Massive Star Formation
  The enormous radiative and mechanical luminosities of massive stars impact a
vast range of scales and processes, from the reionization of the universe, to
the evolution of galaxies, to the regulation of the interstellar medium, to the
formation of star clusters, and even to the formation of planets around stars
in such clusters. Two main classes of massive star formation theory are under
active study, Core Accretion and Competitive Accretion. In Core Accretion, the
initial conditions are self-gravitating, centrally concentrated cores that
condense with a range of masses from the surrounding, fragmenting clump
environment. They then undergo relatively ordered collapse via a central disk
to form a single star or a small-N multiple. In this case, the pre-stellar core
mass function has a similar form to the stellar initial mass function. In
Competitive Accretion, the material that forms a massive star is drawn more
chaotically from a wider region of the clump without passing through a phase of
being in a massive, coherent core. In this case, massive star formation must
proceed hand in hand with star cluster formation. If stellar densities become
very high near the cluster center, then collisions between stars may also help
to form the most massive stars. We review recent theoretical and observational
progress towards understanding massive star formation, considering physical and
chemical processes, comparisons with low and intermediate-mass stars, and
connections to star cluster formation.
