What is the mechanism causing the visual supernova explosion in a massive star's core collapse?

Answer

A powerful rebound shockwave generated after the imploding core reaches nuclear density

When the collapsing iron core reaches nuclear density, it becomes incredibly stiff, halting the implosion and causing the infalling outer layers to violently crash into it, generating a rebound shockwave that tears the star apart.

What is the mechanism causing the visual supernova explosion in a massive star's core collapse?
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