These are two exciting and interdisciplinary questions at the interface of nuclear physics and astrophysics. Elements heavier than iron are synthesized by the slow (s-) and rapid (r-) neutron capture processes. The r-process produces half of the heavy elements up to bismuth and all the uranium and thorium. In the r-process, nuclei capture neutrons on time scales faster than beta decays. The necessary extreme neutron density is exactly what makes this process a big challenge with a high potential for new discoveries across different research fields. The nuclear physics challenge is to describe the properties and reactions of nuclei towards the neutron dripline. From the astrophysics perspective, the challenge is to find the environment(s) with high neutron density and where matter is ejected on short time scales. This points to explosive events involving the most neutron-rich objects in the universe: neutron stars. The r-process was observed in 2017 with the gravitational wave detection (GW170817) from the merger of two neutron stars and its electromagnetic kilonova counterparts. Moreover, core-collapse supernovae, that mark the explosive end of the life of massive stars and the birth of neutron stars, could be the main contribution at early times in the universe.

We use the freshly synthesized heavy elements in neutron star mergers and supernovae to explore these fascinating, high-energy explosions and the extreme nuclear physics involved, with the goal of determining the origin and history of heavy elements in the Universe. This is possible only now with the great advances of all fields involved: multimessenger astronomy with gravitational waves and electromagnetic counterparts, advanced simulations of astrophysical environments with improved microphysics, new experiments producing the most exotic neutron-rich nuclei, and the next-generation of telescopes reaching the light of faintest and oldest stars. Our work builds on these exciting developments and combines advanced long-time supernova simulations including detailed microphysics (dense matter equation of state (EOS) and neutrino interactions), nucleosynthesis and kilonova calculations probing the structure and reactions of the most exotic nuclei, and observations of abundances from the oldest stars in the Milky Way (MW) and in orbiting dwarf spheroidal galaxies. We aim to use neutron star merges and core-collapse supernovae as cosmic laboratories for nuclear physics to establish their contributions to the heavy element synthesis.