What structure halts the inward implosion of a massive star's core just before a core-collapse supernova?

Answer

An incredibly stiff core formed when the shrinking core reaches nuclear density.

During the catastrophic gravitational collapse of a massive star's core, the matter shrinks until it reaches nuclear density. This creates an extremely rigid core that momentarily stops the inward rush of the outer stellar layers.

What structure halts the inward implosion of a massive star's core just before a core-collapse supernova?
starstellar evolutionsupernovamass