Astronomy, the most ancient science, began with the study of
the Sun, the Moon, and the visible planets.  The modern
astronomer is still centrally concerned with recording
position, brightness, motion, and other directly observable
features of celestial objects and with predicting their motion
according to the laws of CELESTIAL MECHANICS.  Astrophysics, a
19th- and 20th-century outgrowth of classical astronomy, uses
quantum mechanics, relativity theory, and molecular, atomic,
nuclear, and elementary-particle physics to explain observed
celestial phenomena as the logical result of predictable
physical processes.  The astrophysicist seeks to characterize
the constituents of the universe in terms of temperatures,
pressures, densities, and chemical compositions.  Although the
term astronomer is still used, virtually all astronomers have
been trained in astrophysics.  The broad aim of modern
astronomy is to develop encompassing theories of the origin,
evolution, and possible destiny of the universe as a whole, a
field of endeavor that is known as COSMOLOGY.
 
NEW WINDOWS ON THE UNIVERSE
 
In the 20th century advances in astronomy have been so rapid
that the second half of the century can be considered a golden
age.  Traditional optical astronomy has been revolutionized by
the development of new techniques of FAINT OBJECT DETECTION,
including more sensitive photographic emulsions and a plethora
of electronic imaging devices.  Using a standard TELESCOPE, the 
optical astronomer can now see fainter and more distant objects 
than ever before.  In addition the astronomer is no longer
limited to observing the visible light from celestial bodies.
New instruments now allow the study of the heavens in entirely
new regions of the SPECTRUM.
 
Radio Astronomy
 
In 1931 Karl G. JANSKY of the Bell Telephone Laboratories
discovered extraterrestrial radiation at radio wavelengths and
launched the field of RADIO ASTRONOMY.  During the 1930s Grote
REBER, a radio engineer, further investigated celestial radio
radiation, and single-handedly brought radio astronomy to the
attention of professional astronomers.  As a result of
theoretical investigations by astronomers in the Netherlands
during World War II, an observable radio line, emitted by
neutral hydrogen atoms in space, was predicted at a wavelength
of 21 cm.  Detection of this line caused radio astronomy to
advance rapidly after the war.  Today, radio telescopes
throughout the world are used to study radio emission from the
stars, the planets, the interstellar medium in the Galaxy, and
extragalactic sources.  Achievements in radio astronomy include 
the mapping of galactic structure and the discovery of quasars, 
pulsars, and a large number of complex organic molecules in
interstellar space.  RADAR ASTRONOMY has also been used within
the solar system to determine, for example, the rotational
periods of Venus and Mercury.
 
Infrared Astronomy
 
Although scientists have known since the time of William
Herschel in the late 18th century that infrared radiation from
celestial objects can be detected, it was not until the late
1950s and early 1960s that INFRARED ASTRONOMY became the
subject of intensive research.  Sensitive detectors were
developed that allowed astronomers to explore the infrared
region of the spectrum.  Infrared astronomy has been helpful in 
studying the very young or evolved stars that are commonly
associated with dense clouds of dust.  
 
Ultraviolet, X-Ray, and Gamma-Ray Astronomy
 
In 1957 the USSR launched the first satellite, thus beginning
the space age.  Few other disciplines have benefited from
artificial SATELLITES to the extent that astronomy has.  (See
SPACE EXPLORATION.) For the astronomer, the atmosphere presents a 
murky or opaque barrier through which observations of the far 
infrared, ultraviolet, X-ray, and gamma-ray spectral regions
are difficult or impossible.  Satellites and, to a limited
extent, high-altitude balloons and rockets have become
platforms from which to observe these spectral regions.  Since
1962, the United States and other nations have launched several 
orbiting observatories devoted to observing the ultraviolet and 
X-ray regions (see OAO;  OSO;  HIGH ENERGY ASTRONOMICAL
OBSERVATORY;  UHURU).  These studies have resulted in better
understanding of very hot stars and have produced strong
evidence of the existence of black holes (see BLACK HOLE).  The 
impact of extraterrestrial observations on astronomy in all
parts of the wavelength spectrum is being extended in the 1990s 
by a continuing program of space astronomy supported by the
SPACE SHUTTLE (see GAMMA-RAY ASTRONOMY;  SPACE TELESCOPE;
ULTRAVIOLET ASTRONOMY;  X-RAY ASTRONOMY).
 
