Astronomy Colloquium

JWST is providing an unprecedented direct glimpse of the first galaxies in formation. This promises answers to longstanding questions in astrophysics, from the details of reionization to early metal enrichment and black hole growth. However, the answers to these questions are inextricably tied to models of the earliest massive star populations. There are many reasons to suspect that these first generations of massive stars were substantially different from their present-day counterparts. This seeming impasse is also a unique opportunity to directly test and calibrate our understanding of these populations. I will discuss how a new wave of deep JWST observations is revolutionizing our view of galaxies in formation in the reionization era and beyond, and challenging simplified pictures of these objects and their stellar contents. Interpreting this deep spectroscopy requires uniting it with improved modeling and with results from spatially resolved observations in the local Universe. I will show how work much closer to home is shedding light on the often-puzzling results that JWST continues to assemble. Finally, I will highlight the crucial roles that upcoming and next-generation facilities such as UVEX, the ELTs, and HWO will play in completing the story of massive stars and their impact across cosmic time.