HOW MANY GALAXIES ARE THERE?
A galaxy is a system of stars, dust and gas held together by gravity. They are scattered throughout the universe. Astronomers aren't sure how many galaxies there are but they have photographed millions of them through telescopes. They believe there may be billions of them.
A galaxy may have a diameter from a few thousand to a half 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 while small galaxies have fewer than a billion.
Our solar system lies near 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.
Astronomers tell us that galaxies are distributed unevenly in space. Some are found alone in space but most are clustered in groups. Some groups of galaxies have just a few members, others have several thousand.
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 looks like a huge pinwheel with bright spiral arms that coil out from the center bulge. Dark clouds of duet and gas lie throughout the disk. 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 generally becomes fainter toward its outer regions.
Ail galaxies rotate, but ellipticals rotate more slowly than spirals. Eilipticais also have less dust and gas than spirals do. Scientists say that gravity slowly forms the dust and gas into stars.
Astronomers say 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.
Because galaxies are moving away from each other, most astronomers believe that the universe is expanding constantly.
Scientists have suggested several theories about the origin of galaxies. According to one theory, 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.
Another theory states that new galaxies constantly appear as old ones move apart.
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 wavelength of light coming from as object moving away from the Earth in space.