THE SOLAR SYSTEM
 
The achievements of astronomy and astrophysics are evident in
the rapidly growing knowledge of the extraterrestrial
environment, from the SOLAR SYSTEM to the most remote galaxies. 
The solar system, as it is known today, comprises the Sun and
nine planets, in order of increasing distance from the Sun:
MERCURY, VENUS, EARTH, MARS, JUPITER, SATURN, URANUS, NEPTIUNE
and PLUTO.  The last, Pluto, was discovered in 1930 by Clyde
TOMBAUGH, an astronomer at the Lowell Observatory.
 
Planets, Asteroids, and Comets
 
Except for Mercury and Venus, each planet has from 1 to more
than 20 natural satellites (see SATELLITE), including Pluto,
whose moon was not discovered until 1978.  The four gas-giant
planets Jupiter, Saturn, Uranus and Neptune all have ring
systems made up of vast swarms of small icy and rocky fragments 
circling those planets in the plane of their equators.  By far
the most spectacular of these ring systems is that of Saturn;
the others are less developed.
 
Between the orbits of Mars and Jupiter lies a belt containing
thousands of minor planets, or asteroids (see ASTEROID).  The
orbits of most of the asteroids restrict them to the region
between Mars and Jupiter, but exceptions of various kinds
exist--including orbits that cross the Earth's orbit or lie
still closer to the Sun.
COMETS can attain distances 150,000 times greater than that
from the Earth to the Sun.  In 1950 the Dutch astronomer Jan
Oort speculated that the solar system is surrounded by a cloud
of comets, most of which never enter the inner regions of the
solar system.  The orbits of only a few are disturbed
sufficiently to bring them near the Earth.  HALLEY'S COMET,
known since 240 BC, swings around the sun once every 76 years
and was visited by probes in 1985-88, during its most recent
swing.
 
Flyby space probes involving most of the planets, and surface
landings on the Moon, Venus, and Mars, have transformed
planetary astronomy.  No longer must observations be made at
great distances;  on-site measurement of numerous physical
properties is now possible.  In studying the planets, the
astronomer must also enlist the aid of the chemist, the
geologist, and the meteorologist.  In spite of the great
increase in knowledge, however, the probes and landings have
raised more questions than they have answered.  The origin of
the solar system, for example, remains unknown.  The VENERA and 
Magellan missions to Venus and the VIKING landers on Mars
indicate that life as it is known on Earth does not exist on
either planet, nor is life possible on the Moon.
 
The Sun
 
The Sun is a star with a surface temperature of 5,800 K and an
interior temperature of about 15,000,000 K.  Because the Sun is 
the nearest star and is easily observed, its chemical
composition and surface activity have been intensely
investigated.  Among the surface features of the Sun are
SUNSPOTS, prominences, and flares.  It is now known that the
maximum number of sunspots occurs approximately every 11 years, 
that their temperature is approximately 4,300 K, and that they
are related to solar magnetic activity in a cycle taking about
22 years to complete.  Studies of historical records have also
shown long-scale variations in sunspot numbers.
 
Predictions based on theory indicate that energy-generating
processes deep within the Sun and other stars should produce a
certain number of chargeless, weightless particles called
neutrinos (see NEUTRINO).  Efforts to detect solar neutrinos
have thus far indicated a far lower rate of neutrino production 
than current theory seems to require, and revisions of theory
may in time prove necessary.  On the other hand, physicists and 
cosmologists are equally interested in the concept that at
least some forms of neutrino have mass and undergo
transformations inside the Sun.
 
