An atmosphere is a collection of gases that surrounded a celestial body that possessed sufficient mass, and therefore sufficient gravity to hold it. Examples included many planets, moons, and large asteroids. Almost all species in the galaxy needed some form of atmosphere to breathe. During the height of the Galactic Empire's colonial period, a system of classification for atmospheres was established. This system, which was afterward adopted by nearly all subsequent governments in the galaxy, separates atmospheres into four distinct categories: