Expert: James Gort - 12/19/2006

Question
what happens if the core is able to produse iron before imploding

Answer
Hi Matt,

If a star is more massive than about 8 times the solar mass, it is able to produce iron through silicon burning (fusion).  Fusion stops with iron, because heavier elements require more energy to fuse than they give off (they're endothermic).  When iron production is nearly complete (all silicon is used up), core temperatures are about 8 billion degrees K (differs depending on the actual mass), and the iron starts to be destroyed by photodisintegration.  High energy photons break up the iron to form helium, and the helium is then broken up into protons and neutrons.  The free protons then combine with free electrons to produce more neutrons (and lots of neutrinos - much energy escapes in the form of neutrinos).  The star's core was previously supported from collapse by the great luminosity of the core (silicon burning) and electron degeneracy (if you don't understand that term, please ask, and I'll answer it in a later response).  But now there is less luminosity as silicon burning stops and fewer free electrons, so nothing is left to support the great weight of the star above the core.  The star starts a free-fall collapse, until the inner core is about three times as dense as an atomic nucleus!  This density stops the collapse abruptly, and it rebounds.  The rebounding material collides with the infalling material, forming an accretion shockwave.    The material becomes even hotter from the collision, and the shockwave continues its rise to the surface (carrying much of the core material with it, since the colliding material is now so dense that nothing (not even neutrinos) can easily penetrate it (it actually forms an expanding wall).  When it reaches the surface, the thermal energy of the core (including the kinetic energy of the expelling material) is released.  This is the mechanism of a Type II Supernova, where the total luminosity can exceed a billion times the luminosity of the sun.

Hope that helps.

Prof. James Gort