Definition and basic nature of stars

Summary

A star is a large, luminous spheroid of plasma held together by its own gravity. The closest star to Earth is the Sun, and many other stars are visible to the naked eye, though their vast distances make them appear as fixed points of light. Stars are grouped into constellations and asterisms, and astronomers use standardized catalogues and designations to identify and describe them. A star’s life begins when gravity causes a gaseous nebula—mostly hydrogen and helium, with small amounts of heavier elements—to collapse. Over most of its active lifetime, a star shines because thermonuclear fusion in its core converts hydrogen into helium, releasing energy that travels through the star and radiates outward. The star’s initial mass largely determines its evolution and eventual fate. When core fusion can no longer continue, the star’s core becomes a stellar remnant: typically a white dwarf, a neutron star, or (for sufficiently massive stars) a black hole. Stars also produce and recycle chemical elements through stellar nucleosynthesis and mass loss or supernova explosions, enriching the interstellar medium and enabling the formation of new stars.

Key Points

• 1

  A star is a self-gravitating, luminous plasma object.

• 2

  Stars form from the gravitational collapse of hydrogen- and helium-rich gas clouds.

• 3

  Core thermonuclear fusion powers a star for most of its lifetime, and initial mass largely determines its evolution and end state.

• 4

  When fusion stops, stars leave remnants such as white dwarfs, neutron stars, or black holes, and they enrich space with heavier elements.