Gas is thermally stable for .
Quiescent interstellar gas in dynamical equilibrium can be assumed to be in in thermal equilibrium, i.e., radiative heating is balanced by radiative cooling. Under such circumstances gas at different temperatures and densities can coexist if the pressure is the same in each phase. If the net volumetric cooling is written as where n is the particle density, is the radiative cooling function, and is the heating function. The functional dependence on n reflects that fact that radiative cooling is usually dominated by optically thin emission lines excited by collisions at densities less than the critical density, i.e., the rate of excitation is proportional to n , and the heating rate is proportional to the particle density, e.g., due to absorption of star light or cosmic ray ionization. This implies that .