The ExaStar project is working to develop a code suite for multi-physics simulations of stellar explosions such as supernovae and neutron star mergers. The codes model the coupled physics of magneto-hydrodynamics, radiation transport, general relativity, nuclear reactions and equation of state. The science goals are to further understand the origin of heavy elements within these astrophysical events and the physics of neutrinos and dense matter under the extreme conditions of gravity and density.