What specific element's formation in the core halts fusion and triggers the catastrophic gravitational collapse in massive stars?

Answer

Iron

Iron is described as the cosmological dead end because fusing it consumes energy rather than releasing it, causing the outward thermal pressure supporting the star to vanish instantly when the iron core forms.

What specific element's formation in the core halts fusion and triggers the catastrophic gravitational collapse in massive stars?
astronomystarstellar evolutionsupernovamass