Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass. The gases are attracted by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, and thus have very deep atmospheres (see gas giants).

The term stellar atmosphere is used for the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Relatively low temperature stars may form compound molecules in their outer atmosphere. Earth's atmosphere protects living organisms from ultraviolet rays.