Supernovas occur whenforms in a stars core

Fusion

Explanation:

Supernova Definition:

The supernova occurs after fusion begins to pile up iron in the high-mass star's core. Because iron fusion cannot release energy, the core cannot hold off the crush of gravity for long. In the instant that gravity overcomes degeneracy pressure, the core collapses and the star explodes.

Fun Fact!

Approximately one supernova occurs every second. Supernova happens more often than you might think: One occurs somewhere in the universe every second!