A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally “milky”, a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (109) stars to giants with one trillion (1012) stars, each orbiting its galaxy’s center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are believed to have supermassive black holes at their centers. The Milky Way contains about 200–400 billion stars and may contain more than 100 billion planets. There are probably more than 170 billion galaxies in the observable universe. Most of them are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space occupies most of the volume of the universe, but even that is mostly empty if not for plasma filling intergalactic filaments between galaxies along which they travel great distances at speeds close to that of light.