  Globular clusters were thought to be simple stellar populations, but recent
photometric and spectroscopic evidence suggests that the clusters' early
formation history was more complicated. In particular, clusters show
star-to-star abundance variations, and multiple sequences in their
colour-magnitude diagrams. These effects seem to be restricted to globular
clusters, and are not found in open clusters or the field. In this paper, we
combine the two competing models for these multiple populations and include a
consideration of the effects of stellar collisions. Collisions are one of the
few phenomena which occur solely in dense stellar environments like
(proto-)globular clusters. We find that runaway collisions between massive
stars can produce material which has abundances comparable to the observed
second generations, but that very little total mass is produced by this
channel. We then add the contributions of rapidly-rotating massive stars (under
the assumption that massive stars are spun up by collisions and interactions),
and the contribution of asymptotic giant branch stars. We find that collisions
can help produce the extreme abundances which are seen in some clusters.
However, the total amount of material produced in these generations is still
too small (by at least a factor of 10) to match the observations. We conclude
with a discussion of the additional effects which probably need to be
considered to solve this particular problem.
