The ultimate stage of heavy stars (>8 Mʘ) takes place when
the iron-nickel core is being built. These elements, being the most stable of all isotopes, can not burn any further. The core builds up until it reaches the 'Chandrasekhar mass' (about 1.4 Mʘ) where the electron-degeneracy pressure is exceeded. The core collapses and, when the thermal pressure reaches the nucleon degeneracy level, bounces at 10-20% of the speed of light. A few-millisecond neutronization phase occurs when electrons are captured on protons, generating a massive burst of electron neutrinos (1% of the total neutrino energy, as much as the total photon energy), which turns the 10-20 km core into a neutron star of the density of an atomic nucleus:
Get your head out of the raw milk, it blurs your vision.