What happens if the mass of the core is greater than this limit? The core remnant continues to contract and the momentum, and hence the kinetic energies, of the electrons increase into the relativistic realm. Now,
so if the electrons have a kinetic energy of greater than 0.8 MeV, the inverse beta reaction,
where n is a neutrino, can proceed.
Beta decay does not occur, because the lowest energy states are already filled; there is nowhere for the beta electrons to go! As well, the escaping neutrino carries energy away from the star, reducing the degeneracy pressure, and increasing the rate of compression. This in turn increases the rate of the inverse beta decay.
Neutrons are also fermions, so they obey the Pauli exclusion principle. The neutrons are attracted to each other by the strong force, but those at higher energies have enough kinetic energy to break free (“evaporate”) from the nucleus. The reverse (“condensation”) also occurs. As the star collapses further, the fermi energies of the neutrons increase until the strong force is incapable of holding the nuclei together. The star is then composed almost entirely of a degenerate neutron gas. In actual fact, the internal structure of a neutron star may well be more complicated than this.
It is doubtful that a neutron star contains an interior of uniform consistency, nor may it be entirely composed of neutrons. The exact composition will depend on the mass of the star, and hence the density of the matter. A neutron star may typically be only 10 kilometres in diameter, so the densities considered are going to be extremely high. The following is a possible model for the internal nature of a neutron star.
- (1) The surface layer of the star is the region with density less than 109kg m-3. It may be composed of closely packed solid atomic polymers of Fe56. The very strong surface magnetic fields often present in neutron stars may polarise the atoms, causing them to take on a cylindrical nature causing the material to behave like a one dimensional solid with highly parallel and zero perpendicular conductivity in relation to the direction of polarisation.
- (2) The outer crust has a structure similar to that of a white dwarf, with relativistic electrons in a neutron Coulomb lattice.
- (3) The inner crust has densities increasing from about 4.3×1014 kg m-3. The neutron lattice begins to dissolve moving further down and forms a neutron gas.
- (4) At densities greater than 2×1017 kg m-3 the neutrons become a liquid, and have the properties of a superfluid. Any protons still present would also be superfluid, and indeed be superconducting.
- (5) Whether a distinct core of a neutron star exists and what it may be composed of is uncertain. This is because the behaviour of matter at such high densities and energies is difficult to model, even using experimental evidence from particle accelerators. It has been postulated that a solid neutron core may exist, or even quark matter.