  Based on the properties of uniform nuclear matter at the nuclear saturation
density and basic thermodynamic relations, we first re-study the composition of
matter on the surface of normal neutron stars and hybrid stars. It is found
that the hybrid stars are composed of uniform hadronic matter on the surface
rather than heavy nuclei. Then we use the Walceka model and the self-consistent
NJL model to describe the equation of state of low-density hadrons and quark
matter at high densities respectively, and the P--interpolation method is
employed to connect the equation of state at the extreme densities to study
hybrid stars. As a result, we find that the obtained hybrid star mass-radius
relation and tidal deformability meet the latest astronomical data. More
importantly, we find that the hybrid stars we obtained can be self-bound rather
than gravitationally bound, which is completely different from previous related
studies.
