Very Massive and Supermassive Stars: Evolution and Fate

This chapter reviews the properties and evolution of very massive stars (M > 100 M _⊙) in the context of recent stellar evolution models. At the end of the chapter, we summarize the properties, evolution, and fate of supermassive stars (M > 10, 000 M _⊙). Since very massive stars have very large convective cores during the main sequence phase, their evolution is never far from a chemically homogeneous evolution, even without rotation-induced mixing. Their evolution is thus not so much affected by rotational mixing, but more by mass loss through strong stellar winds. All very massive stars at metallicities close to solar end their life as hydrogen-free Wolf-Rayet stars. At solar metallicity, mass loss is so strong that even if a star is born with several hundred solar masses, it will end its life with less than 50 M _⊙. This means that their fate will be similar to normal massive stars. At the metallicity of the Large Magellanic Cloud (LMC) and lower, on the other hand, mass loss is weaker and might allow stars to undergo pair-instability supernovae or pulsation pair-instability supernovae. These supernovae are expected to be very rare but very bright and able to explain a subset of slowly evolving super-luminous supernovae. Supermassive stars might be the progenitors of intermediate-mass or super massive black holes or explain anti-correlations between the abundance of specific chemical elements in star clusters. In rare circumstances they might explode due to a general relativistic instability. Supermassive stars formation and their evolution, however, are very uncertain and there is no solid evidence for their existence. If they exist, supermassive stars evolve close to the Eddington limit, and thus their mass loss is probably very strong and makes their fate similar to very massive stars.