Abstract

Massive stars are the engines that drive cosmic evolution. Their intense radiation fields ionize and heat gas within galaxies. The resultant emission lines depict the physical conditions of galaxies across cosmic time. Perhaps most astonishingly, massive stars likely generated the largest phase change of neutral hydrogen in cosmic history: cosmic reionization. While massive stars truly shape cosmic history, their properties are challenging to quantify. Here, I present my observational campaigns from the Hubble Space Telescope and the Keck Telescopes that use local galaxies to constrain the production of ionizing photons from massive stars and describe how ionizing photons can escape galaxies. My upcoming programs will combine Hubble Space Telescope and McDonald observations to fill gaps in our theoretical understanding of the atmospheres of low-metallicity massive stars, the elusive unseen ionizing continuum of massive stars, and how ionizing photons escape local galaxies. I will use these observations to develop the first empirically-motivated massive star models at extremely-low metallicity that theoretical models will use to determine the sources of cosmic reionization. The next generation of observatories, in particular the Giant Magellan Telescope, will use my observations and models to empirically determine how massive stars contributed to cosmic reionization and shaped the early universe.