A“Q but were rationalized and re-ordered as a temperature
sequence later, after astrophysicists learned how to interpret spectra correctly.
The outcome was the list of basic types still used: O, B, A, F, G, K and M (see the
table). The main classes have up to 10 subdivisions, indicated by the numbers 0
to 9 (e.g. A0, K5).

Spectral Temperature
type range Principal features of visible spectrum

> 25 000 K
O Relatively few absorption lines. Ionized helium,
doubly ionized nitrogen, triply ionized silicon.
Hydrogen weak.
B 11 000“25 000 K Neutral helium, singly ionized oxygen and
magnesium. Hydrogen stronger than in O stars.
A 7500“11 000 K Strong hydrogen. Singly ionized magnesium,
silicon, iron, titanium, calcium, etc. and some
neutral metals.
F 6000“7500 K Hydrogen weaker and neutral metals stronger
than in A stars. Singly ionized calcium iron,


G 5000“6000 K Ionized calcium most conspicuous. Many
lines of ionized and neutral metals, and of CH
K 3500“5000 K Neutral metals predominate. CH molecule.
< 3500 K
M Strong lines of neutral metals and of TiO

The luminosity classes, introduced in 1943, are:
Ia Luminous supergiants
Ib Less luminous supergiants
II Bright giants
III Normal giants
V Dwarfs/main sequence

In addition to these basic spectral and luminosity types, which cover the
vast majority of stars, additional classi¬cations have been introduced for more
unusual stars. These include ¤ S stars and ¤ carbon stars, ¤ brown dwarfs and
¤ white dwarfs. ¤ Hertzsprung“Russell diagram.
spectrogram A permanent record of a ¤ spectrum, taken as a photograph or with
an electronic detector.
spectrograph An instrument for recording a ¤ spectrum.
spectroheliograph An instrument for obtaining images of all or part of the Sun in
light of one particular wavelength.
spectrometer An instrument for observing a spectrum and measuring features in
it by direct observation.
spectroscope An instrument for observing a ¤ spectrum visually.
spectroscopic binary A ¤ binary star the double nature of which is revealed by its
spectrum even though the two stars cannot be separated visually. In the
spectrum of a double-lined spectroscopic binary, features of both stars are
detected. Their spectral lines shift relative to each other in a regular way
because of the ¤ Doppler effect, as the two stars orbit around each other. In a
single-lined spectroscopic binary, the two stars differ greatly in luminosity so
only the spectrum of the brighter one can be discerned. Its double nature is
given away because its lines change wavelength in a periodic way when
measured relative to a standard spectrum.
spectroscopy The study and interpretation of ¤ spectra.
spectrum (pl. spectra) The result of dispersing a beam of electromagnetic radiation
so that components with different wavelengths are spread out in order of
increasing or decreasing wavelength. The most familiar example of a spectrum
is the natural rainbow, when sunlight is dispersed into its component colors
by raindrops. The full spectrum of ¤ electromagnetic radiation encompasses, in


order of decreasing wavelength, radio waves, microwaves, infrared radiation,
visible light, ultraviolet radiation, X-radiation and gamma radiation.
There are three principal types of spectra “ continuous, emission line and
absorption line. If brightness is plotted against wavelength as a graph, a
continuous spectrum is a smooth line, with no sharp spikes or dips. Emission
lines show as relatively narrow spikes or peaks of intensity. Absorption lines
are relatively narrow dips in the intensity of a continuous spectrum. All three
kinds can be present in the spectrum of the same object.
Continuous spectra result from ¤ black body radiation or ¤ synchrotron
radiation, for example. Spectral lines are created when discrete packets of
energy (quanta), corresponding to precise wavelengths, are emitted or
absorbed by atoms or molecules. ¤ absorption line, continuous spectrum, emission
line, spectral line.
Spica (Alpha Virginis) The brightest star in the constellation Virgo, which is about
260 light years away. It is an ¤ eclipsing binary, of magnitude 1.0 varying
by about 0.1 magnitude with a period of 4.014 days. The primary is a
blue-white ¤ B star with a mass about 11 times the Sun™s. Spica means ˜˜an
ear of corn.™™
spicules Spike-like structures in the Sun™s ¤ chromosphere that may be observed at
or near the limb. They change rapidly, having a lifetime of five to ten minutes.
Typically, spicules are 1000 km (600 miles) across and 10 000 km (6000 km)
long. They are concentrated along the boundaries of the cells that make up the
Sun™s ¤ supergranulation pattern.
spider The bars supporting the secondary mirror in the tube of a reflecting
telescope. Diffraction by the spider causes spikes around images of bright stars.
Spindle Galaxy Popular name for the edge-on galaxy NGC 3115 in the
constellation Sextans. Its shape is reminiscent of a spindle wound with yarn.
It is a highly evolved galaxy with no obvious evidence of dust.
spiral galaxy A ¤ galaxy with spiral arms. ¤ Edwin Hubble divided spiral galaxies
into two broad groups: those with a central bar (SB galaxies) and those without
(S). Each group he additional subdivided into three categories, a, b and c. Sa and
SBa galaxies have tightly wound arms and a relatively large central bulge. Sc
and SBc galaxies have loose arms and a small central bulge, while types Sb and
SBb are intermediate between the two extremes.
Our own ¤ Galaxy (the Milky Way) is a barred spiral. Its structure is fairly
typical: young stars and interstellar material are concentrated in a disk,
particularly in the spiral arms, and around the disk there is a spherical halo
containing old stars and globular clusters.
The spiral arms are not permanent rigid structures. As stars and
interstellar material orbit around the center of the galaxy, they create
temporary regions of enhanced density in a spiral pattern. The arms are


The arms of the nearby spiral galaxy M81 in the constellation Ursa Major are highlighted
in this Spitzer Space Telescope infrared image.

actually made up of different stars and gas clouds at different eras of time.
¤ Hubble classi¬cation.
Spitzer, Lyman (1914“1997) Spitzer was an American astrophysicist who worked
on how matter behaves in interstellar space and inside stars. He was a pioneer in
the study of the ¤ interstellar medium. In 1946 he proposed the construction of a
large telescope in space, and advocated the idea for decades. He was the principal
driving force behind the ¤ Hubble Space Telescope. Spitzer spent his professional
career at Princeton University, where he succeeded Henry Norris ¤ Russell as
director of the Observatory. The ¤ Spitzer Space Telescope was named in his honor.
Spitzer Space Telescope A NASA space telescope for ¤ infrared astronomy, which
was launched on August 25, 2003, initially on a two-and-a-half-year mission.
With its 0.85-m (33-inch) mirror, it was the largest infrared telescope ever
launched into space. It carried instruments for imaging and spectroscopy in
the wavelength range 3“180 mm. To reduce the amount of coolant it needed to
carry, the telescope was placed in an orbit around the Sun, trailing behind
Earth. In this orbit, compared with being in orbit around Earth, it cooled more
quickly and was less affected by Earth™s own infrared emission. The telescope
was named in honor of Lyman ¤ Spitzer, Jr (1914“1997), a distinguished
American astrophysicist, who was the first person to propose putting a large
telescope in space. Initially it had been referred to as the Space Infrared
Telescope Facility or SIRTF.
Sponde A small outer moon of Jupiter discovered in 2001. Its diameter is about
2 km (1 miles).

