Galaxy

A galaxy is a massive system, bound by gravity, made up of stars, an interstellar medium and dark matter. The number of stars in a galaxy ranges between ten million and one trillion, depending on its size; there are many multiple stars systems, star clusters and various types of interstellar clouds. There is an estimated number of 100 billion galaxies in the observable universe.

The galaxies are classified according to their apparent shapes:


 * elliptical galaxies
 * dwarf galaxies
 * irregular galaxies
 * spiral galaxies
 * barred spiral galaxies
 * starburst galaxies