An atmosphere (from Greek ἀτμός (atmos), meaning "vapour", and σφαῖρα (sphaira), meaning "sphere"[1][2]) is a layer of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if its gravity is high and the atmosphere's temperature is low.
The atmosphere of Earth is mostly composed of nitrogen. It also contains oxygen used by most organisms for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis. The atmosphere helps protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. Its current composition is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.
The term stellar atmosphere describes the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Stars with sufficiently low temperatures may form compound molecules in their outer atmosphere.
