  Massive stars produce so much light that the radiation pressure they exert on
the gas and dust around them is stronger than their gravitational attraction, a
condition that has long been expected to prevent them from growing by
accretion. We present three-dimensional radiation-hydrodynamic simulations of
the collapse of a massive prestellar core and find that radiation pressure does
not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities
channel gas onto the star system through non-axisymmetric disks and filaments
that self-shield against radiation, while allowing radiation to escape through
optically-thin bubbles. Gravitational instabilities cause the disk to fragment
and form a massive companion to the primary star. Radiation pressure does not
limit stellar masses, but the instabilities that allow accretion to continue
lead to small multiple systems.
