A large, spherical celestial body consisting of a mass of gas that is hot enough to sustain nuclear fusion and thus produce radiant energy. Stars begin their life cycle as clouds of gas and dust called nebulae and develop, through gravitation and accretion, into increasingly hot and dense protostars. In order to reach the temperature at which nuclear reactions are ignited (about 5 million degrees K), a protostar must have at least 80 times the mass of Jupiter. For most of its life a star fuses hydrogen into helium in its core, during which period it is known as a dwarf star and is classed according to its surface temperature and luminosity (or spectral type) on a continuum called the main sequence in the Hertzsprung-Russell diagram. When a star exhausts the hydrogen in its core, it typically develops into one of several non-main-sequence forms depending on how massive it is. Smaller stars, with masses less than eight times that of the Sun, become red giants and end their lives, after blowing away their outer layers, as white dwarfs. More massive stars become supergiants and end their lives, after exploding in a supernova, as either a neutron star or ablack hole.

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