The earth's galaxy is part of a group of 20 galaxies called the Local Group. The earth's galaxies and the Andromeda galaxy are the two largest with each having millions of stars. They all share a common direction of motion that some scientist think is because of a supercluster hidden by view. Superclusters can be up to 300 million light years across. Besides stars and planets, galaxies contain clusters of stars, atomic hydrogen gas, molecular hydrogen, complex molecules, composed of hydrogen, nitrogen, carbon, silicon and cosmic rays.
Galaxies come in several forms.
Some are circular shaped with a bright center. They are called ellipticals and have a large number of old stars and a few new stars. These galaxies come in all sizes. Another type are the flattened or spiral galaxies. They contain some old stars but have a large amount of new, bright stars.