Then, carbon is converted to oxygen, which is burnt to neon, which is burnt to magnesium, which is fused to silicon, finally fused to iron. All these transitions are smooth. Now, iron fusion does not release energy. The fusion reactions stop at iron, since it is at the peak of the binding energy curve versus the atomic number. The iron core starts contracting in on itself and its' temperature rises to 10 billion Kelvin.
Photons go on splitting the nuclei, until only protons and neutrons remain.
This reduces the thermal energy and contraction rates increase. The protons and electrons combine together to form neutrons, releasing neutrinos. The gravitational collapse slows down due to the neutron degeneracy pressure. The core overshoots the point of equilibrium and thus starts to re-expand. This entire process takes place in only a few seconds.