Fundamental questions in nuclear physics such as, “What is the nature of hot and dense matter?” and “What is the origin of r-process nuclei?” are deeply connected to neutron star observations. In this talk, I will explain how we are answering these questions using a combination of data from nuclear experiments, data from neutron star observations, theoretical models of hot and dense matter, and numerical simulations. Answering these questions demands a level of effort beyond that of a single research group, so I explain how progress is being made through collaborations like the Nuclear Physics for Multi-Messenger Mergers Focused Research Hub. I summarize recent results on both the speed of sound and the composition of dense matter. I show how we are enhancing astrophysical simulations by improving the nuclear physics input. Finally, I show how we are using machine learning to accelerate progress.

Massive stars, by which we mean those stars evolving through all the stable nuclear burning stages, play a fundamental role in the evolution of the Universe. Therefore, a good knowledge of how they evolve is required in order to shed light on many topical subjects like, e.g., the chemical evolution of the galaxies and the nature of the sources of gravitational waves. In the last years three main questions – still debated – rose in the community working on massive stars. They concern (1) the compactness of massive stars at the presupernova stage as an indicator of their explodability, (2) the nature of the remnant after the explosion, and in particular the maximum mass of a stellar black hole, and (3) the so called “Red Supergiant (RSG) problem”, i.e. the lack of observed supernovae associated with the observed most luminous RSGs. In this colloquium I will address these questions by firstly reviewing our current understanding of the presupernova evolution of massive stars.

TBA