Stellar population

A group of stars characterized by shared properties such as age, chemical composition, or formation history. Stellar populations help trace how galaxies formed and evolved.

Metallicity: The abundance of elements heavier than helium in an astronomical object, often measured relative to hydrogen. Metal abundance separates populations formed from chemically enriched gas from older, less enriched stars.
Stellar population synthesis: A method for predicting a stellar system’s integrated light from models of its stars. It translates assumed population ages and compositions into observable galaxy spectra and colors.
Galaxy formation and evolution: The processes by which galaxies originate, grow, and change over cosmic time. Stellar populations preserve evidence of when and from what material a galaxy formed.
Population I stars: Relatively young, metal-rich stars commonly associated with galactic disks. They exemplify populations formed from gas enriched by earlier generations of stars.
Stellar age: The time elapsed since a star formed. Age distinguishes newly formed stars from populations that preserve earlier eras of galactic history.
Integrated light: The combined radiation from unresolved stars and other sources in an astronomical system. Distant populations are often studied through their combined light rather than individual stars.
Globular cluster: A dense, gravitationally bound cluster containing many old stars. Its stars provide a relatively contained population for studying age and chemical variation.
Population II stars: Generally old, metal-poor stars commonly found in galactic halos and globular clusters. They contrast with Population I stars in typical age, metallicity, and galactic location.
Stellar evolution: The changes in a star’s structure and emitted light over its lifetime. A population’s observed stars depend on how stellar evolution changes their brightness and lifetimes.
Isochrone: A curve on a color–magnitude diagram showing stars of different masses at the same age and composition. Isochrones connect observed stellar sequences to population age and chemical composition.

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