Galaxies

A galaxy is a large collection of stars, gas, and dust isolated in space and held together by its own gravity. Most galaxies contain large amounts of dark matter that contribute to the gravity that holds galaxies together. The Milky Way is our home galaxy. It contains more than 100 billion stars and is approximately 100,000 light years in size. Galaxies are classified by shape as spiral, elliptical, or irregular. Spiral galaxies have a disk, galactic bulge, and core. Spiral galaxies may or may not have a central bar of stars. Spiral galaxies tend to have two or more spiral arms. Elliptical galaxies do not have spiral arms and in most cases are not flattened into a disk. The largest are giant elliptical galaxies that can extend a million light years across and have up to ten trillion stars. Similarly, the smallest galaxies known are dwarf elliptical galaxies, which can be up to ten percent the size of the Milky Way Galaxy. Irregular galaxies lack any structure. Galaxies are often found in groups containing several individual to several dozen galaxies. Larger collections are known as galaxy clusters.