Heavy elements like Gold and Uranium are produced by the rapid neutron capture process (r-process). Extreme neutron-rich conditions  are necessary for this process, which is linked to neutron stars. Two possible astrophysical scenarios have been suggested as the site of the r-process: core-collapse supernovae (where neutron stars are born) and neutron star mergers. In 2017,  gravitational waves were detected from the merger of two neutron stars, GW170817, opening a new multimessenger era. Observations of this event revealed that heavy, neutron-rich nuclei were produced. These nuclei are unstable and their decay leads to a kilonova light curve. In the talk, I will present the nucleosynthesis in neutron star mergers and discuss the critical contribution of core-collapse supernovae to the chemical history of the universe.