Kim Kalber, age 13, of Champaign, I11., for her question:
HOW MANY GALAXIES ARE THERE?
A galaxy is a system of stars, dust and gas held together by gravity. Astronomers do not know just how many galaxies there are in the universe but they estimate that there are billions. Millions of galaxies have already been photographed through telescopes.
Galaxies range in diameter from a few thousand light years to more than half a million light years. A light year is the distance that light travels in a year: about 5.88 trillion miles.
Large galaxies have more than a trillion stars. Some of the small galaxies have fewer than a billion.
Our solar system lies at the edge of a galaxy called the Milky Way. Only three galaxies outside the Milky Way can be seen from the earth without a telescope. Andromeda Nebula is the name of the galaxy that can be seen by people in the Northern Hemisphere. It is more than two million light years away.
Two galaxies, the Small and Large Magellanic Clouds, can be seen without telescopes by people in the Southern Hemisphere. They are about 200,000 light years from Earth.
Astronomers tell us that galaxies are distributed unevenly in space. Some are found alone in space, but most are clustered in groups. Groups of galaxies range in size from a few members to several thousand.
Most of the galaxies are moving away from each other at tremendous speeds. In addition, the galaxies farthest from the earth appear to travel the fastest. Therefore, most scientists say the universe is expanding constantly.
Galaxies are classified according to appearance and shape. There are two main types: spiral galaxies and elliptical galaxies. A spiral galaxy is shaped like a disk with a bulge in the center. The Milky Way is a spiral galaxy. Elliptical galaxies range in shape from round to flattened globes.
All galaxies rotate, but elliptical galaxies rotate more slowly than spirals. Elliptical galaxies also have less dust and gas than spirals do. Astronomers believe that gravity slowly forms the dust and gas into stars.
There are two theories about the origin of galaxies.
One theory states that new galaxies constantly appear as old ones move apart.
A second theory suggests that masses of gas formed soon after the universe began to expand billions of years ago. Gravity slowly condensed these masses into galaxies. No new galaxies have been formed since then, and so all galaxies are about the same age.
Galaxies give off many kinds of radiation, including radio waves, ultraviolet light, visible light and X rays. Astronomers study this radiation with optical and radio telescopes and other instruments.
Astronomers estimate the distance and motion of a galaxy by measuring its red shift. Red shift is the change in the wave length of light coming from an object moving away from the earth in space.