sporadic meteor

Spitzer, Jr

sporadic meteor A ¤ meteor that does not belong to an identified
¤ meteor shower.
Sputnik Three unmanned spacecraft launched into Earth orbit by the Soviet
Union in 1957 and 1958. Sputnik 1, launched on October 4, 1957, was the
first man-made object to orbit around the Earth.
Square of Pegasus Four bright stars making the shape of a large square. They are
Alpha, Beta and Gamma Pegasi, and Alpha Andromedae. (Alpha Andromedae
was formerly known as Delta Pegasi.)
SS433 A peculiar star, which is number 433 in a catalog compiled by C. Bruce
Stephenson and Nicholas Sanduleak. It is a binary system in which a ¤ neutron
star is dragging material from a more massive normal companion.
SS433 is located 18 000 light years away inside the ¤ supernova remnant
W50, which believed to be about 40 000 years old. It appears as a fourteenth-
magnitude star in the constellation Aquila. In 1976 it was found to be a source
of X-rays, and radio emission was detected the following year. The optical
spectrum reveals a complex situation with periodic variations and evidence for
a pair of jets traveling at a quarter the speed of light.


The neutron star has 0.8 times the Sun™s mass. Its companion is an ¤ O star
or ¤ B star. It is losing material very quickly and is currently put at 3.2 solar
masses. The two orbit each other in 13 days. Material from the larger star
streams into an ¤ accretion disk around the neutron star. Excessive heating
causes some of the material to be blasted out from the central hole of the
ring-shaped accretion disk to form the jets. The disk wobbles slightly, like a
spinning top, over a period of 164 days. This creates a helical pattern in the
radio emission from the jets and a regular 164-day cycle in the apparent
velocity of the jets as viewed from Earth.
S star A cool giant star of basic ¤ spectral type K or M that, unlike ordinary K and M
stars, has absorption in its spectrum caused by the molecule zirconium oxide
(ZrO), and often the molecules lanthanum oxide (LaO), yttrium oxide (YO) and
vanadium oxide (VO) as well. The zirconium and other heavier elements are
the products of nuclear reactions in the star™s interior that have risen up to the
star A large, luminous ball of gas generating energy internally by nuclear fusion.
The minimum amount of material required for a star is about
one-twentieth the mass of the Sun. Below this limit, the core cannot get hot
enough to sustain fusion reactions. The most massive stars known are about
100 solar masses. A star™s mass is the factor that chie¬‚y determines its
temperature and luminosity while it is an ˜˜ordinary star™™ on the ˜˜main
sequence™™ of the ¤ Hertzsprung“Russell diagram.
Stars are made mostly of hydrogen, with helium as their other major
constituent. In the Sun, which is in many ways a typical star, 94 percent of
atoms are hydrogen, 5.9 percent helium and less than 0.1 percent other
elements. By weight, 73 percent is hydrogen, 25 percent helium, 0.8 percent
carbon and 0.3 percent oxygen, the remaining 0.9 percent being all the other
elements. ¤ binary star, spectral type, stellar evolution, variable star.
starburst galaxy A galaxy with an exceptionally high rate of star formation.
Starburst galaxies often emit excessive amounts of infrared radiation, which
may account for over 90 percent of the total energy they give out.
star cloud An area of sky, particularly in the ¤ Milky Way, where there are large
numbers of stars close together.
star cluster A group of stars that formed together. There are two main types,
¤ open clusters and ¤ globular clusters. Very young stars are often found in loose
groupings called stellar ¤ associations.
Stardust A NASA mission launched on February 7, 1999, which flew through the
¤ coma of comet ¤ Wild 2, taking images and collecting a sample of cometary
dust. Using an Earth flyby for ¤ gravity assist, Stardust flew within 237 km
(147 miles) of the comet and collected its samples on January 2, 2004. The
sample was successfully returned to Earth in January 2006.


The starburst galaxy NGC 908.

Star clouds of the Milky Way in the
constellation Sagittarius.

starquake A sudden cracking in the outer crust of a ¤ neutron star, similar to an
earthquake. A starquake causes an abrupt change in the neutron star™s period
of rotation, which is called a ˜˜glitch.™™
steady-state theory One of two rival theories of cosmology debated in the
mid-twentieth century, the other being the ¤ Big Bang theory. ¤ Fred Hoyle was
one of its main advocates. The steady-state theory assumes that the universe
is the same everywhere for all observers at all times. It accounts for the
observed expansion of the universe by postulating that new matter is
continuously created to fill the voids left as existing galaxies move apart. The
discovery of the ¤ cosmic background radiation in 1963 was a major setback for

stellar evolution

The open star cluster NGC 625, which lies in the Small Magellanic Cloud.

the theory and the Big Bang theory is now generally supported by the
overwhelming majority of astrophysicists.
stellar evolution The sequence of changes that happen to a star through its
lifetime, from its birth out of the interstellar medium to final extinction.
Stars form in clusters in the clouds of gas and dust between the stars.
Gravity pulls the material of a newly forming star together. The material
falling in releases energy and the center heats up until the temperature is high
enough for the nuclear fusion of hydrogen into helium. The time taken for a
˜˜protostar™™ to become a fully ¬‚edged star depends on its mass. A star of 10
solar masses takes only 300 thousand years, compared with 30 million years
for a star the same mass as the Sun.
The nuclear ˜˜burning™™ of hydrogen in a star™s core continues until the
hydrogen runs out in the core where it is hot enough. During this phase, the
star is on the main sequence of the ¤ Hertzsprung“Russell diagram. How long this
phase lasts depends on its mass. The Sun will be a main-sequence star for a
total of about 10 billion years (of which about half has passed) but a star three
times more massive spends only 500 million years in this phase.
When hydrogen burning in the core stops because the fuel is used up, the
interior of the star adjusts to the new situation. The now inert helium core

stellar evolution

Comet Wild 2 from data collected by the Stardust spacecraft. To create this image, a
short-exposure image showing surface detail was overlain on a long-exposure image taken
just 10 seconds later showing jets.

In stellar evolution, the ultimate fate of a star depends on its mass to begin with.

