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GALAXIES
A 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. Galaxies range in diameter from a few thousand to a half-million lightyears. Small galaxies have fewer than a billion stars. Large galaxies have more than a trillion.
The Milk Way has a diameter of about 100,000 light-years. The solar system lies about 25,000 lightyears from the center of the galaxy. There are about 100 billion stars in the Milky Way. The
Andromeda Galaxy is one of 3 galaxies visible with the naked eye. (2 million light years away)
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 wide.
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 or so. 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 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 dust.
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. 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 electromagnetic spectrum. 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.
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 quasar.
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 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.
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 years.
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. 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.