# Hubble Space Telescope analysis of stellar populations within the globular cluster G1 (Mayall II) in M31

## TL;DR

This study uses Hubble Space Telescope data to analyze the complex stellar populations of the massive globular cluster G1 in M31, revealing multiple populations with varied chemical compositions and an extended blue horizontal branch.

## Contribution

It provides the first multi-wavelength evidence of multiple stellar populations and helium variation in G1, similar to known Galactic globular clusters.

## Key findings

- Red giant branch shows significant spread indicating multiple populations.
- Presence of an extended blue horizontal branch in G1.
- Similarity of G1's features to Galactic clusters NGC6388 and NGC6441.

## Abstract

In this paper we present a multi-wavelength analysis of the complex stellar populations within the massive globular cluster Mayall II (G1), a satellite of the nearby Andromeda galaxy projected at a distance of 40 kpc. We used images collected with the Hubble Space Telescope in UV, blue and optical filters to explore the multiple stellar populations hosted by G1. The $m_{\rm F438W}$ versus $m_{\rm F438W}- m_{\rm F606W}$ colour-magnitude diagram shows a significant spread of the red giant branch, that divides $\sim 1$ mag brighter than the red clump. A possible explanation is the presence of two populations with different iron abundance or different C+N+O content, or different helium content, or a combination of the three causes. A similar red giant branch split is observed also for the Galactic globular cluster NGC6388. Our multi-wavelength analysis gives also the definitive proof that G1 hosts stars located on an extended blue horizontal branch. The horizontal branch of G1 exhibits similar morphology as those of NGC6388 and NGC6441, which host stellar populations with extreme helium abundance (Y>0.33). As a consequence, we suggest that G1 may also exhibit large star-to-star helium variations.