New HST Views at Old Stellar Systems

TL;DR

This paper discusses recent HST observations revealing complex stellar populations and helium enrichment in globular clusters, as well as insights into the age and composition of galactic bulges and halos.

Contribution

It presents new HST imaging results that uncover multiple stellar populations and age compositions in globular clusters and galactic structures.

Findings

Globular clusters host multiple, helium-rich stellar populations.

Galactic bulges are dominated by old, coeval stellar populations.

M31 halo contains a metal-rich, younger stellar component.

Abstract

HST has recently revealed that many among the most massive globular clusters harbor multiple stellar populations, and --most surprisingly-- some of them are extremely helium rich. How these clusters managed to generate such complex stellar populations, and what processes let to so dramatic helium enrichment, is today one of the most exciting puzzles in the astrophysics of stellar systems. HST has also been instrumental in demonstrating that both the bulge of our own Galaxy and that of M31 are dominated by old stellar populations, coeval to galactic globular clusters. Ultradeep HST imaging has also demonstrated that a major component of the M31 halo is metal rich and much younger than old globular clusters.