Stephan™s Quintet

contracts rapidly. In the process, gravitational energy is released, which heats
the surrounding layers of hydrogen to the point where hydrogen burning
recommences, but in a shell around the core. The result of the new outpouring
of energy is to push the outer layers of the star farther and farther outwards. As
this gas expands, it cools, and the star becomes a red giant. The combined
effect of the increase in size and decrease in temperature is to maintain a more
or less constant luminosity overall. Meanwhile, the helium core continues to
contract until its temperature reaches a hundred million degrees. This is high
enough for the fusion of helium into carbon and oxygen to begin.
Eventually, all the helium in the star™s core is consumed. What happens
next depends on the mass of the star. In more massive stars, the core contracts
again after each fuel has been exhausted. This raises the temperature
suf¬ciently to ignite a new, heavier, nuclear fuel. Ultimately, a situation can be
reached in which the central core has been converted to iron, while around the
core, in a series of shells, silicon, oxygen, carbon, helium and hydrogen are
being burnt simultaneously. Once a star has developed an iron core of about
one solar mass no new reactions are possible. At this stage, the core contracts
until it implodes catastrophically, setting off a ¤ supernova explosion. The
naked core that remains becomes a ¤ neutron star or ¤ black hole.
In lower-mass stars, such as the Sun, the central temperature never gets
high enough to progress beyond the burning of hydrogen and helium in shells
around the core. The outer layers of the star become unstable and separate
from the core to form expanding shells of gas, called a ¤ planetary nebula, that
gradually disperses into space. In fact, most stars lose a signi¬cant amount of
material in the form of stellar ˜˜winds,™™ which are stronger during the later
phases of evolution. The remaining core cools and shrinks, becoming more
and more compressed, until it is about the size of Earth. It becomes a ¤ white
dwarf. A white dwarf has no ongoing internal source of energy. It simply cools
This outline of stellar evolution is for single stars. Membership of a binary
or multiple system may profoundly in¬‚uence the course of a star™s evolution if
it exchanges matter with one of its close companions.
stellar wind Mass loss from a star in the form of a continuous outflow of gas. All
stars lose some gas but the rate is highest for the hottest stars. Through winds
blowing at hundreds or even thousands of kilometers per second, they can lose
a significant fraction of their original mass over their lifetimes. ¤ solar wind.
Stephano A small outer moon of Uranus discovered in 1999. It is about 20 km
(12 miles) across.
Stephan™s Quintet (NGC 7317, 7318a and b, 7319 and 7320) A group of five
galaxies lying close together in the sky, which were first noted by Edouard
Stephan in 1877. Subsequent measurements of their recession velocities show

Steward Observatory


that NGC 7320 is much closer than the others and just happens to lie along the
line of sight by chance. The other four share a common speed and appear to be
physically associated.
Steward Observatory The observatory of the University of Arizona. It operates
telescopes at a number of sites in Arizona. The largest is the Bok Telescope, a
2.29-m (90-inch) on ¤ Kitt Peak, opened in 1969. There is a 1.54-m (60-inch)
reflector and a 42-cm (16-inch) ¤ Schmidt camera at Mount Bigelow in the
Catalina Mountains, 54 km (34 miles) from Tucson. 1.5-m and 1.0-m telescopes
are located at Mount Lemmon, and the ¤ Heinrich Hertz Submillimeter Telescope
together with the 1.8-m Lennon reflector are at the ¤ Mount Graham
International Observatory.
Stickney The largest crater on Mars™s inner moon, ¤ Phobos. It is 10 km (6 miles)
across, over a third of Phobos™s largest diameter of 28 km. Stickney was the
maiden name of the wife of Asaph ¤ Hall (1829“1907), the American
astronomer who discovered Phobos and Mars™s other moon, Deimos.
Stonehenge A prehistoric stone monument in the UK, thought to be of
astronomical significance. Located 130 km (80 miles) west of London,
Stonehenge is one of the finest of all neolithic sites. It was constructed in three
phases, commencing with a bank and ditch around 2800 bc. The surviving
group of sandstone megaliths in a circle 30 m (100 feet) in diameter was erected
in about 2000 bc. Some of the stones form foresight and backsight markers
that indicate crucial rising and setting points for the Sun and Moon with
considerable accuracy. Astronomers have shown how observations made at
Stonehenge would have enabled its users to predict solar and lunar eclipses

submillimeter-wave astronomy

with certainty. If the astronomical interpretation is a correct one, it implies
that the designers of Stonehenge had recorded, or remembered through an
oral tradition, observations extending over many centuries.
stony-iron meteorite A major category of ¤ meteorite, consisting of a mixture of
metallic and stony elements. There are two main types: pallasites and
mesosiderites. Pallasites consist of olivine grains enclosed by metal. Typically
they contain twice as much olivine as metal by volume. Mesosiderites contain
silicates and metal, in roughly equal proportions.
stony meteorite A ¤ meteorite consisting entirely of stony material. Stony
meteorites are divided into two main classes: ¤ chondrites and ¤ achondrites.
More than 90 percent of the meteorites seen to fall (as opposed to those found
by chance) are stony.
stratosphere The region of Earth™s atmosphere above the ¤ troposphere. It lies
between heights of about 15 and 50 km (9 and 30 miles). From the bottom to
the top of the stratosphere, the temperature increases from about 240 K to
270 K.
Stratospheric Observatory for Infrared Astronomy ¤ SOFIA.
Struve, Otto (1897“1963) Otto belonged to the fourth generation of a notable
dynasty of Russian astronomers founded by his great-grandfather Friedrich
Struve (1793“1864). He spent his professional career in the USA where he went
in 1921. He became director of the ¤ Yerkes Observatory (1932“47) and of the
¤ McDonald Observatory (1939“50), which he helped to found. As first director of
the ¤ National Radio Astronomy Observatory (1959“62) he encouraged the search
for extraterrestrial intelligence. Struve™s astronomical work was mainly
concerned with the ¤ interstellar medium and he discovered that it contains
¤ ionized hydrogen.
Subaru Telescope An 8.3-m telescope at the ¤ Mauna Kea Observatories in Hawaii
for the National Astronomical Observatory of Japan. Construction began in
1991 and it opened in 1999. It operates in both the visual and infrared spectral
regions. Subaru is the Japanese word for the ¤ Pleiades.
Submillimeter Array A submillimeter-wave telescope at the ¤ Mauna Kea
Observatories in Hawaii. The telescope consists of eight movable dishes, each
6-m (20 feet) across.
Submillimeter Telescope Observatory A facility at the ¤ Mount Graham
International Observatory run jointly by the University of Arizona and the Max-
Planck-Institut fur Radioastronomie in Bonn, Germany. Its instrument is the
10-m (33-foot) Heinrich Hertz Telescope, which operates in the submillimeter
waveband between 0.3 and 1 mm. First observations were made in 1994.
submillimeter-wave astronomy The study of ¤ electromagnetic radiation from
celestial sources in the wavelength band between 0.3 and 3 millimeters. It uses
a combination of techniques from radio astronomy and infrared astronomy.

Submillimeter Wave Astronomy Satellite

Sudbury Neutrino Observatory. A view
of the structure from the outside.

The telescopes have to be located at particularly dry places and high elevations,
because water vapor in Earth™s atmosphere absorbs strongly at these
wavelengths, and the astronomical signals are mostly weak. However, this
region of the spectrum is important for a number of studies in astronomy,
including the ¤ cosmic background radiation, regions of star formation and
molecules in interstellar clouds.
There are only a few submillimeter telescopes in operation. One is the
¤ James Clerk Maxwell Telescope situated at the ¤ Mauna Kea Observatories, Hawaii,
as is the ¤ Submillimeter Array. The California Institute of Technology™s
Submillimeter Observatory (CSO), a 10.4-m (84-foot) telescope with a
segmented mirror, is also at the same site. The Swedish“ESO Submillimeter
Telescope (SEST) is at the ¤ European Southern Observatory (ESO), La Silla, Chile.
The re¬‚ector has a diameter of 15 m (49 feet) and consists of 176 panels that can
be adjusted separately. The ¤ Heinrich Hertz Submillimeter Telescope is at Mount
Graham in Arizona.
In 1998, the ¬rst orbiting observatory for submillimeter wave astronomy
was launched from the USA “ the ¤ Submillimeter Wave Astronomy Satellite (SWAS) “
with the prime objective of studying the composition of interstellar clouds.
Submillimeter Wave Astronomy Satellite (SWAS) A small NASA satellite
launched in December 1998 to detect and measure the abundance of water,
molecular oxygen, carbon monoxide and atomic carbon in a wide variety of
astronomical objects. It was put into ˜˜hibernation™™ in July 2004 but reactivated
in 2005 to make observations in support of the ¤ Deep Impact mission.
subsolar point The point on a body in the solar system from which observers
would see the Sun directly overhead.
Sudbury Neutrino Observatory (SNO) A detector for neutrinos from the Sun and
other astronomical sources located 2 km (1.25 miles) underground in a nickel


