Multiphase interstellar medium

Interstellar gas occupies distinct thermal and ionization regimes because heating, cooling and compression depend on density, radiation and composition. Cold and warm neutral gas can coexist over a finite pressure range, while ionized gas, molecular clouds and hot shock-heated regions add further components. turbulence, magnetic stress and finite cooling times mean that these are dynamically exchanging phases, not necessarily a common global equilibrium.

A useful optically thin model subtracts volumetric heating from radiative cooling. At equilibrium its net cooling vanishes. Pressure-confined temperature perturbations are governed by the derivative of net cooling per unit mass at fixed pressure, so a negative derivative is unstable. The heating law, species abundances and radiation field must be specified before predicting an equilibrium curve. thermal conduction and finite sound-crossing times limit the isobaric approximation.

Dilute shock-heated interstellar plasma with characteristic temperatures around $10^6\mathrm{K}$. supernova energy injection and subsequent cooling are important to its evolution; temperature alone does not determine its volume filling factor.

Diffuse ionized gas near $10^4\mathrm{K}$, maintained by ionization and heating balanced by radiative losses. It is physically distinct from warm gas that remains mostly neutral despite a similar kinetic temperature.

Diffuse mostly neutral atomic gas with temperatures below about $10^4\mathrm{K}$. It can share a pressure range with the cold neutral medium where the thermal equilibrium curve has two stable branches.

The cold predominantly atomic neutral phase of interstellar gas, with characteristic temperatures of order $10^2\mathrm{K}$. It is denser than the warm neutral phase at comparable thermal pressure. Its existence depends on radiation, grain heating and atomic-line cooling; one universal density or temperature does not define every environment.