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.