The interior structure of the Sun.

mine near Sudbury, Ontario, Canada. It is based on a tank of 1000 tonnes of
heavy water and detects faint flashes of light emitted as neutrinos are stopped
or scattered. The observatory began operation in 1999. ¤ neutrino astronomy.
Summer Triangle The three bright stars ¤ Vega, ¤ Altair and ¤ Deneb, which are
particularly conspicuous in the summer evening sky.
Sun The central star of the solar system. Within the range stars cover, the Sun is of
medium size and brightness, though the vast majority of stars in the solar
neighborhood are smaller and less luminous. It is a dwarf star of ¤ spectral type
G2 with a surface temperature of about 5700 K. Like all stars, it is a globe of hot
gas and its energy source is nuclear fusion taking place in the center, where the
temperature is 15 million K. Four million tonnes of solar material are
annihilated each second as hydrogen is converted into helium.
Overlying the core is the radiation zone, where high-energy radiation
produced in the fusion reactions is converted to light and heat. Over the
radiation zone is a convection zone in which currents of gas ¬‚ow upwards,
release energy at the surface, then ¬‚own again to be reheated. These
circulating currents create the Sun™s mottled appearance, or ¤ granulation. The
surface layers, or ¤ photosphere, from which the light we see comes, are some
hundreds of kilometers thick.


The layer over the photosphere is the ¤ chromosphere, visible as a glowing
pinkish ring during a total solar eclipse. ¤ Spicules and ¤ prominences erupt
through the chromosphere. The thinnest, outermost layers, forming the solar
¤ corona, merge into the interplanetary medium. ¤ coronal hole, coronal mass
ejection, ¬‚are, solar activity, solar cycle, solar wind, sunspot.

Properties of the Sun

1.989 · 1030 kg (332 946 Earth masses)
6.96 · 105 km (109 Earth radii)
Effective temperature 5785 K
3.9 · 1026 W
Apparent visual magnitude
Absolute visual magnitude 4.79
7 150
Inclination of equator to ecliptic
Synodic rotation period 27.275 days
Sidereal rotation period 25.380 days

sundial A time-keeping instrument consisting of a shadow stick (or gnomon) and a
dial on which the shadow cast by the Sun falls. The dial is graduated in hours.
A sundial measures apparent ¤ solar time. There are many different types of
design for sundials of varying degrees of sophistication.
sundog An alternative name for a ¤ mock Sun or parhelion.
sungrazer A ¤ comet that approaches very close to the Sun and passes through the
Sun™s outer layers. Around a dozen long-period comets, which have other
orbital characteristics in common, form a well-established group of sungrazers.
They are also known as the ˜˜Kreutz group™™ after the Dutch astronomer
Heinrich Kreutz (1854“1907) who, in 1888, was among the first to note the
similarity between the orbits of some of the brightest comets ever observed.
Many of the hundreds of comets discovered near the Sun by the orbiting
¤SOHO solar observatory are sungrazers.
sunspot A relatively dark region on the Sun where the temperature is lower than
in the rest of the ¤ photosphere. Sunspots occur where the magnetic field in the
photosphere is thousands of times stronger than the average field for the Sun.
The strong magnetic field inhibits the upward flow of hot gas, and so has a
cooling effect.
Sunspots can occur individually but often form pairs of opposing magnetic
polarity or in larger groups. A typical sunspot is around twice the size of Earth
and lasts for about a week. Large groups may contain up to 100 spots, extend
over hundreds of thousands of kilometers and persist for up to two months.

superior planet

A large sunspot group imaged by the SOHO spacecraft on March, 30 2001.

In the dark central part of the sunspot, called the ˜˜umbra,™™ the
temperature is about 4200 K compared with the 5700 K of the photosphere. The
umbra forms a depression a few hundred kilometers below the general level of
the photosphere, giving rise to the ¤ Wilson effect. The outer and brighter part
of a sunspot, the ˜˜penumbra,™™ consists of aligned bright grains on a darker
background which look like a border of rays around the spot. The penumbra is
the sloping sides of the depression formed by the spot. ¤ butter¬‚y diagram,
solar activity.
super bubble A large, expanding, bubble of luminous interstellar gas hundreds of
light years across. Super bubbles contain gas that is tenuous but hot. They are
created by ¤ stellar winds streaming from massive hot stars, and by ¤ supernova
supercluster A concentration of clusters of galaxies. About 50 are known. On
average they contain 12 rich galaxy clusters, though the largest have many
more. Superclusters are hundreds of millions of light years across.
supergiant One of the largest, most luminous stars known. Supergiants can be up
to 500 times larger than the Sun and many thousands of times more luminous.
There are supergiants of all ¤ spectral types. They are massive stars (mass
greater than about 10 times the Sun™s) in an advanced state of ¤ stellar evolution.
A supergiant is likely to become a ¤ supernova.
supergranulation A large-scale pattern of cells of moving gas on the Sun.
Super-Kamiokande A neutrino detector located 1 km (3000 feet) underground at
the Kamioka mine in Japan. It is the successor to the earlier Kamiokande
(Kamioka nucleon decay experiment). The main element of the detector is a
tank containing 50 000 tonnes of water. Sensors record flashes of light, emitted
when high-velocity charged particles travel through the water. ¤ neutrino
superior conjunction The position of either Mercury or Venus when it lies on the
far side of the Sun as viewed from the Earth.
superior planet Any of the major planets whose orbits lie outside that of Earth™s “
Mars, Jupiter, Saturn, Uranus and Neptune.

superluminal motion

A supernova that exploded near the nucleus of the spiral galaxy NGC3310 in 1991. This
picture shows the central region of the galaxy before (left, April 1987) and close to
maximum brightness (right, April 5, 1991).

superluminal motion Motion apparently faster than light. The separation on the
sky of the components of some radio sources is increasing at a rate that is
apparently equivalent to as much as 10 times the speed of light when the
distance of the source is taken into account. Speeds in excess of that of light,
however, are physically impossible. In reality, the effect is a purely geometrical
one caused by one component traveling almost directly towards us at almost
the speed of light.
supernova (pl. supernovae) A catastrophic stellar explosion in which so much
energy is released that the supernova alone can outshine an entire galaxy of
billions of stars. In addition to the energy given off as radiation, 10 times more

Denoted N 63A, this supernova remnant is a member of N 63, a star-forming region in the
Large Magellanic Cloud.


