Young stellar groupings and clusters have hierarchical patterns
ranging from flocculent spiral arms and star complexes on the
largest scale to OB associations, OB subgroups, small loose
groups, clusters and cluster subclumps on the smallest scales.
There is no obvious transition in morphology at the cluster
boundary, suggesting that clusters are only the inner parts of the
hierarchy where stars have had enough time to mix. The power-law
cluster mass function follows from this hierarchical structure:
n(M)∼M-b for b∼2. This value of b is independently required by
the observation that the summed IMFs from many clusters in a
galaxy equals approximately the IMF of each cluster.