STAR

A star is a luminous, self-gravitating sphere of plasma that generates energy by nuclear fusion in its core. Most stars form when dense regions of molecular clouds collapse under gravity, forming a protostar that heats and contracts until hydrogen fusion begins.

Energy produced by hydrogen fusion, mainly through the proton-proton chain in smaller stars and the CNO cycle in more massive ones, powers a star’s luminosity. The energy is transported outward by radiation and convection and is radiated across the electromagnetic spectrum. A star’s surface temperature determines its color and spectral type, ranging from hot blue O-type stars to cool red M-type dwarfs; the Sun is a G-type star.

Stellar properties span wide ranges. Masses go from about 0.08 to over 100 solar masses, with luminosities and radii covering many orders of magnitude. On the main sequence, stars fuse hydrogen in their cores; their lifetimes shorten with increasing mass, so massive stars burn brighter but exhaust fuel quickly.

The life cycle of a star depends on initial mass. Low- to intermediate-mass stars become red giants and eventually shed outer layers to leave white dwarfs. High-mass stars end their lives in core-collapse supernovae, leaving neutron stars or black holes. The Sun will follow a slower path, expanding into a red giant and ending as a white dwarf several billion years from now.

Observationally, stars are characterized by brightness, color, and distance. Apparent magnitude describes how bright a star appears from Earth, while luminosity and absolute magnitude describe intrinsic brightness. Distances are measured by stellar parallax and other methods. Stars are central to galactic structure, chemical enrichment, and the study of planetary systems.