Definition and basic nature of stars

Summary

A star is a large, self-illuminated spheroid of plasma held together by its own gravity. Its life begins when gravity causes a gaseous nebula—mostly hydrogen and helium, with small amounts of heavier elements—to collapse. The star’s total mass largely determines how it evolves and what it eventually becomes. For most of its active lifetime, a star shines because thermonuclear fusion occurs in its core, typically converting hydrogen into helium. The energy released travels through the star and radiates outward into space. When fusion can no longer continue, the star’s core becomes a stellar remnant—such as a white dwarf, neutron star, or, for sufficiently massive stars, a black hole—while nucleosynthesis and stellar mass loss (including supernova explosions) enrich the surrounding interstellar medium with heavier elements.

Key Points

• 1

  A star is plasma held together by self-gravity and is powered mainly by core thermonuclear fusion.

• 2

  A star forms from the gravitational collapse of a hydrogen- and helium-rich nebula; its mass largely controls its evolution and end state.

• 3

  When fusion stops, the star leaves a remnant (white dwarf, neutron star, or black hole) and can return enriched material to space through winds and supernovae.