So, when no more Si is available to fuse into Fe, gravity crushes the core from being about the size of earth into a sphere 15 km across, in only one tenth of a second. During this time the Weak Force turns protons into neutrons, and a by-product of the reaction is created, a small particle called a neutrino. Neutrinos are produced in great quantities as the Weak Force turns the core into neutrons. Neutrinos don't like to interact with normal matter, so they stream out of the collapsing core at close to the speed of light, as if the star wasn't even there. So, a sudden burst of neutrinos rushes out from the star before any other change occurs on the surface.
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