Very Massive Stars

Abstract: The upper limit to stellar masses has not been established. It is not known whether this is determined by the environments in which stars form or by the internal physics and stability of the stars themselves. I will argue that quasar disks may form stars, and that the mass scales there are larger than in giant molecular clouds. This gives cause to look again at the limits imposed by stellar physics. We have studied pulsational instabilities of solar-metallicity models up to 938 solar masses. Despite some uncertainties in the linear growth rate, we conclude that these modes saturate nonlinearly at small and harmless amplitudes. Vigorous, optically thick winds driven by the Rosseland-mean opacity near 100,000 K are more likely to limit the main-sequence lifetime.