system
Given the following post, respond as if you're commenting on Reddit:
human
Why don’t the largest stars have impossibly short lifespans (a few millennia or less)?

I’ve been fascinated with the most massive stars for some time, and I was looking at their life spans. They didn’t seem as short as they should have. I went ahead and plotted a bunch of stellar lifespans. Below 3 solar masses (M☉), they seemed to follow approximately the formula 12Gyr/M☉³, but between 3M☉ and 30M☉, it seems to transition to something much less extreme. Above 30M☉, the formula appears to be closer to 100Myr*M☉²′³. Going from the 3rd power to 2/3’s power seems like a lot. Is this transition real, or am I doing something wrong? Is this caused by convection currents carrying additional hydrogen to the core to be burned? Is it the Eddington limit and mass loss preventing the stars from shining as brightly as they otherwise would?
gpt
Larger stars (> 20 M☉) have longer lives than they would otherwise (assuming extrapolation from smaller stars) because radiation pressure overtakes gas pressure as the dominant force holding the core up against collapse. Radiation pressure is proportional to the internal temperature to the fourth power. Gas pressure is only proportional to temperature to the first power. Since radiation pressure is negligible to begin with, only the extreme conditions of massive stars can make it significant. Beyond this point, a slight increase in mass yields a large increase in radiation pressure. Since high radiation pressure brings about inflation, which slows the star's reaction rate, the luminosity increase per unit of mass increase is lower. As the equations roughly go, lower mass stars increase luminosity proportionally to mass cubed, and higher mass stars increase luminosity directly proportionally to mass.

This runaway radiation pressure is also responsible for the Eddington limit, beyond which a star will blow off mass, as you mentioned. As far as we can tell, this sets an upper limit for main-sequence stars at about 150 M☉.