What immediately follows when the core of the supergiant becomes predominantly iron?

Answer

The energy source holding the star up vanishes, leading to catastrophic core collapse and a Type II supernova

When the iron core forms, the energy source supporting the star disappears instantly, causing the core to collapse almost instantaneously, resulting in a Type II supernova explosion.

What immediately follows when the core of the supergiant becomes predominantly iron?
Elementhydrogencompositionstarsupergiant