Understanding Stars: Life, Properties, and History

Definition and basic nature of stars

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.

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Etymology of the word star

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