The Birth, Evolution and Death of Star Clusters
  A dense-enough gas-accumulation evolves, over a few Myr of intensifying star formation, to an embedded cluster. If it contains a sufficient amount of mass, O stars form and explosively expel the remaining gas, whereas poorer clusters reduce their embryonic gas content more gradually. The sudden expulsion of gas unbinds most of a rich cluster, but a significant fraction of it can condense by two-body interactions to become an open cluster despite a star-formation efficiency as low as 30 per cent. Poorer clusters survive their gradual mass loss more easily, but have short, relaxation-limited life-times. Pleiades-like clusters may thus form as nuclei of expanding OB associations, by filling their tidal radii and having large (1-2 pc) core-radii. A 'main-sequence' of clusters is thus established. Ultimately, a cluster dies an explosive death through the ever shortening relaxation time, and leaves a remnant that consists of about 4-10 stars arranged in a highly hierarchical and thus long-lived system. Dynamical mass segregation in very young clusters is extremely rapid, and heats a cluster substantially, which is partially off-set by the cooling from the disruption of primordial binaries. 
Pavel Kroupa (Heidelberg)