Albeit rare in absolute numbers, massive stars are shaping our cosmic history as they are connected to many astrophysical key processes. Commonly defined as stars with an initial mass of more than 8 times the mass of our Sun, massive stars are the progenitors of black holes and neutron stars, reaching all nuclear burning stages before eventually undergoing their inevitable core collapse. In their relative short, but wild life, these luminous objects have an enormous impact on their galactic environment, enriching the surrounding medium with momentum, matter and ionizing radiation. This so-called "feedback" of massive stars is a building block for the evolution of galaxies, initiating and inhibiting further star formation. In the "afterlives" of massive stars, black holes and neutron stars can merge with each other, giving rise a to Gravitational Wave events.

Many details of massive stars as well as their impact and evolution are still poorly understood. In fact, the overall picture we draw in textbooks often does not hold once we actually try to bring all the observational and theoretical constraints together. We nowadays know that many massive stars have one or more companion and interactions between massive stars are common. While this gives rise to different evolutionary channels, many challenges remain. Further unconventional puzzle pieces and surprising constraints have arrived from observational frontiers such as the strong metal-enrichment in high-redshift galaxies discovered by JWST or the black hole statistics obtained from Gravitational Waves.

Investigating the massive star puzzle with a combined approach of theory, observation and numerics is at the very heart of my research group at the ARI. Our central tool in this endeavour is the application and development of dedicated stellar atmosphere models. I will briefly introduce the techniques and challenges of atmosphere modelling for hot, massive stars and their winds as well as their empirical and theoretical applications. Afterwards, I will provide an overview about the multi-ranged research efforts in my group, ranging from the spectral analysis of individual stars and the identification of "hidden" companions over theoretical studies on radiation-driven winds up to the generation of synthetic stellar libraries and new predictions for stellar feedback.