Astronomers classify galaxies according to their shape and appearance. A galaxy may be found alone in space, but most are in grouped in formations called clusters. Clusters of galaxies range in size from a few dozen members to several thousand. Galaxies are measured in light years. A light year is 9.5 trillion km, and is the distance light travels in one year.
Galaxies vary in size from a few thousand to a million light years in diameter.
Spiral galaxies, which number about half of all galaxies, are shaped like a disk, resembling a pinwheel with arms which spiral outward as it rotates. Spiral galaxies contain middle-aged stars along with clouds of dust and gas. The Milky Way Galaxy is a spiral galaxy, and is only one of the uncountable galaxies. Elliptical galaxies contain older stars and very little dust and gas. They range in their shape from round to flattened, oval-shaped pancakes.