Supernovae!

Iron won't undergo nuclear fusion. The center of that iron core is squeezed down tightly and becomes degenerate, held up by electron degeneracy pressure, rather than gas pressure.

But at the extremely high temperatures in the core, the photons have enough energy to destroy nuclei:

This process is known as photodisintegration, and it uses energy (endothermic), rather than creating it (exothermic).

All of the sudden we have tons of protons and neutrons running around in the core. In these extreme conditions (T ~ 8x10⁹ K, rho ~ 10¹⁰ gm/cm³), free electrons are captured by protons:

So we get a burst of neutrinos, and the electrons are gone. But the electrons were holding up the star!

Uh oh....

Core collapse

Because free-fall time depends on density, and the density increases as you go deeper in the star, the inner portions react first. At collapse speeds of 70,000 km/s, the inner core collapses from 1 Earth radius to 50 km in one second.

This essentially pulls the rug out from under the upper layers. They begin to collapse, too.

Once the density of the inner core reaches 10¹⁵ gm/cm³, the collapse stops. This is about the density of an atomic nucleus, and occurs because of neutron degeneracy. The core rebounds a bit, sending a blast wave outwards, where it meets....

....the infalling outer layers.

BOOM!

A supernova is formed. The outer layers are blasted off in a violent explosion. When they become optically thin, so the energy can escape, we see the energy flooding out.

How much? The peak optical luminosity is around 10⁹ L_sun, while 100 times more energy comes out in neutrinos (which we don't see). This luminosity rivals that of an entire galaxy.

Here is a before and after picture of Supernova 1987A, the closest (d=50 kpc) supernova to us in modern times:

Here's a supernova (1994D) in a much more distant galaxy:

(Note to the purists: It's a Type Ia, not a Type II...)

Note also that the plural of "supernova" is "supernovae", not "supernovas"...