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heating and cooling balance in the interstellar medium depends on density, ionization, radiation and shielding. Write net cooling per volume as $\mathcal{C}=n^2\Lambda (T)-n\Gamma$. Ultraviolet grain photoelectric heating is important in diffuse neutral gas; photoionization heats ionized regions, cosmic rays penetrate shielded gas, and shock waves from stellar winds and supernovae supply hot gas. Cooling includes collisionally excited atomic lines, fine-structure metal lines at lower temperatures, molecular lines in shielded clouds, and continuum radiation and dust emission where appropriate. The heating efficiency and cooling abundances matter as much as a nominal temperature.

A multiphase interstellar medium can contain a cold neutral medium near $10^2\mathrm{K}$, a warm neutral medium and warm ionized medium near $10^4\mathrm{K}$, a hot ionized medium near $10^6\mathrm{K}$, and still colder molecular clouds. Cold and warm atomic equilibrium branches can coexist at similar thermal pressure over a finite pressure interval; this is a calculated heating/cooling balance, not a requirement that all galactic gas have the same pressure.^[ref]

For net cooling per unit mass $\mathcal{L}=\mathcal{C}/\rho$, an isobaric perturbation at equilibrium obeys $c_p{\partial }_t\delta T=-(\partial \mathcal{L}/\partial T{)}_p\delta T$. Thus a negative constant-pressure cooling derivative is thermally unstable: a cool perturbation cools further and condenses. Sound crossing must be fast enough to maintain pressure balance; thermal conduction suppresses sufficiently short wavelengths. Supernova-driven motions, magnetic pressure, finite cooling times and interfaces make the actual medium dynamic, so equilibrium branches describe useful phases rather than a static collection of perfectly separated components.