The Formation of Massive Stars

Stars more than about 10 times the mass of the Sun are responsible for creating most of the heavy elements in the universe, for governing the evolution of galaxies, and quite possibly for reionizing the universe a few hundred million years after the Big Bang. The formation of these stars can be understood as an extension of the theory of low-mass star formation, generalized to include the effects of interstellar turbulence. However, at least two major problems must be overcome: First, for massive stars, the outward force due to radiation pressure exceeds the inward force due to gravity; how can gas accrete onto the protostar in that case? Circumstellar disks, outflow cavities, and radiative Rayleigh-Taylor instabilities all contribute to the solution of this problem. Second, why does a gas cloud form a single massive star instead of fragmenting into a large number of low-mass stars? This problem can be solved by thermal feedback from the first stars to form in the cloud. These conclusions are validated by means of 3D radiation-hydrodynamic simulations of high-mass star formation. Observational predictions include (1) Massive stars should form in cores with surface densities of order 1 g cm^-2; (2) the stellar initial mass function (IMF) should follow the mass function of cores in the host molecular cloud, scaled down by a factor of a few; (3) and massive, turbulent disks detectable by ALMA and the EVLA should occur around massive protostars.