Star

A star is a luminous sphere of plasma held together by gravity and powered by nuclear fusion. Its mass determines its structure, lifetime, and final fate.

Hydrostatic equilibrium: A balance in which inward gravity is offset by outward pressure within an object such as a star. This balance keeps a star from collapsing or expanding rapidly during stable phases.
Stellar mass: The total amount of matter in a star, usually measured relative to the Sun’s mass. Mass sets a star’s core pressure and temperature, strongly shaping its evolution.
Molecular cloud: A cold, dense interstellar cloud composed mainly of molecular hydrogen, where stars can form. Gravitational collapse within these clouds begins the formation of stars.
Stellar parallax: The apparent shift in a nearby star’s position against distant background objects as Earth orbits the Sun. Parallax provides direct distances to nearby stars.
Supernova: A powerful stellar explosion that can briefly outshine an entire galaxy. Some massive stars end in supernovae that disperse newly formed elements.
Stellar nucleosynthesis: The creation of chemical elements through nuclear reactions inside stars and during stellar explosions. Fusion in stellar interiors builds heavier elements and supplies energy.
Stellar luminosity: The total electromagnetic energy a star emits per unit time. Luminosity measures a star’s intrinsic energy output, unlike its apparent brightness from Earth.
Jeans instability: The condition in which gravity overcomes internal pressure and causes a gas cloud to collapse. It describes when a region of a molecular cloud can fragment and contract into a protostar.
Spectroscopy: The study of how matter emits, absorbs, or scatters electromagnetic radiation across wavelengths. Stellar spectra let astronomers infer properties that cannot be measured by direct sampling.
Neutron star: An extremely dense stellar remnant composed primarily of neutrons, formed in some supernovae. The collapsed core of a massive star can become a neutron star.

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