Life Cycle of a Star

Every star begins the same way: inside a cold, dense cloud of hydrogen and dust called a stellar nebula. Gravity pulls small overdensities together, heat builds, and the cloud fragments collapse into protostars. When the core temperature reaches roughly 10 million kelvin, hydrogen fusion ignites and a main-sequence star is born.

From that point onward, the star's mass at birthdecides almost everything about its future.

Low- and medium-mass stars

A Sun-like star spends billions of years fusing hydrogen into helium in its core. When the core runs out of hydrogen, the outer layers expand into a red giant. Eventually the star sheds its outer atmosphere as an expanding, glowing shell — a planetary nebula (a name coined before the word "planet" was well defined, and unrelated to actual planets). What is left behind is a hot, dense core about the size of Earth: a white dwarf. Given enough time it will fade into a cold "black dwarf", but the universe is not yet old enough for any to exist.

High-mass stars

Stars above roughly eight solar masses fuse hydrogen, then helium, then carbon, oxygen and progressively heavier elements. Once their cores build up iron, no further fusion releases energy. The core collapses in under a second and the outer envelope explodes as a supernova. The remnant is a neutron star for lower-mass collapses or a stellar-mass black hole if the core exceeds roughly three solar masses.

The Sun's future

The Sun is a low-mass star and will not explode. In roughly five billion years it will become a red giant, swallow Mercury and Venus, and eventually shed a planetary nebula, leaving a hot Earth-sized white dwarf behind. Read our full guide to the Sun.