Massive stars are the primary chemical and dynamical drivers of the Universe and are crucial for interpreting phenomena in high-redshift galaxies. Understanding the early reionization epoch, the formation of the first galaxies, the transition to Population II metallicities, and the rates of very luminous supernovae, gamma-ray bursts, and gravitational waves requires a deep knowledge of the most massive stars ever formed. However, the formation and evolution of these very massive stars remain uncertain. This gap in understanding extends to star formation studies in galaxies where individual stars cannot be resolved, blurring the conclusions drawn from such studies. Therefore, it is essential to understand the evolution and formation of the most massive stars in the Milky Way and Local Group before extending our research to more distant galaxies.
Fig. 1. Spectrum of an O star marking the most relevant lines in the BlueMUSE (blue) and MUSE (red) wavelength ranges, with an overlap zone.
Systematic spectroscopic analysis of the most massive star clusters is fundamental for addressing this problem, as it helps to confirm or challenge current stellar evolutionary theories. However, this process is both resource-intensive and incomplete. Spectroscopy of massive stars in star-forming regions is notoriously challenging due to nebular contamination and crowding, and assembling large spectroscopic samples requires significant observing time.