ISF 3.P164

The figure shows the complicated layered structure predicted for neutron stars due to the increase in pressure with depth. If a neutron
star is part of a binary system, its outer surface may have a thin hydrogen atmosphere and a thin ocean mainly consisting of
heavier nuclei produced by thermonuclear burning on the surface. Beneath the ocean of nuclei, the density is such that the neutron
star will have a solid outer crust consisting of nuclei embedded in a degenerate electron gas. A transition from the outer crust to an
inner crust will occur at the level that electron capture reactions raise the neutron chemical potential to the extent that nuclei can
be embedded in a neutron gas. The density of the neutron gas in the inner crust increases with depth and the nuclei, arranged in a
Coulomb lattice in this layer, become larger and more neutron-rich with depth. The transition to the outer core of uniform nuclear
(mainly neutron) matter may proceed through a variety of geometrical “pasta” phases that start as high as the lower boundary of
the inner crust. The nature of the stable phase of the inner core of the neutron star is not known; it could be nuclear matter, quark
matter, or consist of pion or kaon condensates. Various astrophysical observables that provide information about these stellar
regions are indicated in the figure.