Type 2 supernova

 A Type II supernova happens when a massive star (at least ~8 times the Sun’s mass) reaches the end of its life and undergoes core collapse.

Why “stop at iron”?

Inside massive stars, nuclear fusion builds heavier and heavier elements in layers:

Hydrogen → Helium → Carbon → Neon → Oxygen → Silicon → Iron

The key point:

• Fusion releases energy only up to iron (Fe-56).
• Iron has the highest binding energy per nucleon.
• Fusing iron does not produce energy — it absorbs energy.

So once the core becomes mostly iron, fusion stops producing outward pressure.

What Happens Next?

1. The iron core grows to about 1.4 solar masses (the Chandrasekhar limit).
2. Electron degeneracy pressure can no longer support it.
3. The core collapses in milliseconds.
4. Protons + electrons → neutrons + neutrinos.
5. The core rebounds, and a shockwave forms.
6. The star explodes as a Type II supernova.

The outer layers are blown into space.

What’s Left Behind?

Depending on mass:

• Neutron star
• Or black hole

Important Clarification

Fusion in the star stops at iron, but elements heavier than iron (like gold and uranium) are created during the explosion via rapid neutron capture (the r-process).

If you’d like, I can also show you the “onion shell” structure of the star right before collapse.