If the mass of a star exceeds the Chandrasekhar limit , or 1.4 solar masses, when its nuclear fusion can no long sustain the pressure needed to counteract gravity, the star will supernova and collapse past the white dwarf stage, (as electron degeneracy pressure will not be enough to counter gravity) to the stage known as a neutron star.

If the stars mass exceeds three solar masses, then not even neutron degeneracy pressure will be enough to stop the star collapsing and a black hole will result. A typical neutron star will only have a radius of 10 kilometers, and be so dense that a teaspoon full will weigh about a billion tons.

At such huge densities the electrons are forced into the protons, reacting to form a neutron and a neutrino. In neutron star only 0.5 percent of the matter remains as protons and electrons, hence the name 'neutron star'. The material formed is actually 'fluid'; there is little force between neutrons to bind them together, it can't be a solid. It may in fact be a superfluid like liquid helium; The small residual amount of protons and electrons may allow this fluid to be superconducting also. Modeling material at this (and) higher densities is difficult, although direct observations by telescope of effects caused by processes in neutron stars will help prove or disprove the models.

The structure of neutron star is more similar to a planet like earth, than a star like our sun. There is an 'atmosphere' only a few centimeters thick, caused by evaporation of the surface material, then a solid crust, perhaps only a kilometer thick, followed by a mantle of solid and neutron superfluid, followed by a pure superfluid. It's hypothesized a massive example may even have a solid, crystalline core.

Types of neutron stars include Pulsars and Magnetars. Stephen Baxter has written a book about life inside a neutron star.