galaxy is a system of stars, dust, and gas held together by gravity. Our solar
system is in a galaxy called the Milky Way. Scientists estimate that there are
more than 100 billion galaxies scattered throughout the visible universe.
Astronomers have photographed millions of them through telescopes. The most
distant galaxies ever photographed are as far as 10 billion to 13 billion
light-years away. A light-year is the distance that light travels in a vacuum in
a year -- about 5.88 trillion miles (9.46 trillion kilometers). Galaxies range
in diameter from a few thousand to a half-million light-years. Small galaxies
have fewer than a billion stars. Large galaxies have more than a trillion.
The Milky Way has a diameter
of about 100,000 light-years. The solar system lies about 25,000 light-years
from the center of the galaxy. There are about 100 billion stars in the Milky
Only three galaxies outside the Milky Way are visible with the unaided eye.
People in the Northern Hemisphere can see the Andromeda Galaxy, which is about 2
million light-years away. People in the Southern Hemisphere can see the Large
Magellanic Cloud, which is about 160,000 light-years from Earth, and the Small
Magellanic Cloud, which is about 180,000 light-years away.
Groups of galaxies
Galaxies are distributed unevenly in space. Some have no close neighbor.
Others occur in pairs, with each orbiting the other. But most of them are found
in groups called clusters. A cluster may contain from a few dozen to several
thousand galaxies. It may have a diameter as large as 10 million light-years.
Clusters of galaxies, in turn, are grouped in larger structures called
superclusters. On even larger scales, galaxies are arranged in huge networks.
The networks consist of interconnected strings or filaments of galaxies
surrounding relatively empty regions known as voids. One of the largest
structures ever mapped is a network of galaxies known as the Great Wall. This
structure is more than 500 million light-years long and 200 million light-years
Shapes of galaxies
Astronomers classify most galaxies by
shape as either spiral galaxies or elliptical galaxies. A spiral galaxy is
shaped like a disk with a bulge in the center. The disk resembles a pinwheel,
with bright spiral arms that coil out from the central bulge. The Milky Way is a
spiral galaxy. Like pinwheels, all spiral galaxies rotate -- but slowly. The
Milky Way, for example, makes a complete revolution once every 250 million years
New stars are constantly forming out of gas and dust in spiral galaxies.
Smaller groups of stars called globular clusters often surround spiral galaxies.
A typical globular cluster has about 1 million stars.
Elliptical galaxies range in shape from almost perfect spheres to flattened
globes. The light from an elliptical galaxy is brightest in the center and
gradually becomes fainter toward its outer regions. As far as astronomers can
determine, elliptical galaxies rotate much more slowly than spiral galaxies or
not at all. The stars within them appear to move in random orbits. Elliptical
galaxies have much less dust and gas than spiral galaxies have, and few new
stars appear to be forming in them.
Galaxies move relative to one another, and occasionally two galaxies come so
close to each other that the gravitational force of each changes the shape of
the other. Galaxies can even collide. If two rapidly moving galaxies collide,
they may pass right through each other with little or no effect. However, when
slow-moving galaxies collide, they can merge into a single galaxy that is bigger
than either of the original galaxies. Such mergers can produce spiral filaments
of stars that can extend more than 100,000 light-years into space.
Galaxies of a third kind, irregular
galaxies, lack a simple shape. Some consist mostly of blue stars and puffy
clouds of gas, but little dust. The Magellanic Clouds are irregular galaxies of
this type. Others are made up mostly of bright young stars along with gas and
Emissions from galaxies
All galaxies emit (give off) energy as
waves of visible light and other kinds of electromagnetic radiation. In order of
decreasing wavelength (distance between successive wave crests), electromagnetic
radiation consists of radio waves, infrared rays, visible light, ultraviolet
rays, X rays, and gamma rays. All these forms of radiation together make up the
The energy emitted by galaxies comes from various sources. Much of it is due
to the heat of the stars and of clouds of dust and gas called nebulae. A variety
of violent events also provide a great deal of the energy. These events include
two kinds of stellar explosions: (1) nova explosions, in which one of the two
members of a binary star system hurls dust and gas into space; (2) much more
violent supernova explosions, in which a star collapses, then throws off most of
its matter. One supernova may leave behind a compact, invisible object called a
black hole, which has such powerful gravitational force that not even light can
escape it. Another supernova may leave behind a neutron star, which consists
mostly of tightly packed neutrons, particles that ordinarily occur only in the
nuclei of atoms. But some supernovae leave nothing behind.
The intensity of the radiation emitted
by a star at various wavelengths depends on the star's surface temperature. For
example, the sun, which has a surface temperature of about 5500 ¡C (10,000 ¡F)
emits most of its radiation in the visible part of the electromagnetic spectrum.
Radiation of this type, whose intensity depends on temperature as it does for
the sun and other normal stars, is called thermal radiation.
A small percentage of galaxies called active galaxies emit tremendous amounts
of energy. This energy results from violent events occurring in objects at their
center. The distribution of the wavelengths of the emissions does not resemble
that of normal stars, and so the emissions are known as nonthermal radiation.
