  We explore, through a simplified, semi-analytic model, the formation of dense
clusters containing massive stars. The parent cloud spawning the cluster is
represented as an isothermal sphere. This sphere is in near force balance
between self-gravity and turbulent pressure. Self-gravity, mediated by
turbulent dissipation, drives slow contraction of the cloud, eventually leading
to a sharp central spike in density and the onset of dynamical instability. We
suggest that, in a real cloud, this transition marks the late and rapid
production of massive stars.
  We also offer an empirical prescription, akin to the Schmidt law, for
low-mass star formation in our contracting cloud. Applying this prescription to
the Orion Nebula Cluster, we are able to reproduce the accelerating star
formation previously inferred from the distribution of member stars in the HR
diagram. The cloud turns about 10 percent of its mass into low-mass stars
before becoming dynamically unstable. Over a cloud free-fall time, this figure
drops to 1 percent, consistent with the overall star formation efficiency of
molecular clouds in the Galaxy.
