The Composition of Dense Matter from the Cooling of Neutron Stars*

I present our new comprehensive analysis of neutron star observations and nuclear structure experiments in order to obtain novel constraints on the neutron-to-proton ratio of cold dense matter in beta-equilibrium. The data set includes neutron star mass and radius observations, gravitational wave observations which constrain the tidal deformability, and observations of the cooling of isolated and accreting neutron stars. We find that the proton fraction in dense matter must be large enough to allow direct Urca to occur, but that neutron superfluidity and proton superconductivity suppress the direct Urca process except for larger neutron star masses. We also find that the neutron star radius is strongly constrained by the cooling data.