Bulk viscosity and the Urca process

In nuclear matter, bulk viscosity arises from a phase lag between an imposed density oscillation and beta reequilibration of the particle content of the nuclear matter, which proceeds via the Urca process.  When bulk viscosity is sufficiently strong, which happens when the reequilibration rate nearly matches the frequency of the density oscillation, it can noticeably dampen the oscillation.  I will first discuss the Urca process in neutron stars and then will explain the role of the Urca process in determining the bulk viscosity.  I will conclude with the possible impact of the bulk viscosity on neutron star mergers.