  Understanding the origin of high-mass stars is central to modern
astrophysics. We shed light on this problem with simulations using a novel,
adaptive-mesh, ray-tracing algorithm. These simulations consistently follow the
gravitational collapse of a massive molecular cloud core, the subsequent
build-up and fragmentation of the accretion disk surrounding the nascent star,
and, for the first time, the interaction between its intense UV radiation field
and the infalling material. We show that ionization feedback can neither stop
protostellar mass growth nor suppress fragmentation. We discuss the effects of
feedback by ionizing and non-ionizing radiation on the evolution of the stellar
cluster. The accretion is not limited by radiative feedback but by the
formation of low-mass companions in a process we call "fragmentation-induced
starvation". This behavior consistently reproduces the observed relation
between the most massive star and the total mass of stars in a cluster. We show
that magnetic fields reduce the star formation rate and lead to the formation
of more massive stars.
