What cause a neutron star to form?

Answer 1

The strong gravity of the core of a dead high-mass star causes a neutron star to form. When the high-mass star becomes a supernova and leaves a core behind, the core no longer undergo fusion. Without fusion, gravity starts to push the core inward until most protons and electrons are crushed into neutrons, a neutron star forms. If the core is too massive, the neutron star would collapse and become a black hole.