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

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 Lsun, 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"...