  Bulk viscosity of neutron star cores containing hyperons is studied taking
into account non-equilibrium weak process $n+n \rightleftharpoons p+\Sigma^-$.
Rapid growth of the bulk viscosity within the neutron star core associated with
switching on new reactions (modified Urca process, direct Urca process, hyperon
reactions) is analyzed. The suppression of the bulk viscosity by superfluidity
of baryons is considered and found out to be very important.
