IIRC, under the influence of gravity matter gets more dense until you get a critical mass. That's the tipping point. That's when nuclear fusion starts and is self sustaining. The energy released can grow until it hits a more or less steady state, like the Sun with its variations. Any corrections are appreciated.
The Wiki article: http://en.wikipedia.org/wiki/Stellar_evolution , seems to be pretty well written. Gases condense, getting hotter via simple thermodynamics, become brown dwarfs, which may fuse at least some deuterium-deuterium. This is a relatively “common” state for lightweight stars. If massive enough, the collapse can continue further to time when hydrogen can start fusing. Though it’s not mentioned in the article, I suspect that may well be close to a “light switch” regime: several percent in mass (given similar composition) may delineate too small to remain compressed despite additional internal pressure outward added by that fusion, from the stars massive enough to continue their collapse. It is probably a very small window of mass that will resist collapse, or even “oscillate” - I’ve never seen suggestions of the latter occurring.
I don’t recall seeing any data regarding this, though, and would love to hear from someone involved in the field. It is too bad we don’t have RadioAstronomer around any more due to The Purge. He would surely know, or be able to obtain, the answer for us.