On the Evolution of Globular Clusters

The evolution of globular clusters is dominated by dynamical relaxation, produced by two-body en- counters, and by gravitational shocks, which occur whenever a cluster passes through the galactic plane; the variation with Z of the gravitational acceleration, g(Z), produces a change in acceleration across the cluster which increases the random kinetic energy of the cluster stars. Observational data are used to evaluate these two effects; at present dynamical relaxation is mnch more important for the observed stars in most clusters. For the less massive stars, which presumably are farther from the center of each cluster, the effect of gravitational shocks is considerably increased These two effects together have tended to cause the escape from the observed clusters of most of the less massive stars, possibly accounting for the low mass4uminosity ratio observed in clusters. The Population II stars in the disk may consist, in significant part, of stars formerly in globular clusters; some clusters may have dissipated entirely. Since the relaxation times for about half of the clusters appear to be shorter than 10 years, collapse of the central cores of these systems appears unavoidable within less than 1010 years from now, leading probably to massive supernovae.