 Galaxy evolution

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.^[ref]