The Surveyor III spacecraft is examined on the lunar surface by Charles Conrad Jr., Apollo
12 Commander.

energy goes into the motion of material blown out by the explosion, and a
hundred times as much is carried off by neutrinos.
Supernovae are classi¬ed according to their spectra as Type I or Type II.
Type II supernovae have hydrogen lines in the spectra while Type I supernovae
do not. Type I supernovae are additionally subdivided into Types Ia, Ib and Ic.
Type Ia have a strong line due to ionized silicon. Type Ib have strong neutral
helium lines. Type Ic have neither.
The progress of all Type I supernovae is very similar: their luminosity
increases steadily for about three weeks then declines systematically over
six months or longer. However, Types Ib and Ic are fainter than Ia overall. Type
II supernovae are more varied. Type Ia supernovae occur in all types of
galaxies. Types Ib and Ic are seen only near sites of recent star formation in
spiral galaxies. Type II supernovae occur only in the arms of spiral galaxies.
This suggests that types Ib and Ic are more closely linked with Type II than
with Ia.
The progenitors of Type Ia supernovae are thought to be ¤ white dwarfs in
binary systems that have dragged material from their companions onto
themselves. So much heat is generated it detonates a nuclear explosion that
probably destroys the white dwarf. The nuclear reactions create about one
solar mass of an unstable isotope of nickel, which decays to iron over a

Supernova 1987A

period of months. This radioactive decay accounts for the observed decline in
light output.
Type II supernovae appear to be stars of eight solar masses or more that
have run the course of ¤ stellar evolution and totally exhausted the nuclear fuel
available in their cores. At this stage their structure is like that of an onion,
consisting of concentric spherical shells in which different nuclear reactions
are taking place. Once silicon burning starts in the central core, instability
develops within a day because the iron created cannot fuse into heavier
elements without an input of energy. With no energy being generated, there is
no pressure to balance the weight of the overlying layers.
When the crunch comes, the core collapses in less than a second. The rate
of collapse accelerates as iron nuclei break up and neutrons form. However,
the implosion cannot continue inde¬nitely. At the density of nuclear matter,
there is sudden resistance. The imploding material bounces back and an
outward shock wave is generated. The outer layers of the star are blown
outwards at thousands of kilometers per second, leaving the core exposed as a
¤ neutron star. In some cases, a stellar ¤ black hole is probably created.
The progenitors of Types Ib and Ic are likely to be massive stars that have
evolved and already lost their outer envelopes of hydrogen, exposing a core
made of heavier elements. As with Type II, they explode when the central iron
core collapses.
The material ejected in the explosion forms an expanding ¤ supernova
remnant. The neutron stars can sometimes be detected as ¤ pulsars through their
radio emission and, in some cases, by pulsed light and X-ray emission as well.
The explosion of supernovae serves to enrich the chemical composition of
the interstellar medium from which subsequent generations of stars are
created. Very old stars contain much lower quantities of the elements heavier
than hydrogen and helium than are found in the Sun and solar system and
many of these heavier elements can be created naturally only in the explosion
of a supernova. Supernova shock waves may also trigger star formation as they
propagate through interstellar clouds.
Supernovae in our own Galaxy are fairly rare events: only ¬ve have been
observed visually in the last thousand years. Radio emission from other
remnants has been detected, but the light from those outbursts was concealed
by dust clouds. The nearest supernova of recent times was ¤ Supernova 1987A in
the Large Magellanic Cloud.
Supernova 1987A (SN 1987A) A ¤ supernova in the Large ¤ Magellanic Cloud
discovered on February 24, 1987 when it was about sixth magnitude. It was the
nearest and brightest supernova observed since 1604. The star that exploded
was identified as a twelfth-magnitude blue supergiant, known as Sanduleak “
69 202. Maximum magnitude, reached in mid-May, was near 2.8.


supernova remnant The expanding shell of material created by the ejection of
the outer layers of a star that explodes as a ¤ supernova. Some supernova
remnants are observable visually; others have been detected through their
radio and X-ray emission. A shock wave precedes the material ejected. This
collides with the interstellar gas and heats it. The result is a reverse shock,
moving inwards, which heats the ejected material and the interstellar
material, causing it to emit X-rays. Electrons, accelerated by the shocks, emit
radio waves by the ¤ synchrotron radiation mechanism. The ejected material
breaks up into clumps, so the radiation emitted from the shell often does not
make up a uniform ring.
A small proportion of supernova remnants, including the ¤ Crab Nebula,
have a rather different appearance. In these, the synchrotron radiation coming
from within the shell far outshines any from the shell itself. This type of
supernova remnant has been termed a plerion. An ongoing supply of electrons
traveling at close to the speed of light is needed to account for this emission. In
the Crab Nebula, the pulsar produces the electrons. For plerions where no
pulsar has been detected, it is assumed that we are observing at the wrong
angle to pick up the pulses from the central neutron star. Some other
well-known examples of supernova remnants are ¤ Cassiopeia A, ¤ Kepler™s
Supernova, ¤ Tycho™s Supernova and the ¤ Cygnus Loop.
surface gravity The local value of the acceleration due to gravity experienced by a
free-falling object at the surface of an astronomical body.
Surveyor A series of seven unmanned American spacecraft launched between
1966 and 1968 to soft-land on the Moon. Five were successful. They conducted
experiments to test the Moon™s surface for a subsequent manned landing and
returned a large number of close-up images of the lunar surface.
Suttungr A small outer moon of Saturn in a very elliptical orbit. It was discovered
in 2000 and is about 6 km (3 miles) across.
Swan Nebula An alternative name for the ¤ Omega Nebula.
Swift Short for the Swift Gamma-Ray Burst Explorer, a NASA three-telescope space
observatory launched on November 20, 2004, for a nominal two-year mission.
It was designed to be able to point its gamma-ray, X-ray and ultraviolet
telescopes towards a ¤ gamma-ray burst within minutes of the detection of a
burst. Observations with Swift in 2005 finally resolved the 35-year-old mystery
of what causes short gamma-ray bursts.
Sword of Orion The stars Theta and Iota in the constellation ¤ Orion, which lie
below the three bright stars forming the belt of the mythological figure of
Sycorax One of two small moons of Uranus discovered in 1997 by Brett Gladman
and others, using the ¤ Hale Telescope. It is reddish in color and thought to be a
captured ¤ Kuiper Belt object. Its diameter is estimated to be 190 km (118 miles).

