[vikingerik]

The other reply gave good info, but didn't quite answer this. Once an object's core acquires enough temperature/pressure/density to start protium fusion, it will continue, not burn out. The fusion itself increases the temperature, and the produced helium is denser than hydrogen, so the core compresses more under gravity thus increasing the pressure too. So protium fusion creates its own conditions to continue, so there's no such thing as borderline stardom; once protium fusion starts at all, it will continue and it's now a red dwarf. Fusion doesn't begin or occur everywhere, it's still only at the core, but convection will eventually (10^12 year timescale) cause all the hydrogen to have moved through the fusing region.

[delta_p_delta_x]

Thanks; this is much more concise than I could manage.

I think the key idea is fixed minimum/maximum boundaries for a lot of astrophysical phenomena—stellar protium fusion, the Chandrasekhar limit, Tolman–Oppenheimer–Volkoff limit, etc.

[ctippett]

I appreciated both yours and the parent's comment. I read the shorter answer first, which was accessible enough to a layman like me that I was left wanting more, which made reading your comment immediately after all the more satisfying.

[hparadiz]

It should have been obvious to me (fusion duh!) that the result of the fusion is a higher density material that then increases the conditions for the reaction to take place. Both comments were immensely helpful and interesting! Thank you.