  If dark matter is made of mirror baryons, they are present in all
gravitationally bound structures. Here we investigate some effects of mirror
dark matter on neutron stars and discuss possible observational consequences.
The general-relativistic hydrostatic equations are generalized to spherical
objects with multiple fluids that interact by gravity. We use the minimal
parity-symmetric extension of the standard model, which implies that the
microphysics is the same in the two sectors. We find that the mass-radius
relation is significantly modified in the presence of a few percent mirror
baryons. This effect mimics that of other exotica, e.g., quark matter. In
contrast to the common view that the neutron-star equilibrium sequence is
unique, we show that it depends on the relative number of mirror baryons to
ordinary baryons. It is therefore history dependent. The critical mass for core
collapse, i.e., the process by which neutron stars are created, is modified in
the presence of mirror baryons. We calculate the modified Chandrasekhar mass
and fit it with a polynomial. A few percent mirror baryons is sufficient to
lower the critical mass for core collapse by ~0.1 M_sun. This could allow for
the formation of extraordinary compact neutron stars with low mass.
