The quite complex "Simple Stellar Populations" of Globular Clusters

TL;DR

Globular clusters are more complex than previously thought, exhibiting internal chemical variations and multiple stellar populations, which challenge the traditional view of these objects as simple stellar populations.

Contribution

This paper reviews observational evidence for the complexity of globular clusters, emphasizing chemical inhomogeneities and their implications for formation theories.

Findings

Presence of internal dispersion in iron and light elements.

Complexity correlates with cluster mass, metallicity, and possibly orbit.

Stars may have varying helium contents.

Abstract

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.