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.
