The cloud-intercloud phase-change in the interstellar medium

Pressure equilibrium is not the only condition necessary for the equilibrium of an interstellar cloud-the interface between cloud and intercloud medium as a whole must be in thermal balance as heat diffuses from regions of the interface that are heated excessively to those that would otherwise cool. On the other hand this equilibrium is only attained on a very long time-scale so that earlier treatments of clouds as discrete particles are quite justified. Use of an improved equation of state shows that interstellar clouds are always gravitationally unstable if their mass exceeds 2000 M . Such a figure would indicate star formation to be a more efficient process than hitherto thought but inclusion of effects such as magnetism and rotation would increase this mass and modify this conclusion.