Galaxy

A galaxy is a gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with just a few thousand (103) stars to giants with one hundred trillion (1014) stars, each orbiting their galaxy's own center of mass. Galaxies have historically been categorized according to their visual morphology, including elliptical, spiral, irregular, and starburst. Many galaxies are believed to have black holes at their active center. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times that of our Sun.

The observable universe contains hundreds of billions (more than 1011) of galaxies. Most are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas with an average density less than one atom per cubic meter. The majority of galaxies are organized into a set of associations known as galaxy groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids. (Wikipedia)