Globular clusters as laboratories for stellar evolution

TL;DR

Globular clusters serve as natural laboratories for studying stellar evolution, but recent findings of multiple populations with varying helium levels challenge previous assumptions, prompting a review of observational indicators.

Contribution

This paper reviews how features of the Hertzsprung-Russell diagram can indicate helium abundance and summarizes current constraints on helium variation in globular clusters.

Findings

Presence of multiple populations with different helium abundances.

Identification of features in the Hertzsprung-Russell diagram as helium indicators.

Summary of observational constraints on helium in globular clusters.

Abstract

Globular clusters have long been considered the closest approximation to a physicist's laboratory in astrophysics, and as such a near-ideal laboratory for (low-mass) stellar evolution. However, recent observations have cast a shadow on this long-standing paradigm, suggesting the presence of multiple populations with widely different abundance patterns, and -- crucially -- with widely different helium abundances as well. In this review we discuss which features of the Hertzsprung-Russel diagram may be used as helium abundance indicators, and present an overview of available constraints on the helium abundance in globular clusters.