THE CHEMICAL-COMPOSITIONS AND AGES OF GLOBULAR-CLUSTERS

Recent technological advances have led to a dramatic improvement in th e 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 dispers ion spectra show that abundance variations cannot explain the observed differences. In particular, it seems that NGC 288 must be 2-3 Gyr old er 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 abundanc e remains constant, indicating that the material has undergone varying amounts of nuclear processing. No variations are seen in the abundanc es of iron group elements. Lower dispersion spectra for a large sample of faint stars in 47 Tucanae, obtained with a multi-object optical fi bre system, show that unevolved main sequence stars in that cluster sh are the same CNO variations as the bright giants. The conclusion from all these data is that the intra-cluster CNO variations are neither tr uly primordial nor due to evolutionary mixing. It may be that there wa s a sufficiently extended period of star formation for material from f irst generation stars to be used in later generations, or that some po llution has occurred due to mass loss. Finally, it is noted that if 'p rehistoric' clusters exist with ages of around 50 Gyr, as hypothesised in some cosmological models, these should probably still be rather ob vious and readily recognised.