New Insights into the Evolution of Massive Stars and Their Effects on Our Understanding of Early Galaxies

The observable characteristics and subsequent evolution of young stellar populations is dominated by their massive stars. As our understanding of those massive stars and the factors affecting their evolution improves, so our interpretation of distant, unresolved stellar systems can also advance. As observations increasingly probe the distant Universe, and the rare low metallicity starbursts nearby, so the opportunity arises for these two fields to complement one another, and lead to an improved conception of both stars and galaxies. Here we review the current state of the art in modelling of massive star dominated stellar populations, and discuss their applications and implications for interpreting the distant Universe. Our principle findings include: - Binary evolutionary pathways must be included to understand the stellar populations in early galaxies. - Observations constraining the extreme ultraviolet spectrum of early galaxies are showing that current models are incomplete. The best current guess is that some form of accretion onto compact remnants is required. - The evolution and fates of very massive stars, of the order of 100Msun and above, may be key to fully understand aspects of early galaxies.Comment: Manuscript submitted to Annual Reviews of Astronomy and Astrophysics for future publication in Volume 60. This is the authors' own version. The final version will become available from https://www.annualreviews.org/journal/astro There are 43 pages, 5 figures and 2 table