Stellar astronomy

Stellar astronomy studies stars, including their physical properties, distribution, evolution, and systems. It uses observations and physical models to explain how stars form, change, and end.

Stellar spectroscopy: The analysis of stellar spectra to determine stars’ temperatures, compositions, motions, and physical conditions. Spectral lines reveal chemical abundances and line-of-sight motion in stellar observations.
Stellar structure: The arrangement of a star’s interior layers and the balance of forces and energy transport within them. Structure equations explain how gravity, pressure, and energy flow set a star’s observable properties.
Main sequence: The region of the Hertzsprung–Russell diagram occupied by stars that fuse hydrogen in their cores. Most stars spend the longest part of their lives in this stable phase.
Hertzsprung–Russell diagram: A plot comparing stars’ luminosities with their spectral types or effective temperatures. It organizes observed stars into groups that expose evolutionary stages and physical differences.
Brown dwarf: A substellar object too low in mass to sustain ordinary hydrogen fusion in its core. Brown dwarfs lie below the mass boundary separating stars from substellar objects.
Stellar parallax: The apparent shift in a nearby star’s position as Earth moves around the Sun, used to measure distance. Parallax supplies direct distances that anchor estimates of stellar luminosity.
Hydrostatic equilibrium: A state in which pressure gradients balance gravity, preventing a fluid body from collapsing or expanding rapidly. Most stars remain stable because internal pressure balances their self-gravity.
Red giant: A luminous, expanded star that has exhausted hydrogen in its core and burns fuel in surrounding layers. Red giants show how stars change after leaving the main sequence.
Star cluster: A gravitationally associated group of stars that formed together or remain bound as a system. Clusters provide coeval populations for testing stellar ages and evolution.
Exoplanet: A planet that orbits a star other than the Sun. Exoplanets share stellar systems but are distinguished by their planet-like masses and formation.

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