87 Sylvia

87 Sylvia One of the larger asteroids in the asteroid belt, discovered by
Norman R. Pogson in 1866. It is about 280 km (175 miles) across and located in
the outer part of the asteroid belt. It has two small moons, discovered
in 2001 and 2005, which have been named Romulus and Remus. Romulus is
18 km (11 miles) across and Remus 7 km (4.4 miles). With the discovery of
Remus, Sylvia became the first triple asteroid system known. By observing
the motion of Romulus and Remus in their orbits, Sylvia™s density has been
found to be only 20 percent greater than that of water. This means that it is a
˜˜rubble-pile™™ “ a loose collection of pieces of rock and ice with empty space
between the pieces.
symbiotic stars A ¤ binary star system with an unusual type of combination
spectrum. The spectrum has features of both a cool star and emission lines
characteristic of very-high-temperature gas. The normal interpretation is that
the cool star is losing mass to a ¤ dwarf or ¤ white dwarf companion. Symbiotic
stars are variable because mass is transferred irregularly and glowing gas is
sometimes eclipsed by the large cool star. They are also known as Z
Andromedae stars.
synchronous rotation (captured rotation) The rotation of a moon when its
orbital period and the time it takes to rotate on its axis are the same. In
synchronous rotation, the moon always has the same face towards its parent
planet, as is the case with our own Moon. Synchronous rotation is brought
about by tidal interactions over long periods of time.
synchrotron radiation Electromagnetic radiation emitted by an electrically
charged particle traveling almost as fast as light through a magnetic field. It
was first observed in synchrotron accelerators used by nuclear physicists. It is
the major source of radio emission from ¤ supernova remnants and ¤ radio
galaxies. The spectrum of synchrotron radiation has a characteristic profile very
different from that of the thermal radiation emitted by hot gas, making
synchrotron sources easy to identify.
synodic period For planets, the synodic period is the average time between
successive ¤ conjunctions of a pair of planets, as observed from the Sun. When
only one planet is mentioned, the other is taken to be Earth. For a moon, its
synodic period is the average time between successive conjunctions with the
Sun, as observed from its parent planet.

Synchrotron radiation is emitted when
an electron spirals in a magnetic ¬eld.


A view of Mars from Mars Global
Surveyor showing the large dark area
of Syrtis Major Planum towards the
lower right.

synthetic aperture radar (SAR) A radar technique used, for example, by the
¤ Magellan mission to Venus, in which the echoes from radar pulsars emitted at
a rate of thousands per second are processed by computer to generate a
detailed picture of the structure of the reflecting surface.
Syrtis Major Planum A cratered volcanic plain on Mars, identified with a dark,
triangular feature (Syrtis Major) easily visible in telescopic views of the planet.
syzygy The rough alignment of the Sun, Earth and Moon, or the Sun, Earth and
another planet. Syzygy thus describes both ¤ conjunctions and ¤ oppositions.

Tarantula Nebula (NGC 2070) A large cloud of glowing ¤ ionized hydrogen, 900
light years across, in the Large ¤ Magellanic Cloud.
Tarvos A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and its diameter is about 13 km (8 miles).
Tau Ceti A star similar to the Sun 11.7 light years away. It is one of the nearest
stars known and, at magnitude 3.5, one of very few stars no more massive than
the Sun that is visible to the naked eye. Its ¤ spectral type is G8, and it has about
70 percent of the Sun™s mass. It was one of the ¬rst targets of searches for radio
signals from extraterrestrials, but nothing has ever been found and no planets
have been found in orbit around it.
Taurids A modest annual ¤ meteor shower with twin radiants in the constellation
Taurus. It peaks around November 3. The meteoroid stream responsible is
associated with Comet ¤ Encke.

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


6h 5h 4h


χ ·
… 1,2 Pleiades

κ (See inset below)
ζ +20º
± δ 2,1
θ2,1 γ
σ2,1 π
+10º +10º

µ ο


7 x enlarged Asterope
0º Maia 0º


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 Taurus.

Taurus (The Bull) A conspicuous zodiacal constellation, traditionally seen as the
head and forequarters of a bull. It is possibly one of the most ancient of
constellations. The brightest star in Taurus is the ¬rst-magnitude ¤ Aldebaran,
which appears to belong the ¤ Hyades cluster, though it is in fact in the
foreground. In total, Taurus has 14 stars brighter than fourth magnitude. The
¤ Pleiades cluster and the ¤ Crab Nebula also lie within the boundaries of Taurus.
Taurus A The radio source associated with the ¤ Crab Nebula.
Taurus“Littrow valley The site on the Moon where the Apollo 17 astronauts
landed. It is on the south-east border of Mare Serenitatis, in the region of the
crater Littrow. The valley is completely surrounded by mountains, some more
than 2000 m (6500 feet) high.
Taygeta One of the brighter stars in the ¤ Pleiades.
Taygete A small outer moon of Jupiter discovered in 2000. Its diameter is about
5 km (3 miles).


The Taurus“Littrow valley on the Moon. This perspective view was constructed by overlaying
a Hubble Space Telescope image with a digital-terrain model acquired by the Apollo program.

Teapot A popular name for an ¤ asterism formed by some brighter stars in the
constellation Sagittarius.
Teide Observatory An observatory site on the island of Tenerife in the Canary
Islands, shared by the Instituto de Astro¬sica de Canarias with European
partners. The instruments located there include several solar telescopes, a
¤ spectroheliograph, a radio telescope for studying the ¤ cosmic background
radiation and a 1.55-m (61-inch) infrared telescope.
tektite A piece of peculiar natural glass. Tektites are found distributed on Earth™s
surface in four main areas, called ˜˜strewn ¬elds,™™ which are in Australasia, the
Ivory Coast, Moravia and Bohemia in the Czech Republic, and Texas and
Georgia in the USA. Individual tektites can weigh as much as 15 kg (33 lb).
Most, though, are much smaller and their shape and structure suggests that
the molten material from which they formed ¬‚ew through the atmosphere at
high speed. The most popular theory for their origin is that they were created
from terrestrial material when the impacts of large meteorites melted and
ejected rock at the impact sites. Their ages and links to known impact
structures support this theory.
telescope An instrument to collect light “ or any other kind of ¤ electromagnetic
radiation “ from a distant object, bring it to a focus and produce a magni¬ed
image or signal. ˜˜Telescope™™ originally meant an optical instrument but it now
has a much broader meaning in astronomy. Telescopes for observing different
parts of the spectrum “ radio waves or X-rays, for example “ employ widely
differing designs and techniques.
Optical telescopes fall into two main categories, refractors and re¬‚ectors,
according to whether a lens or a mirror is used to collect the light. A refracting


Tektites found in Thailand. The largest of these is 7 cm (3 in) long.

telescope has an objective lens at the front of the telescope tube and either an
eyepiece or equipment (such as a camera) at the back where the image is
formed. In a re¬‚ecting telescope a concave mirror at the back of the tube
collects the light.
The objective of a refracting telescope is usually a compound lens, made of
two or more elements cemented together, with a relatively long focal length.
This helps reduce the problems with lenses that affect the quality of the image
but it also means that refractors tend to be long and bulky. Small refractors are
good for amateurs but very large lenses are dif¬cult to make and mount, and
they absorb too much light for astronomical purposes. The world™s largest
refractor has an objective lens 101 cm (40 inches) in diameter and is at the
¤ Yerkes Observatory.
All large astronomical telescopes are re¬‚ectors. Re¬‚ectors are also popular
with amateurs, being less expensive than refractors and easier to make. In a
re¬‚ector, the light converges towards a focal point in front of the main mirror,
called the ¤ prime focus. However, it is usually diverted, by means of a
secondary mirror, to a more convenient place. Several arrangements are in
common use. The ¤ Newtonian telescope, ¤ Cassegrain telescope, ¤ coude focus and
¤ Nasmyth focus all have different applications. Large multi-purpose
professional telescopes usually offer observers a choice of foci. The Newtonian
focus is used only on amateur visual telescopes.
The primary mirrors in re¬‚ecting telescopes are usually made from glass
or a ceramic material that does not expand or contract when the temperature

