Disclaimer: I'm not a physicist I've never quite grasped interstellar gas clouds (i.e. the material for new stars) and how they work. If they were too sparse, then you'd expect them to just dissipate. If they were too dense, then you'd expect them to collapse spontaneously. But yet they seem to be fairly stable over long time scales. What is the source of this stability? One answer I came across is that the gas pressure in the cloud balances the gravitational tendency to collapse. If the cloud operates similarly to a familiar gas, then such a collapse would imply a significant amount of cooling, since the thermal energy had been sufficient to oppose collapse beforehand. But, yet, they do collapse and form new stars. An alternative is that the cloud was not in equilibrium, and therefore it's prone to seek equilibrium by collapsing. Is that possible? Thanks. --Edit The popular description I've come across seems to imply that a shock may cause a pocket of gas to become compressed, and then gravity just takes over. The calculation of gravity interactions are no doubt hard to do. I'm not even bothering to attempt these. But in a familiar gas, there are weak attractions that would seem to be roughly analogous. For there to be a kind of dual equilibrium, where a cloud can be stable and yet collapse spontaneously to some more favorable equilibrium, seems to require some kind of nonlinear interaction.