Sun

The Sun’s past and future have been inferred from theoretical models of stellar structure. During its first 50 million years, the Sun contracted to approximately its present size. Gravitational energy released by the collapsing gas heated the interior, and when the core was hot enough, the contraction ceased and the nuclear burning of hydrogen into helium began in the core. The Sun has been in this stage of its life for about 4.5 billion years.

Enough hydrogen is left in the Sun’s core to last another 4.5 billion years. When that fuel is exhausted the Sun will change: as the outer layers expand to the present size of the orbit of the Earth or beyond, the Sun will become a red giant, slightly cooler at the surface than at present, but 10,000 times brighter because of its huge size. The Earth may not be swallowed up, however, for it may have spiralled outwards, in response to a loss of mass by the Sun. The Sun will remain a red giant, with helium-burning nuclear reactions in the core, for only about half a billion years. It is not massive enough to go through successive cycles of nuclear burning or a cataclysmic explosion, as some stars do. After the red giant stage it will puff off its outer layers to form a planetary nebula, while the core will shrink to a white dwarf star, about the size of the Earth, and slowly cool for several billion years.