Life Cycle of Stars

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Stars form in nebulae, where gravity collapses clouds of gas and dust into protostars; when the core is hot enough, fusion begins. A star spends most of its life on the main sequence, fusing hydrogen. Low-mass stars like the Sun become red giants, then shed their outer layers as planetary nebulae and leave white dwarfs.

Massive stars become red supergiants and explode as supernovae, leaving neutron stars or black holes. Supernovae scatter heavy elements, which form new stars, planets and us. Bigger stars live shorter lives.

A star like the Sun spends about 10 billion years on the main sequence, but a star with 20 times the Sun's mass burns through its fuel in only a few million years because its core is far hotter and fusion runs at a much faster rate. When hydrogen in the core runs out, the core contracts and heats up while the outer layers swell into a red giant. Once helium fuses to carbon and oxygen and the Sun's fuel is spent, the remaining core cools slowly as a white dwarf about the size of Earth.

Stars heavier than about eight solar masses fuse heavier elements up to iron, then the core collapses within a second, rebounding as a supernova. All elements heavier than iron, such as gold and uranium, come from supernovae and neutron star mergers, and so the calcium in your bones and the iron in your blood were forged in stars. Supernovae also serve as standard candles for measuring cosmic distances. A frequent misconception is that the Sun will explode as a supernova; it is far too small. It will end as a white dwarf inside a planetary nebula.