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For stars that undergo core collapse (leading to a core-collapse supernova or direct blackhole formation), the final stage of nuclear burning is silicon burning. Most online references I've found state that an iron core is the final stage of silicon burning, but a few references and stuff I vaguely remember from my classes state that the final core is an iron-nickel core, with Ni-56 being the actual final product of silicon burning and then decaying to Fe-56.

In the final core that undergoes core collapse and creates the explosion we call a supernova, what fraction of the core is iron vs. nickel?

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  • $\begingroup$ Good question. I posted relevant half-life info & a couple of links here. $\endgroup$ – PM 2Ring Feb 15 at 17:49
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    $\begingroup$ The half life of Ni-56 is 6.1 days, the half life of Co-56 is 77 days. $\endgroup$ – Jon Custer Feb 15 at 17:56
  • $\begingroup$ @JonCuster, so the answer you're giving is that only a very small fraction of the core is iron? (Given that silicon burning occurs for only a few days.) $\endgroup$ – NeutronStar Feb 15 at 19:02
  • $\begingroup$ @Joshua - I think that one needs to follow all the options for what the isotope yields of silicon burning and then what happens, because there is plenty going on in the core. But, if you only consider the Si-28 + Si-28 -> Ni-56 reaction, the only half-lives to really consider are how Ni-56 decays down to Fe-56. But, there is more going on. $\endgroup$ – Jon Custer Feb 15 at 19:13
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Given the high temperatures, Ni decays to Fe 56 within minutes. So, you will find mostly iron. In case of a star the size of Betelgeuse, supernova occurs at roughly 4 to 5 hours after silicon is exhausted in the core. By that time, shell silicon burning has already moved too far. But don't expect there to be only iron. As temperature increases, even iron is no longer stable. You will find a mixture of elements close to iron. Neutronization occur (electrons merge with protons, creating neutron rich isotopes). Neutronozation occurs mostly in the inner part of the core. The shell, which burned silicon for 4 hours, is now mostly made of Fe 56. So, the inner part of the core is made of exotic, neutron rich elements, the outer part is a mixture dominated by Fe 56, while the shell is almost entirely made of Fe 56.

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    $\begingroup$ The high temperature doesn't affect the beta decay of Ni-56 by very much. As mentioned above by Jon Custer, and myself in the linked post, the half-life of Ni-56 is about 6 days. Ni-56 beta decays to Co-56, which beta decays with a half-life of 77 days to Fe-56. So those decays mostly occur in the expanding gas cloud after the collapse, and are primarily responsible for the light production of the supernova, as described (for example) in the paper Light Curves of Type I Supernovae. $\endgroup$ – PM 2Ring Sep 10 at 16:20

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