So the core is no longer just a dense clump of iron atoms; it’s now essentially a single atom a few miles across: a giant ball of neutrons all shoved together as close as they can get. Those neutrons naturally repel each other. Not through any electric repulsion, since they are, after all, neutral, but through a combination of the vagaries of the strong nuclear force and their own version of degeneracy pressure. Neutrons are fermions too, which means they can only be squeezed down so tightly before they refuse to go any further. And since neutrons are much, much heavier than electrons (almost
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