The ages and compositions of old clusters

Abstract

From published color-magnitude diagrams of 23 globular and open clusters we have measured the mean color index or absolute magnitude at various points along the principal sequences. By relating these quantities to the analogous parameters derived from theoretical isochrones, it is possible to solve simultaneously for age, heavy-element abundance, and helium abundance. This analysis leads us to conclude that there is no correlation of globular cluster age with composition and that the oldest open clusters are about one-half the age of the globular clusters. Although the substantial observational and theoretical uncertainties prevent a conclusive statement, the evidence suggests that the open clusters have a higher helium abundance than the globular clusters, that the globulars have an age of about 16.6 +- 0.5 Gyr, and that the metal-rich clusters 47 Tuc and M71 have (Fe/H)approx.-0.9. Changes in the metallicity scale or mixing length could substantially alter these values. Nevertheless, globular cluster ages as young as 10 Gyr would require large corrections and would yield cluster properties inconsistent with observation. If the Hubble constant is as large as 100 km s/sup -1/ Mpc/sup -1/, then fundamental changes in stellar evolution theory are required.