The chemical compositions and ages of globular clusters

Abstract

Recent technological advances have led to a dramatic improvement in the quality of photometric and spectroscopic data obtainable on stars in globular clusters. Evidence from CCD-based colour magnitude diagrams points to clear differences in age between some clusters. High dispersion spectra show that abundance variations cannot explain the observed differences. In particular, it seems that NGC 288 must be 2–3 Gyr older than NGC 362. The same spectra show that although there is a spread in some molecular band strengths in NGC 362, the total C+N+O abundance remains constant, indicating that the material has undergone varying amounts of nuclear processing. No variations are seen in the abundances of iron group elements. Lower dispersion spectra for a large sample of faint stars in 47 Tucanae, obtained with a multi-object optical fibre system, show that unevolved main sequence stars in that cluster share the same CNO variations as the bright giants. The conclusion from all these data is that the intra-cluster CNO variations are neither truly primordial nor due to evolutionary mixing. It may be that there was a sufficiently extended period of star formation for material from first generation stars to be used in later generations, or that some pollution has occurred due to mass loss. Finally, it is noted that if ‘prehistoric’ clusters exist with ages of around 50 Gyr, as hypothesised in some cosmological models, these should probably still be rather obvious and readily recognised.