# 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.

## Key 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.