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The Stellar Life Cycle

The Birth of a Star

Stars are born in vast clouds of gas and dust known as nebulae. As gravity pulls the matter in a nebula together, it begins to rotate and form a protostar. The protostar is a hot, dense ball of gas that is not yet self-sustaining, and it relies on the infalling material from the nebula to continue growing.

As the protostar grows, its core becomes denser and hotter, and nuclear fusion reactions begin to take place. These reactions release enormous amounts of energy, which causes the star to become self-sustaining and to shine. The star will remain on the main sequence for most of its life, burning hydrogen in its core and producing helium as a byproduct.

The Main Sequence

The main sequence is a period of stability in a star's life when it is burning hydrogen in its core and producing helium as a byproduct. The star's temperature, luminosity, and size are relatively constant during this phase.

The length of time a star remains on the main sequence depends on its mass. More massive stars burn through their hydrogen fuel more quickly than less massive stars, so they have shorter main sequence lifetimes.

The Red Giant Phase

When a star runs out of hydrogen fuel in its core, it begins to fuse helium in a shell around the core. This causes the star to expand and cool, and it becomes a red giant. The red giant phase is a relatively brief period in a star's life, and it ends when the star's core collapses and it begins to fuse heavier elements.

The End of a Star's Life

The end of a star's life depends on its mass.

• Low-mass stars: These stars end their lives as white dwarfs. A white dwarf is a hot, dense remnant of a star that has lost its outer layers. White dwarfs eventually cool and become black dwarfs.
• Medium-mass stars: These stars end their lives as neutron stars. A neutron star is a small, dense remnant of a star that has collapsed under its own gravity. Neutron stars are very hot and rotate very quickly.
• Massive stars: These stars end their lives as black holes. A black hole is a region of space with such strong gravity that nothing, not even light, can escape.