Galaxies

Galaxies are tremendously large collections of stars, gas, and dust held together by the weakest force in the universe, gravity. There can be anywhere from a few million to nearly a trillion stars in individual galaxies. The structure of a galaxy defines its classification both by the size of its nucleus and the shape of its arms (or lack thereof). Galaxy classes include spiral, barred-spiral, elliptical, peculiar, and combinations thereof. It is now well known that at the center of most galaxies is a super massive black hole containing thousands to millions of stellar masses compressed by gravity to an infinitesimally small point.