The Nature and Nurture of Star Clusters
  Star clusters have hierarchical patterns in space and time, suggesting
formation processes in the densest regions of a turbulent interstellar medium.
Clusters also have hierarchical substructure when they are young, which makes
them all look like the inner mixed parts of a pervasive stellar hierarchy.
Young field stars share this distribution, presumably because some of them came
from dissolved clusters and others formed in a dispersed fashion in the same
gas. The fraction of star formation that ends up in clusters is apparently not
constant, but may increase with interstellar pressure. Hierarchical structure
explains why stars form in clusters and why many of these clusters are
self-bound. It also explains the cluster mass function. Halo globular clusters
share many properties of disk clusters, including what appears to be an upper
cluster cutoff mass. However, halo globulars are self-enriched and often
connected with dwarf galaxy streams. The mass function of halo globulars could
have initially been like the power law mass function of disk clusters, but the
halo globulars have lost their low mass members. The reasons for this loss are
not understood. It could have happened slowly over time as a result of cluster
evaporation, or it could have happened early after cluster formation as a
result of gas loss. The latter model explains best the observation that the
globular cluster mass function has no radial gradient in galaxies.
