The construction of initial data for neutron stars requires that
the state of the neutron-star matter be specified before the
gravitational data can be determined. Of course, a solution of
the gravitational constraint equations can be found in principal
for any given energy density
and momentum density
. But, with neutron-star solutions, we are usually interested in
situations where the matter is in (or nearly in) hydrostatic
equilibrium and the gravitational fields are also in (or nearly
in) equilibrium.