  The collapse of massive molecular clumps can produce high mass stars, but the
evolution is not simply a scaled-up version of low mass star formation.
Outflows and radiative effects strongly hinder the formation of massive stars
via accretion. A necessary condition for accretion growth of a hydrostatic
object up to high masses M > 20 M_sun (rather than coalescence of optically
thick objects) is the formation of and accretion through a circumstellar disk.
Once the central object has accreted approximately 10 M_sun it has already
evolved to core hydrogen-burning; the resultant main sequence star continues to
accrete material as it begins to photoevaporate its circumstellar disk (and any
nearby disks) on a timescale of 100,000 years, similar to the accretion
timescale. Until the disk(s) is (are) completely photoevaporated, this
configuration is observable as an ultra-compact HII region (UCHII). The final
mass of the central star (and any nearby neighboring systems) is determined by
the interplay between radiation acceleration, UV photoevaporation, stellar
winds and outflows, and the accretion through the disk.
  Several aspects of this evolutionary sequence have been simulated
numerically, resulting in a "proof of concept". This scenario places strong
constraints on the accretion rate necessary to produce high mass stars and
offers an opportunity to test the accretion hypothesis.
