A star such as our own Sun was born when enormous clouds of dust and gas, stretching over 2 light years, collapsed under the force of gravity. As this material was compressed it gradually warmed up to temperatures of 15 million degrees. At this point nuclear fusion began in its core and energy in the form of heat and light travelled out from the centre. As long as there is still material within the star the nuclear fusion process can continue with hydrogen being fused to form helium.
When, however, it has used up its core hydrogen fuel, the star starts to fuse helium and eventually other elements. During this time the star will expand into a red giant or super-giant, becoming tens or even hundreds of times larger. Eventually, no more fuel will be available and the star will collapse under gravity to form a white dwarf star only a few thousand kilometres in diameter.
Life Cycle Menu Large Star Evolution