Oops

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 (in the tail of the blackbody distribution) 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 an outrageously large burst (N ~10⁵⁷) of neutrinos, and the electrons are gone. But the electrons were holding up the star!

NO MORE SUPPORT
Core free falls
Whats is the time scale?
Well we already did this and noted the freefall time goes as inverse square root of the density
Let's calibrate this with respect to the sun.
The sun has a mean density of about 1.4 grams per cc and has a free fall time of 30 minutes.
The degenerate electron core has a density of 10⁷ grams per cc so the free fall time will be 30 minutes * 10^-3.5 = 0.57 seconds! Say what, Batman?