Simplified core collapse scenario: (a) A massive, evolved star has onion-layered shells of elements undergoing fusion. An inert iron core is formed from the fusion of Silicon in the inner-most shell. (b) This iron core reaches Chandrasekhar-mass and starts to collapse, with the outer core (black arrows) moving at supersonic velocity (shocked) while the denser inner core (white arrows) travel sub-sonically; (c) The inner core compresses into neutrons and the gravitational energy is converted into neutrinos. (d) The infalling material bounces off the nucleus and forms an outward-propagating shock wave (red). (e) The shock begins to stall as nuclear processes drain energy away, but it is re-invigorated by interaction with neutrinos. (f) The material outside the inner core is ejected, leaving behind only a degenerate remnant.
File:Core collapse scenario.png
Illustration by R.J. Hall. Redrawn in Inkscape by Magasjukur2