Over the course of their lifetime, stars burn hydrogen into helium and depending on their mass, helium into heavier elements (referred to as metals). The helium-to-metal enrichment ratio thus provides important insights on stellar nucleosynthesis and Galactic chemical evolution. Attempts to measure this quantity have proven challenging over the past four decades, with estimates ranging anywhere between 1 and 10 for stars in the Galactic disc. It is not clear if this broad range of values is due to uncertainties in the data, or to intrinsic scatter across different stellar populations. The availability of precise distances and metallicities from ongoing stellar surveys allows us to tackle this problem anew.

In this project, you will exploit the fact that for unevolved, low mass stars on the Hertzsprung-Russell diagram, the width of the main-sequence depends on both the metal and helium content. Using distances from the Gaia satellite, and metallicities from the GALAH survey, you will build a sample of low-mass stars and pinpoint their location on the Hertzsprung-Russell diagram. By comparing against stellar models computed for different mixtures of helium and metal, you will use this sample of stars to derive the helium-to-metal enrichment ratio and to investigate if and how it varies as function of Galactic stellar populations.