A galaxy is a large gravitationally bound system of stars, interstellar gas and dust, plasma and unseen dark matter. Typical galaxies contain 10 million to one trillion (107 to 1012) or more stars, all orbiting a common center of gravity.
http://commons.wikimedia.org/wiki/Image:Andromeda_galaxy.jpg
This image is in ultraviolet waves, a Galaxy Evolution Explorer observation of the large galaxy in Andromeda, Messier 31.
