THE TIME-SCALE FOR CORE COLLAPSE IN SPHERICAL STAR-CLUSTERS

The collapse time for a cluster of equal-mass stars is usually stated to be either 330 central relaxation times (t(rc)) or 12-19 half-mass r elaxation times (t(rh)). But the first of these times applies only to the late stage of core collapse, and the second only to low-concentrat ion clusters. To clarify how the time depends on the density profile, the Fokker-Planck equation is solved for the evolution of a variety of isotropic cluster models, including King models, models with power-la w density cusps of rho similar to r(-gamma), and models with nuclei. T he collapse times for King models vary considerably with the cluster c oncentration when expressed in units of t(rc) or t(rh), but vary much less when expressed in units of t(rc) divided by a dimensionless measu re of the temperature gradient in the core. Models with cusps have lar ger temperature gradients and evolve faster than King models, but not all of them collapse: those with 0<gamma<2 expand because they start w ith a temperature inversion. Models with nuclei collapse or expand as the nuclei would in isolation if their central relaxation times are sh ort; otherwise their evolution is more complicated. Suggestions are ma de for how the results can be applied to globular clusters, galaxies, and clusters of dark objects in the centers of galaxies.