Late Stage Nuclear Burning in High Mass Stars

If the mass of a star is > ~ 8 M_sun, the temperature and density of the C/O core can rise to levels where carbon-burning can occur:

which creates a central "ashpile" of oxygen and other heavy metals. As this ashpile contracts, its temperature and density can rise, igniting oxygen burning:

This oxygen ashpile can contract, and, well, you get the idea. The star's core takes on an onion-skin structure of nuclear burning:

Less and less energy is produced per nuclear reaction in the nucleosynthesis of these high mass elements. So each burning phase lasts a shorter and shorter amount of time:

For a 15 M_sun star:
Reaction	Timescale
Hydrogen burning	10 million years
Helium burning	1 million years
Carbon burning	300 years
Oxygen burning	200 days
Silicon burning	2 days

Silicon burning makes iron, and iron won't burn to make more massive elements. It is the most stable nucleus. No more nuclear energy is available to the star. Now what?