Then the core becomes a neutron star. At these high densities, neutron formation dominates over decay. 3 M is a best guess. The figure shows what a neutron star is thought to look like. The crust contains nuclei.
Neutrons exist below the crust.
These mean no one really knows what goes on there. Quarks are the elementary particles that make up neutron and protons. In a star, the energy input comes from nuclear reactions. Consider now the ignition of He in a low mass red giant. He burning starts and the core is heated.