Spectroscopy of Main Sequence Stars in Globular Clusters
  Although globular clusters are generally chemically homogeneous, substantial abundance variations are sometimes seen even among unevolved main sequence stars, especially for the CNO group of elements. Multi-object intermediate-dispersion spectroscopic systems are now being used to determine the patterns of abundance variations for large samples of stars, while high dispersion spectrographs on 8m-class telescopes are providing good spectra of individual faint stars. In some circumstances the spectra of many similar stars can be combined to yield extremely high S/N spectra. The sample of clusters which has been studied remains small, but it seems that many of the more metal-rich clusters must have experienced some sort of self-enrichment, either in a proto-cluster stage, or through successive episodes of star formation or some other processes early in their lives. The metal-poor clusters provide equally clear evidence for internal mixing and dredge-up of processed material within evolving red giant stars. 
Russell Cannon, Gary Da Costa, John Norris, Laura Stanford and Barry   Croke