The most powerful such object is a quasar, which emits a huge amount of radio,
infrared, ultraviolet, X-ray, and gamma-ray energy. Some quasars emit 1,000
times as much energy as the entire Milky Way, yet look like stars in
photographs. Quasar is short for quasi-stellar radio source. The name comes from
the fact that the first quasars identified emit mostly radio energy and look
much like stars. A radio galaxy is related to, but appears larger than, a
A Seyfert galaxy is a spiral galaxy that emits large amounts of infrared rays
as well as large amounts of radio waves, X rays, or both radio waves and X rays.
Seyfert galaxies get their name from American astronomer Carl K. Seyfert, who in
1943 became the first person to discover one.
Some active galaxies emit jets and blobs of highly energetic, electrically
charged particles. These particles include positively charged protons and
positrons and negatively charged electrons. Electrons and protons are forms of
ordinary matter, but positrons are antimatter particles. They are the antimatter
opposites of electrons -- that is, they have the same mass (amount of matter) as
electrons, but they carry the opposite charge. See Antimatter.
The cause of the intense activity in active galaxies is thought to arise from
a colossal black hole at the galactic center. The black hole can be as much as a
billion times as massive as the sun. Because the black hole is so massive and
compact, its gravitational force is powerful enough to tear apart nearby stars.
The resulting dust and gas fall toward the black hole, adding their mass to a
disk of matter called an accretion disk that orbits the black hole. At the same
time, matter from the inner edge of the disk falls into the black hole. As the
matter falls, it loses energy, thereby producing the radiation and jets that
shoot out of the galaxy.
The Milky Way is not
an active galaxy, but it does have a powerful source of radiation called
Sagittarius A* at its center. The cause of this radiation may be a black hole a
million times as massive as the sun.
Origin of galaxies
Scientists have proposed two main kinds of theories of the origin of
galaxies: (1) bottom-up theories and (2) top-down theories. The starting point
for both kinds of theories is the big bang, the explosion with which the
universe began 10 billion to 20 billion years ago. Shortly after the big bang,
masses of gas began to gather together or collapse. Gravity then slowly
compressed these masses into galaxies.
The two kinds of theories differ concerning how the galaxies evolved.
Bottom-up theories state that much smaller objects such as globular clusters
formed first. These objects then merged to form galaxies. According to top-down
theories, large objects such as galaxies and clusters of galaxies formed first.
The smaller groups of stars then formed within them. But all big bang theories
of galaxy formation agree that no new galaxies -- or very few -- have formed
since the earliest times.
Astronomers have found evidence of what conditions were like before the
galaxies formed. In 1965, American physicists Arno Penzias and Robert Wilson
detected faint radio waves throughout the sky. According to the big bang theory,
the waves are radiation left over from the initial explosion. The strength of
the radio waves appeared to be very nearly the same in every direction. But in
1992, a satellite called the Cosmic Background Explorer (COBE) detected tiny
differences in the strength of radio waves coming from different directions. The
differences in strength arise from tiny increases in the density of matter in
the universe shortly after the big bang. The small regions of increased density
had a stronger gravitational force than the surrounding matter. Clumps of matter
therefore formed in these regions; and the clumps eventually collapsed into
Most astronomical observations made to date support big bang theories.
According to these theories, the universe is still expanding. Two kinds of
observations strongly support the idea of an expanding universe. These
observations indicate that all galaxies are moving away from one another and
that the galaxies farthest from the Milky Way are moving away most rapidly. This
relationship between speed and distance is known as the Hubble law of recession
(moving backward), or Hubble's law. The law was named after American astronomer
Edwin P. Hubble, who reported it in 1929.
Astronomers estimate the speed at
which a galaxy is moving away by measuring the galaxy's redshift. The redshift
is an apparent lengthening of electromagnetic waves emitted by an object moving
away from the observer. A redshift can be measured when light from a galaxy is
broken up and spread out into a band of colors called a spectrum. The spectrum
of a galaxy contains bright and dark lines that are determined by the galaxy's
temperature, density, and chemical composition. These lines are shifted toward
the red end of the spectrum if the galaxy is moving away. The greater the amount
of redshift, the more rapid the movement.
Scientists estimate the distance to galaxies by measuring the galaxies'
overall brightness or the brightness of certain kinds of objects within them.
These objects include variable stars as well as supernovae.
Evolution of spiral galaxies
Astronomers do not understand clearly how galactic spirals evolved and why
they still exist. The mystery arises when one considers how a spiral galaxy
rotates. The galaxy spins much like the cream on the surface of a cup of coffee.
The inner part of the galaxy rotates somewhat like a solid wheel, and the arms
trail behind. Suppose a spiral arm rotated around the center of its galaxy in
about 250 million years -- as in the Milky Way. After a few rotations, taking
perhaps 2 billion years, the arms would "wind up," producing a fairly continuous
mass of stars. But almost all spiral galaxies are much older than 2 billion
According to one proposed solution to the mystery, differences in
gravitational force throughout the galaxy push and pull at the stars, dust, and
gas. This activity produces waves of compression. A familiar example of waves of
compression are ordinary sound waves. Because the galaxy is rotating, the waves
seem to travel in a spiral path, leading to the appearance of spiral arms of
dense dust and gas. Stars then form in the arms.
Contributor: Kenneth Brecher, Ph.D., Professor of Astronomy and Physics,
How to cite this article: To cite this article, World Book recommends the
following format: Brecher, Kenneth. "Galaxy." World Book Online Reference
Center. 2005. World Book, Inc.