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The star becomes a white dwarf, with a core mostly composed of two different physical systems: (1) Degenerate electron gas and (2) Fe nuclei. As the system contracts, the Fe nuclei absorb electrons and each combination of electrons and protons inside the nucleus becomes a neutron + a neutrino. This will replace most protons in the Fe nuclei with neutrons, but these are unstable, and when the density becomes around 10E11 grams per cubic centimeter, nuclei will dissolve into a relativistic, degenerate electron gas.