Galaxies are large systems of stars and gas (and dark matter) that lie outside the Milky Way (the galaxy in which we reside). A typical large galaxy, like the Milky Way, is about 100 thousand light years across and contains 100 billion stars. Some galaxies are much smaller, less than 10% the size of the Milky Way, while others are more than ten times bigger. The nearest large galaxy to ours is the Andromeda galaxy (M31) which lies about 2.5 million light years away. It is part of our Local Group of galaxies, several dozen mostly small galaxies, along with Andromeda and the Milky Way, that comprises our local neighborhood. Most galaxies lie much farther away than Andromeda, and far beyond the bounds of the Local Group. The brighter ones in our night sky, which can be found in the Messier and New General catalogs, are typically tens of millions to hundreds of millions of light years distant. Much fainter galaxies lie beyond, stretching all the way back to the edge of the observable universe, with lookback times more than 12 billion years in the past. Use the links at left to explore some of the properties of galaxies and the observational characteristics we use to try to understand them.