  We study the bulk viscosity in neutron star cores due to modified Urca
processes involving nucleons, electrons and muons and analize its reduction by
singlet-state or triplet-state superfluidity of nucleons. In combination with
the results of our previous paper on the bulk viscosity due to direct Urca
processes, a realistic description of the bulk viscosity in superfluid neutron
star cores is obtained. Switching off direct Urca processes with decreasing
density in a nonsuperfluid matter lowers the bulk viscosity by 3-5 orders of
magnitude. The presense of muons opens additional source of bulk viscosity due
to muon Urca processes and lowers the threshold density of the electron direct
Urca process. The superfluidity may strongly reduce the bulk viscosity and
affect thus damping of neutron star vibrations.
