The quite complex “Simple Stellar Populations” of globular clusters

There is compelling observational evidence that globular clusters (GCs) are quite complex objects. A growing body of photometric results indicate that the evolutionary sequences are not simply isochrones in the observational plane -as believed until a few years ago- from the main sequence, to the subgiant, giant, and horizontal branches. The strongest indication of complexity comes however from the chemistry, from internal dispersion in iron abundance in a few cases, and in light elements (C, N, O, Na, Mg, Al, etc.) in all GCs. This universality means that the complexity is intrinsic to the GCs and is most probably related to their formation mechanisms. The extent of the variations in light elements abundances is dependent on the GC mass, but mass is not the only modulating factor; metallicity, age, and possibly orbit can play a role. Finally, one of the many consequences of this new way of looking at GCs is that their stars may show different He contents.