Telescopio Nazionale Galileo

changes. The surface must be carefully ¬gured to the required shape, either
part of a sphere or part of a paraboloid, to an accuracy of a fraction of the
wavelength of light. A thin layer of aluminum is then deposited onto the glass
to make the re¬‚ecting surface.
In the design of the most modern large telescopes, ¤ active optics
allow light-weight mirrors to be kept accurately in shape by an array of
computer-controlled supports. This also makes it possible to have mirrors
composed of a number of separate segments or parts.
Both the light-gathering power and the ¤ resolving power of a telescope
depend on the diameter of its ˜˜aperture™™ “ the mirror or lens that collects the
light. Astronomers continually aspire to larger instruments to reach fainter
limiting magnitudes and achieve resolution of greater detail, though some of
these objectives are also achieved with more sensitive detectors and with
¤ interferometers.
Magnifying power is not of great signi¬cance, except with small amateur
telescopes for visual use. The magni¬cation for visual observing is changed by
employing different ¤ eyepieces. The maximum magni¬cation is usually governed
by ¤ seeing conditions rather than the limit of performance of the telescope. The
images formed by astronomical telescopes are inverted. Since the introduction of
a lens to rectify the image would serve no useful purpose and would absorb
valuable light, astronomers prefer to work directly with inverted images.
The mounting of an astronomical telescope is important because it has to
be easily pointed at its targets and able to follow them across the sky. Very
small amateur telescopes and modern computer-controlled telescopes employ
¤ altazimuth mountings. Before the advent of computerized controls, the most
practical method was the ¤ equatorial mounting. Many older telescopes are on
equatorial mounts, and the system remains popular for amateur instruments.
¤ adaptive optics, radio telescope.
Telescopio Nazionale Galileo A 3.5-m re¬‚ecting telescope at the ¤ Observatorio
del Roque de los Muchachos, in the Canary Islands. It was commissioned by Padua
University, Italy, as a national facility for Italian astronomers and completed in
1997. It is modeled on the European Southern Observatory™s ¤ New Technology
Telescopium (The Telescope) An insigni¬cant southern constellation introduced
by Nicolas L. de Lacaille in the mid-eighteenth century. It contains only one star
as bright as third magnitude.
Telesto A small satellite of Saturn, discovered in 1980 when the planet™s rings
were edge-on (and thus invisible) as viewed from Earth. Along with another
small moon, ¤ Calypso, it is in the same orbit as the larger moon ¤ Tethys,
294 660 km (183 090 miles) from Saturn. Telesto measures 30 · 25 · 15 km
(19 · 16 · 9 miles).


Tethys imaged by the Cassini
spacecraft in natural colour.

terminator The boundary between the illuminated and unilluminated parts of the
surface of a planet or moon. Someone at the terminator would be experiencing
dusk or dawn.
Terrestrial Planet Finder A proposed NASA space mission for launch after 2010,
designed to study the formation, development and characteristics of extrasolar
terrestrial planets The inner rocky planets (Mercury, Venus, Earth and Mars),
which are relatively small and made largely of solid rock and metal.
Tethys A moon of Saturn discovered by Giovanni Cassini in 1684. It is almost
spherical with a diameter of 1060 km (665 miles). Its low density, only 1.1
times that of water, suggests that at least half of the interior must be ice.
Images from the ¤ Voyager spacecraft show the surface to be heavily cratered,
though there are regions of lower crater density where the surface has been
changed in the past. Two notable features are the large crater Odysseus, which
is 400 km (250 miles) across, and Ithaca Chasma, a valley more than 2000 km
(1250 miles) long that cuts round three-quarters of Tethys. It is 100 km wide
and several kilometers deep.
Tethys shares its orbit at a distance of 294 660 km (183 090 miles) from
Saturn with two very small moons, Telesto and Calypso.
Thalassa A small moon of Neptune discovered during the ¬‚yby of ¤ Voyager 2 in
August 1989. Its diameter is about 80 km (50 miles).
Tharsis Ridge A raised volcanic area on Mars, 10 km (6 miles) above the average
level for the planet. Three large volcanoes, with peaks up to 27 km (17 miles)
high, lie in a line along the ridge. They are Arsia Mons, Pavonis Mons and
Ascraeus Mons.
Thebe A small inner moon of Jupiter, discovered by Stephen P. Synnott in 1979. It
measures 110 · 90 km (68 · 56 miles).


The three volcanoes of the Tharsis
Ridge on Mars, and Olympus Mons to
the upper left, all veiled with thin

Thelxinoe A small outer moon of Jupiter discovered in 2003. Its diameter is about
2 km (1 mile).
Themis family One of the ¤ Hirayama families of asteroids, located at a distance of
3.13 AU from the Sun. The members of the family are all of the carbonaceous
type, suggesting that they all come from the same parent body. Their prototype
is 24 Themis, which has a diameter of 228 km (142 miles) and was discovered
in 1853 by Annibale de Gasparis.
Themisto A small moon of Jupiter, probably ¬rst sighted in 1975 but positively
con¬rmed in 2000. Its diameter is about 8 km (5 miles).
Theophilus A large lunar crater to the north-west of Mare Nectaris, overlapping
another large crater, Cyrillus. Theophilus is 100 km (60 miles) in diameter and its
terraced walls rise 5 km above the ¬‚oor. A complex central peak rises to 2.2 km.
third contact In a total or annular ¤ eclipse of the Sun, the point when the edges of
the Moon™s disk and Sun™s ¤ photosphere are in contact at the end of totality or
the annular phase. In a lunar eclipse, third contact occurs when the Moon
starts to leave the full shadow (umbra) of the Earth. The term also applies to
the similar stage in a ¤ transit or ¤ occultation.
third quarter The phase of the Moon when half the visible disk of the waning
Moon appears illuminated. Third quarter occurs when the celestial ¤ longitude
of the Moon is 270 greater than the Sun™s.
Thrymr A small outer moon of Saturn in a very elliptical orbit. It was discovered in
2000 and is about 6 km (3 miles) across.
Thuban (Alpha Draconis) A third-magnitude star in the constellation Draco, which
is 310 light years away. It is a rare example of a white giant star. Despite its
designation as Alpha, it is only the seventh-brightest star in Draco. About 5000
years ago Thuban was the nearest bright star to the north celestial pole. Since
then the north pole™s position among the stars has changed because of
¤ precession. Derived from Arabic, Thuban means ˜˜dragon.™™


Thyone A small outer moon of Jupiter discovered in 2003. Its diameter is about
4 km (2.5 miles).
tides The movements of ¬‚uids, or stresses induced in solid objects, by a cyclical
change in the overall gravitational forces acting upon them. On Earth,
¬‚uctuating ocean tides are governed by the daily, monthly and annual
variations in the combined gravitational force of the Sun and Moon. These
variations arise from Earth™s rotation, the Moon™s orbital motion around Earth
and Earth™s orbital motion around the Sun.
time zone A geographical region in which civil time is reckoned to be the same.
Time zones are roughly based on longitude bands 15 wide, corresponding to a
one-hour difference in local time. There are, however, considerable deviations
from regular lines of longitude in the boundaries of time zones in order to
take account of where land is and centers of habitation. The difference
between most adjacent time zones is one hour, but there are some
instances of half-hour differences aimed at minimizing deviations from
local time.
Titan The largest moon of Saturn and the second-largest in the solar system
(after Ganymede). It was discovered in 1655 by Christiaan Huygens.
Titan is 5151 km (3200 miles) across and orbits Saturn at a distance of
1221 850 km (759 220 miles). It is surrounded by a thick atmosphere, mainly of
nitrogen but also containing methane. The surface pressure is 1.5 times greater
than atmospheric pressure at the surface of the Earth and hurricane strength
winds blow in the lower atmosphere. The action of sunlight on the methane
and other substances in the atmosphere, such as carbon monoxide, produces a
layer of opaque orange-colored haze 200 km (125 miles) above the surface.
Because of the haze, ordinary telescopes cannot see Titan™s surface. However,

Three views of Titan. The ¬rst image (left), a natural color composite, shows approximately
what Titan would look like to the human eye. The second (center) is a near-infrared image that
penetrates through the hazy atmosphere and down to the surface. The third view (right),
which is a false color composite, was created by combining two infrared images with a visible
light image.


