= Solution
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 is
$$
\dot\rho_\star=\epsilon_{\rm ff}\frac{\rho}{t_{\rm ff}},\qquad t_{\rm ff}=\sqrt{\frac{3\pi}{32G\rho}},
$$
where 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>, $\Sigma_{\rm SFR}\propto\Sigma_g^N$. 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> $t_{\rm dep}=M_g/\Psi$ 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. \b[A galactic rate reflects supply, cloud formation and feedback together, rather than only the free-fall time of its densest <gas>.]<Star Formation in the Milky Way and Nearby Galaxies by Robert C. Kennicutt Jr. and Neal J. Evans II>[Sections3–6: diagnostics, <gas> relations and physical interpretation]{ref}
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