Douglas Goldstein, age 13, of Great Falls, Mont., for his question:
HOW MANY GALAXIES ARE THERE?
A galaxy is a system of stars, dust and gas held together by gravity. Galaxies are scattered throughout the universe. Astronomers do not know how many galaxies there are, but they have photographed millions of them through telescopes and they believe there are billions.
Galaxies range in diameter from a few thousand to half a million light years. A light year is the distance that light travels in one yeear: about 5.88 trillion miles.
Large galaxies have more than a trillion stars. Small galaxies have fewer than a billion.
Scientists classify galaxies 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 disk resembles a huge pinwheel, with bright spiral arms that coil out from the central bulge. Dark clouds of dust and gas lie throughout the disk.
Our own galaxy, the Milky Way, is a spiral galaxy.
Elliptical galaxies range in shape from round to flattened globes. The light from an elliptical galaxy is brightest in the center and gradually becomes fainter toward its outer region.
All gallaxies rotate, but ellipticais rotate more slowly than spirals. Ellipticals also have less dust and gas than spirals do. Scientists believe that gravity slowly forms the dust and gas into stars.
Astronomers have discovered that almost all 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 believe the universe is expanding constantly.
Only three galaxies outside the Milky Way can be seen from the earth without a telescope. They can be seen most easily on clear, dark nights and away from bright lights.
People in the Northern Hemisphere can see the Andromeda Nebula, a galaxy more than 2 billion light years away. People in the Southern Hemisphere can see two other galaxies: the Small and the Large Magellanic Clouds, about 200,000 light years away.
Scientists have proposed several theories about the origin of galaxies. According to one theory, masses of gas formed soon afer 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.
Another theory states that new galaxies constantly appear as old ones move apart.
Galaxies are distributed unevenly in space. Some galaxies are found alone in space but most are clustered in groups. Groups of galaxies range in size from a few members to several thousand.
Scientists estimate the distance and motion of a galaxy by mesasuring its red shift. Red shift is the change in the wave length of light coming from an object moving away form the earth in space.