THE STARS
 
The accumulation of precise data on some of the nearer STARS
early in the 20th century enabled Ejnar HERTZSPRUNG and Henry
Norris RUSSELL, working independently, to plot a graph of
brightness and color, two basic stellar properties.  When they
plotted intrinsic stellar brightness on one axis and stellar
color (equivalent to surface temperature) on the other axis,
Hertzsprung and Russell found that, instead of being scattered
over the graph, the stars fell into distinct regions:  a
heavily populated, diagonal band, known as the MAIN SEQUENCE,
that varies from bright, hot, blue stars to faint, cool, red
ones;  a horizontal band containing bright, cool, red stars
(the giants);  and a sparsely populated, horizontal band
containing very luminous stars of all colors (the supergiants). 
In honor of these scientists, graphs of the type they plotted
are called HERTZSPRUNG-RUSSELL DIAGRAMS, or simply H-R
diagrams.
 
The features found on the H-R diagrams are a key to modern
astrophysics, because they are basic to an understanding of
STELLAR EVOLUTION.  The star's initial mass determines exactly
the position of the star on the main sequence.  The star
gradually changes, however, thus changing its position on the
H-R diagrams.  As the hydrogen that fuels the star's fusion
reaction becomes depleted, the outer layers of the star expand, 
and it enters the giant phase.  Eventually they become unstable 
and begin to lose mass--some smoothly, others catastrophically,  
depending on their masses.  Most stars pulsate smoothly;  some
may brighten rapidly in older age, blowing material off into
space to form a PLANETARY NEBULA.  A few giant, unstable stars
explode as supernovas (see SUPERNOVA).  In any case, the end is 
inevitable and evolution proceeds to the stellar graveyard.
The most common result of evolution is the WHITE DWARF;  A
large star would end up as a NEUTRON STAR (PULSAR) and,
possibly, a black hole.
 
THE GALAXIES
 
The solar system is located in the outer regions of our GALAXY. 
From the Earth, the visible part of our Galaxy is seen in the
night sky as the Milky Way.  The Galaxy is actually a flattened 
disk about 100,000 light-years wide, surrounded by a spherical
halo approximately 200,000 light-years in diameter and by a
much larger corona.  In all, this system contains stars, gas,
and dust in quantities equivalent to more than 1,000 billion
solar masses.  The Sun takes about 200 million years to orbit
the galactic center, approximately 30,000 light-years from the
Sun in the direction of the constellation Sagittarius.
 
Before the pioneering work of Harlow SHAPLEY in 1917,
astronomers believed that the Sun was near the center of the
galaxy.  Besides correctly locating the solar system, Shapley
showed that globular star clusters (see CLUSTER, star) form a
halo about the center of the Galaxy.  The structure of the
Galaxy has been mapped by now, using the distances of extremely 
luminous stars as well as radio observations of the 21-cm
(8.27-in) line of the hydrogen spectrum, and has been shown to
take the form of a typical spiral galaxy.  Three basic types of 
galaxies exist:  spirals, such as the Milky Way and the
Andromeda galaxy;  irregulars, such as the Magellanic clouds;
and ellipticals.  Representatives of the elliptical galaxies
exist at both extremes of galactic size.  A dwarf elliptical
may contain only a few million stars;  a giant elliptical may
contain trillions (see EXTRAGALACTIC SYSTEMS).
The Galaxy is a member of a gravitationally bound cluster of
galaxies known as the Local Group.  The ANDROMEDA GALAXY and
the MAGELLANIC CLOUDS are conspicuous members of this group,
which contains some 20 galaxies.  As clusters of galaxies go,
this is rather sparse.  Other clusters, such as the one in the
direction of the constellation Virgo, can contain more than
1,000 galaxies.  Gerard de Vaucouleurs indicated the possible
existence of a local supercluster of galaxies, a cluster of
clusters of which the Local Group is a member.
 
