Massive Star Formation: A Tale of Two Theories
  The physical mechanism that allows massive stars to form is a major unsolved problem in astrophysics. Stars with masses $\gtsim 20$ $\msun$ reach the main sequence while still embedded in their natal clouds, and the immense radiation output they generate once fusion begins can exert a force stronger than gravity on the dust and gas around them. They also produce huge Lyman continuum luminosities, which can ionize and potentially unbind their parent clouds. This makes massive star formation a more daunting problem than the formation of low mass stars. In this review I present the current state of the field, and discuss the two primary approaches to massive star formation. One holds that the most massive stars form by direct collisions between lower mass stars and their disks. The other approach is to see if the radiation barrier can be overcome by improved treatment of the radiation-hydrodynamic accretion process. I discuss the theoretical background to each model, the observational predictions that can be used to test them, and the substantial parts of the problem that neither theory has fully addressed. 
Mark R. Krumholz (Department of Astrophysical Sciences, Princeton   University)