  Close encounters and physical collisions between stars in young dense
clusters may lead to the formation of very massive stars and black holes via
runaway merging. We examine critically some details of this process, using
N-body simulations and simple analytical estimates to place limits on the
cluster parameters for which it expected to occur. For small clusters, the mass
of the runaway is effectively limited by the total number of high-mass stars in
the system. For sufficiently dense larger clusters, the runaway mass is
determined by the fraction of stars that can mass segregate to the cluster core
while still on the main sequence. The result is in the range commonly cited for
intermediate-mass black holes, such as that recently reported in the Galactic
center.
