Les Jackson, age 13, of Nashua, N.H., for his question:
IS THERE MORE THAN ONE KIND OF GALAXY?
A galaxy is a system of stars, dust and gas held together by gravity. Galaxies are scattered throughout the universe. Astronomers tell us there are two main types of galaxies: 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 form the central bulge. Dark clouds of dust and gas lie throughout the disk.
An elliptical galaxy may be shaped either round or look like a flattened globe. The light from an elliptical galaxy is brightest in the center and gradually becomes fainter toward its outer regions.
All galaxies rotate, but ellipticals rotate more slowly than spirals. Ellipticals also have less duet and gas than spirals do. Scientists believe that gravity slowly forms the dust and gas into stars.
Our solar system lies near the edge of a galaxy called the Milky Way. The Milky Way is a spiral galaxy.
Astronomers do not know how many galaxies there are in the universe. But they have photographed millions of galaxies through telescopes and they believe there are billions.
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. They 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.
Astronomers have found 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.
Only three galaxies outside the Milky Way can be seen from the earth without a telescope.
People in the Northern Hemisphere can see the Andromeda Nebula, a galaxy more than 2 million light years away. People in the Southern Hemisphere can see two other galaxies: the Small and Large Magellanic Clouds, about 200,000 light years away.
These three visible galaxies can be seen most easily on clear, dark nights and away from bright city lights. These galaxies appear as small, hazy patches of light.
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.
Galaxies range in diameter from a few thousand to half a million light years. A light year is the distance that light travels in a year: about 5.88 trillion miles.
Largo galaxies have more than a trillion stars and small galaxies have fewer than a billion.