Life Cycles of Small- & Large-Mass Stars

Birth

• All stars are born in the same way, though their size is determined by the amount of matter in their birthplace. Gravity pulls together the hydrogen in a nebula -- a massive cloud of gas and dust -- and the hydrogen begins to rotate. The spinning gas increases its speed, begins to glow and forms a protostar. When the protostar reaches 15 million degrees, nuclear fusion begins and the new star stabilizes.

Becoming a Red Giant

• The star's nuclear reaction converts its hydrogen to helium. As the supply of hydrogen begins to run out, the core of the star contracts while the outer part of the star begins to expand and cool. The cooler gas glows red, giving this phase its name.

Death of a Small-Mass Star

• After becoming a red giant, the core of a small star no longer has enough gravity to hold its outer layers together. Eventually, these outer layers escape and spread, becoming a planetary nebula. This nebula may be visible for as long as a million years. When the remaining star core can't sustain fusion, it becomes a white dwarf. Eventually the star cools until it is no longer visible and becomes a black dwarf.

Death of a Large-Mass Star

• A large-mass star has a shorter life span than a small-mass star because it burns its fuel more quickly. After it becomes a red giant (sometimes called a red supergiant), the star eventually destabilizes. Its core is so massive that the entire star implodes, becoming a supernova. If the star's remaining mass is about 1.4 times that of the sun, it will continue to collapse until it forms a neutron star. If its remaining mass is even larger, about three times that of the sun, it will collapse until it forms a black hole.