Cosmology
 
The nature of the spiral nebulae, as vast, remote collections
of suns, was not understood until 1929 when Edwin P. HUBBLE
identified a variable star in the Andromeda galaxy and
determined its distance.  In 1912 Vesto M.  Slipher had
discovered that the spiral nebulae are receding from the Earth
at high velocities.  With a knowledge of the distances of these 
objects, Hubble was able to demonstrate a relationship between
a galaxy's RED SHIFT (a DOPPLER EFFECT observed in the galaxy's 
spectrum, indicating its recessional velocity) and its
distance:  the farther away a galaxy is, the greater is its red 
shift (see HUBBLE's CONSTANT).  This is considered good
evidence for the BIG BANG THEORY mentioned below.
 
Since Hubble's computations, a continuous effort has been made
to extend the boundaries of the observable universe to more
remote, higher red-shift objects.  Observation of such distant
galaxies contributes to greater understanding of the origin and 
possible fate of the universe.  The search for higher red
shifts took an unexpected turn when, in 1960, two radio sources 
were identified with what appeared to be stars.  This was
surprising, because stars were not expected to be such strong
radio sources, and the spectra of these objects could not be
associated with any type of star.  It was not until 1963 that
Maarten Schmidt correctly interpreted the spectra as having
enormous red shifts.  These objects, known as quasars, are
still the subject of great controversy.  It is not known
whether their red shifts are attributable to their great
distance, as is true of normal galaxies, or to other physical
phenomena.
 
Other evidence has given astronomers a good idea of the origin
of the universe--the concern of COSMOLOGY.  In 1964 A. A.
Penzias and R. W. Wilson discovered an isotropic microwave
BACKGROUND RADIATION characteristic of that emitted by a
blackbody at 3 K.  This BLACKBODY RADIATION, whose existence
has since been confirmed by numerous observations, is believed
to be an artifact of the big bang with which most astronomers
believe the universe began.  According to the big bang theory,
a single, cataclysmic event occurred about 20 billion years ago 
that disrupted the dense mass composed of all matter and
radiation in the universe.  The matter then dispersed, cooled,
and condensed in the form of stars and galaxies.  Most
cosmologists now accept the big bang theory rather than the
rival STEADY-STATE THEORY, and are attempting to account for
the exotic physical events that would have been involved in the 
very first moments of the big bang (see INFLATIONARY THEORY).
They have yet to determine, however, whether the universe will
expand indefinitely or will ultimately collapse upon itself and 
perhaps repeat the process indefinitely.
 
Cynthia E. Irvine
Bibliography:  Abell, G.  O., Realm of the Universe (1984);
Ferris, T., Coming of Age in the Milky Way (1988);  Field, G.
B., and Chaisson, E.  J., The Invisible Universe (1985);
Fredrick, L.  W., and Baker, R.  H., An Introduction to
Astronomy, 9th ed.  (1981);  Harwit, Martin, Cosmic Discovery
(1984);  Illingworth, Valerie, The Macmillan Dictionary of
Astronomy, 2d ed.  (1985);  Jastrow, Robert, and Thompson,
Malcolm R., Astronomy:  Fundamentals and Frontiers, 4th ed.
(1984);  Lovell, Sir Bernard, and Smith, Sir F.  Graham, The
Guide to Modern Astronomy (1987);  Mitton, Jacqueline, Key
Definitions in Astronomy (1982);  Pasachoff, Jay M., Astronomy, 
2d ed.  (1983);  Preston, R., First Light (1988);  Schaaf, F.,
The Starry Room (1988);  Riordan, Michael, and Schramm, David
N., The Shadows of Creation:  Dark Matter and the Structure of
the Universe (1991);  Trefil, J.  S., The Moment of Creation
(1983);  Unsold, Albrecht, and Baschek, R.  B., The New Cosmos, 
3d ed.  (1983);  Zeilik, Michael, Astronomy:  The Evolving
Universe, 4th ed.  (1985).

General Astronomy The Galaxy Space Exploration Gravity
Magnetism Meteors Novas! Black Holes!!! Comets Asteroids The Sun & Planets
Solar System & The Sun Mercury Venus Earth
Mars Jupiter Saturn Uranus Neptune Pluto

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