  In the framework of the relativistic mean-field theory, we have considered
the equation of state of superdense nuclear matter, taking into account an
effective scalar-isovector delta-meson field. The effect of the delta-meson
field on the characteristics of a Maxwell-type quark phase transition has been
studied. The quark phase is described with the aid of the improved version of
the MIT (Massachusetts Institute of Technology) bag model, in which
interactions between the u, d, s quarks inside the bag are taken into account
in the one-gluon exchange approximation. For different values of the bag
parameter B, series of neutron star models with a quark core have been built.
Stability problems for neutron stars with an infinitesimal quark core are
discussed. An estimate is obtained for the amount of energy released in a
catastrophic transformation of a critical neutron star to a star with a
finite-size quark core.
