  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_cl) M_cl^-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.
