I understand that iron and all heavier elements consume more energy to produce than they make, and that is what eventually leads to a supernova. I also understand that a lot of the heavier elements are produced during that supernova. However, what I'm wondering is, before the star goes supernova, does any of the iron fuse with other elements? Yes, there would be a net energy loss, but if there is only small amount of iron in the star, it would probably be able to handle that.
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2 Answers
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The "iron core" in a supernova is actually the end product of a nuclear statistical equilibrium that begins when the silicon core begins to fuse. The fusion of alpha particles onto nuclei is actially exothermic well beyond iron and nickel (see What effects besides "mass defect" cause the alpha ladder beyond iron-56/nickel-56 to be endothermic?). Successive, rapid alpha captures produce nuclei with the same number of protons and neutrons, but at the same time, the competing processes of photodisintegration and radioactive decay work in the other direction. The process is thought to mostly stop at Nickel-56 because the temperatures required to initiate fusion of further alpha particle also result in photons with sufficent energy to photo-disintegrate iron-peak nuclei. The Nickel-56 then undergoes a couple of $\beta^{+}$ decays via Cobalt-56 to Iron-56 because because these heavier nuclei are more stable with $n/p>1$, . However, the core of a supernova just before it explodes is likely to contain a bit of a mixture of iron-peak isotopes.
Before all this happens it is possible for iron and nickel to undergo nuclear reactions if there is an appropriate source of free neutrons. The elements beyond iron in our universe are predominantly created by neutron-capture in either the r-process or the s-process.
The r-process is thought to occur after a core-collapse supernova (or a type Ia supernova) has been initiated. The neutron flux is created by the neutronisation of protons by a dense, degenerate electron gas in the collapsing core.
However, the s-process can occur outside the core of a massive star before it explodes. It is a secondary process because it needs iron nuclei to be present already - that is, the iron that is used for the seed nuclei is not produced inside the star, it was already present in the gas from which the star formed. The s-process in massive stars uses free neutrons produced during neon burning (so at advanced nuclear burning stages beyond helium, carbon and oxygen burning) and results in the addition of neutrons to iron nuclei. This builds up heavy isotopes, which may either be stable or undergo $\beta$ decay and/or further neutron captures to build up a chain of "s-process elements" (e.g. Sr, Y, Ba) all the way up to lead. The overall process is endothermic, but the yields and reaction rates are so small that it has no major influence on the overall energetics of the star. The newly-minted s-process elements are easily blasted into the interstellar medium shortly afterwards when the supernova explodes.
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$\begingroup$ Hi Rob, thank you for answering my question too! One aspect of your answer I thought was really interesting was that the iron needed for the s-process has to come from outside the core of a star. Why is that? Are only certain isotopes present inside of stars? $\endgroup$– caffeinSep 22, 2015 at 16:31
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3$\begingroup$ @caffein I think the problem is that the iron produced in the core is (a) very short-lived and (b) separated from the neon-22 neutron source. It thus never gets the opportunity to participate in the slow s-process, only the rapid r-process when the core collapses on timescales of seconds. $\endgroup$– ProfRobSep 22, 2015 at 21:39
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Yes, but it's slow. (I'm not an expert, so feel free to correct if I miss something important), but once the star is into the later stages, past the helium stage, up to iron, fusion mostly takes place by fusing a helium onto a heavier element, raising each atomic number by 2. That's not the only method but it's the most common.
Iron can also fuse into nickel in this way inside a star and it does in small amounts, but mostly beyond iron, and certainly beyond nickel, heavier elements are created through the S-Process. (short for slow neutron capture process). This happens when a free neutron binds to the atomic nucleus and over time, the addition of neutrons can lead to beta decay, where an electron is ejected and a proton remains - adding to the atomic number.
but if there is only small amount of iron in the star, it would probably be able to handle that.
This is undoubtedly true. The stars that go super-nova are incredibly large and the iron doesn't exactly sink to the core right away. It takes some time. For a star to go kablooie (supernova), it needs an iron core of both enough purity where it's no longer undergoing expansion from nearby fusion, and enough size for it to undergo rapid collapse in a way that effects the star around it almost instantaneously. I'm not clear on the exact process, but it requires way more than just a little iron. As a layman's guess, it might require a Jupiter sized ball of iron. Perhaps a fair bit more than that.