A Voyager 2 image of Titania made in

infrared radiation can penetrate the haze and be detected, and radar aboard
the ¤ Cassini spacecraft is being used to create images of Titan™s surface. The
Huygens probe released by Cassini on January 14, 2005, parachuted down to
Titan™s surface and returned images and other data.
On Titan, where the temperature is only 95 K (À178  C), water is frozen
solid as rock, but under the conditions there, methane could exist as a liquid.
Before the Cassini“Huygens mission, it was thought there might be extensive
seas of liquid methane on Titan. Large seas have not been found, but there is
much evidence for lakes, shorelines and drainage channels. It is possible that
methane rain sometimes falls from the clouds seen in Titan™s atmosphere, or
that liquid wells up from the ground. Radar images have also shown impact
craters and what is thought to be an ˜˜ice volcano.™™
Titania The largest moon of Uranus, discovered by William ¤ Herschel in 1787. It
measures 1578 km (981 miles) across and orbits Uranus at an average distance
of 435 910 km (270 874 miles). The ¬‚yby of ¤ Voyager 2 in 1986 showed Titania
to be peppered with numerous craters, though there are regions where the
crater density is lower. The surface is also scarred by a large number of valleys
and fractures, some of which cut large craters in half. This suggests that some
process has dramatically changed Titania™s surface in the past.
Titius“Bode law (Bode™s law) A mathematical formula that gives approximations
to the distances of the planets from the Sun. The formula is: D ¼ 0.4 þ (0.3 N)
where D is the distance in ¤ astronomical units (AU) and N takes the values 0, 1,
2, 4, 8, . . . , doubling for each successive planet. The relationship holds to
within a few percent for the seven innermost major planets as long as the
value N ¼ 8 is taken to represent the largest asteroid, ¤ Ceres. However, it
breaks down seriously for Neptune and Pluto.

Tombaugh, Clyde (1906“1997)

Clyde Tombaugh, shortly after his discovery of Pluto.

The formula was devised in 1766 by Johann Titius and copied a few years
later by Johann E. Bode, who published it. At that time, none of the ¤ asteroids
had been discovered and the ˜˜gap™™ at 2.8 AU, where the formula predicted that
there should be a planet, convinced astronomers that a small planet would be
found there, which indeed proved to be the case.
Tokyo Astronomical Observatory The former name of a research institute of
the University of Tokyo, which, in a reorganization in 1988, was largely
incorporated into the new National Astronomical Observatory of Japan, which
has its headquarters at the former Tokyo Astronomical Observatory. Its 8-m
(300-inch) optical/infrared instrument, the ¤ Subaru telescope at the ¤ Mauna Kea
Observatories in Hawaii, was opened in 1999.
Tombaugh, Clyde (1906“1997) The American astronomer Tombaugh discovered
¤ Pluto. Though self-taught and with no formal quali¬cations, he was taken
onto the staff of the ¤ Lowell Observatory in 1929 to work on a systematic search
for a planet beyond Neptune. On 18 February 1930 he discovered Pluto on
photographic plates taken a month earlier.

Torino scale

This computer generated image depicts a view of Earth as seen from the surface of the
asteroid Toutatis on 29 November 1996, when it was 5.2 million km (3.3 million miles)

Torino scale A numerical scale, from 0 to 10, indicating the seriousness of the risk
to Earth presented by a ¤ near-Earth object. It was initiated at a meeting in
Torino, Italy, in 1999, and revised in 2005.
totality The phase of a solar or lunar ¤ eclipse during which the Sun is totally
obscured or the Moon totally in the Earth™s shadow.
4179 Toutatis An Earth-crossing asteroid discovered in 1989 by Christian Pollas.
Radar studies have shown that its shape is very irregular and have revealed the
presence of craters and ridges on the surface. It measures 4.7 · 2.4 · 1.9 km
(2.9 · 1.5 · 1.2 miles) and has two distinct lobes. It might even be two bodies in
close proximity. It rotates in a very complex manner. Both its shape and
rotation are thought to be the result of collisions with other bodies. The plane
of Toutatis™s orbit is closer to the plane of Earth™s orbit than that of any other
known Earth-crossing asteroid. It makes frequent close approaches to Earth
and in 2004, passed at about four times the Moon™s distance.
TRACE Abbreviation for Transition Region and Coronal Explorer, a small NASA
satellite launched in April 1998 to study the Sun. Its purpose was to study the
connection between ¬ne-scale magnetic ¬elds and physical structures on the Sun.
Tracking and Data Relay Satellite System (TDRSS) A network of seven satellites
and a ground station in New Mexico used to track NASA spacecraft and relay
their data and commands.
transient lunar phenomenon (TLP) ¤ lunar transient phenomenon.
transit (1) The passage of a star or other celestial object across the observer™s
¤ meridian.
transit (2) The passage of either of the planets Mercury or Venus across the visible
disk of the Sun. Transits of Venus and Mercury do not occur every time these

transit circle

This image of the Sun taken by the
TRACE mission shows loops of hot gas
in the solar corona which span 30 or
more times the diameter of planet

planets are between the Sun and Earth because their orbits are slightly inclined
to the ¤ ecliptic.
Transits of Venus occur at a series of intervals that repeat regularly over a
243-year period, though the pattern of intervals within the 243 years changes
over timescales of hundreds of years. Since 1631, transits of Venus have been
at intervals of 8, 121.5, 8 and 105.5 years, a situation that will continue
until 2984.
Transits of Mercury are more frequent, but their pattern of occurrence is
more complex. Thirteen or fourteen take place each century, always in either
May or November. The dates of transits in the early twenty-¬rst century are May
7, 2003, November 8/9, 2006, May 9, 2016, and November 11, 2019.

Transits of Venus 1631“2125

Date Start time (UT) Duration

1631 December 7 03:49 3h 00 m
1639 December 4 14:56 6h 58 m
1761 June 6 02:01 6h 35 m
1769 June 3/4 19:15 6h 20 m
1874 December 9 01:50 4h 36 m
1882 December 6 13:57 6h 13 m
2004 June 8 05:15 6h 13 m
2012 June 5/6 22:2 6h 31 m
2117 December 11 00:03 5h 41 m
2125 December 8 13:19 5h 35 m


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