= Galaxy evolution
{wiki}
Galaxy evolution is the change of a <galaxy>'s <stellar population>, <gas> supply, structure and <mass> over cosmic time. <Star formation> consumes cold <gas>; evolving <stars> return chemically enriched material, while <stellar feedback> and <active-galactic-nucleus feedback> can heat or expel it. <Galaxy mergers> rearrange stellar <orbits> and may concentrate <gas>, changing both structure and subsequent <star formation>. Observers combine <stellar ages>, <optical spectra> and <star formation rate> diagnostics, then compare populations at different <redshifts>. Such comparisons require care: a distant population is a statistical sample of earlier <galaxies>, rather than a time sequence of the same objects.<Star Formation in the Milky Way and Nearby Galaxies by Robert C. Kennicutt Jr. and Neal J. Evans II>[Sections 3–6: star formation diagnostics, gas content and relations across galactic environments]{ref}
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