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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]

A molecular cloud is a cold region of the interstellar medium in which hydrogen is predominantly molecular hydrogen. Shielding against dissociating ultraviolet radiation helps molecules survive, and radiative cooling allows dense regions to develop. Gravitational collapse within these clouds produces stars, although turbulence, magnetic fields and stellar feedback affect which regions collapse. Cold molecular hydrogen is difficult to detect directly, so observers often map carbon monoxide spectral lines or interstellar dust emission as indirect tracers. Converting either signal into cloud mass requires assumptions about abundance, excitation or dust properties; a bright tracer is not itself a direct weighing of the cloud.^[ref]

Neal J. Evans II studies star formation and the connection between interstellar gas and newly formed stars. His 2012 review with Robert C. Kennicutt Jr. compares ways of estimating gas content and star formation rates, from individual molecular clouds to entire galaxies.

The comparison matters because neither quantity is simply read off a photograph: a measured signal must be converted into gas mass or a star formation rate. The review assembles those measurement prescriptions before comparing the physical relations inferred from them.^[ref]

Galactic star formation requires gas to collect into cold dense regions that can become self-gravitating. Disk-scale gravity, turbulent motions, shear, pressure and magnetic fields affect this collection; formation then proceeds within molecular clouds. A useful local parametrization iswhere the star-formation efficiency per free-fall time summarizes inefficient conversion rather than requiring every cloud to collapse at once.

Across galactic disks the empirical Kennicutt–Schmidt law relates star formation rate surface density to gas surface density, ${\Sigma }_{\mathrm{SFR}}\propto {\Sigma }_g^N$. A slope near $1.4$ is a familiar integrated total-gas fit, while molecular-gas relations in ordinary disks can be closer to linear; the slope and normalization depend on tracer, averaging scale and environment. The gas-depletion time $t_{\mathrm{dep}}=M_g/\Psi$ is the corresponding fuel-consumption scale, not a prediction that inflow and recycling cease.

Massive stars supply ionizing radiation, winds and supernova energy and momentum. This stellar feedback can disperse clouds, sustain turbulence and drive outflows, regulating rather than simply extinguishing all future star formation. Fresh accretion and recycled gas replenish the reservoir. UV and recombination-line emission trace young stars, while infrared emission traces absorbed and re-radiated light; extinction, stellar populations and the assumed initial mass function affect inferred rates. A galactic rate reflects supply, cloud formation and feedback together, rather than only the free-fall time of its densest gas.^[ref]

Robert C. Kennicutt Jr. studies how gas becomes stars in galaxies. His 1998 observational analysis combined normal spiral galaxies and starburst galaxies, finding an approximately $1.4$ power law between disk-averaged star formation rate per unit area and gas surface density. This Kennicutt–Schmidt law connects measurements across whole galaxies to prescriptions used in models of galaxy formation.^[ref]

With Neal J. Evans II, he also reviewed how different observations estimate star formation rates and how the gas–star formation relation changes between molecular clouds and whole galaxies.^[ref]