Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

TL;DR

Recent studies reveal that globular clusters are composed of multiple stellar populations formed through several star formation episodes, challenging the traditional view of them as simple stellar populations.

Contribution

This review summarizes recent progress in understanding multiple populations in globular clusters and discusses new paradigms for their formation and evolution.

Findings

Globular clusters host multiple stellar generations.

Evidence from photometry and spectroscopy supports complex formation histories.

New models explain features like the second parameter problem.

Abstract

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.