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I understand stars fuse atoms into heavier and heavier elements. When the star starts to fuse iron, the amount of energy released is less than the amount of energy it to takes to fuse the atoms. This creates an energy deficiency and the core is unable to maintain its outward pressure against gravity's inward collapse. The star collapses, rebounds against electron degeneracy pressure (I believe?) and goes BOOM. But how much iron is actually fused? And, for how long is the star fusing iron before the collapse?

Also, as a little bonus, I read somewhere recently that the presence of nickel is possible prior to going supernova. I can't find where though. Was that person wrong?

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Wikipedia says the iron-nickel core is inert, and doesn't mention any endothermic iron fusion reactions, just the core collapse, which converts the core to neutrons (and a not insignificant proportion of protons & electrons), and releases a lot of neutrinos. The nickel is radioactive, decaying to cobalt & iron.

The silicon burning phase leading to nickel only lasts about 5 days. The inert core collapses when it exceeds the Chandrasekhar limit of about 1.2 M$\odot$.

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    $\begingroup$ The Chandrasekhar mass for an iron core is about 1.2 solar masses. Indeed nickel is the natural product of alpha fusion, not iron. $\endgroup$ – Rob Jeffries Nov 15 '18 at 14:17
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The collapse of the stellar core has actually 5 steps, and only the first is well known.

1. exotherm fusion

The first is the fusion of the nuclei until iron/nickel. Also the normal life of the star belongs here.

2. endotherm fusion

While the iron-nickel core is inert on the Wikipedia, it is true only in "normal" circumstances. In fact, also iron nuclei can fuse, but it is already an endotherm process: they get away energy, thus they cool the core, accelerating the contraction.

Until iron (and nickel) is created due to nuclear fusion, the nuclear reactions are working against the collapse, because they generate energy (photons) and their pressure tries to avoid the collapse.

After that, this obstacle stops and the core can contract. From a point, also the iron (and nickel) nuclei start to fuse, to create all known elements of the periodical system, and probably even much higher, which can't exist in our low-pressure environment.

However, it is already an endotherm process, thus it takes away energy from the core. Thus, instead of decelerating the contraction, it accelerates.

3. exotherm fission

But it can't balance the temperature elevation due to the gravitational contraction, and over some billions kelvins, the nuclei start to decay.

It is the reverse process what the star did in the previous millions of years. It is exotherm for nuclei higher than iron/nickel and endotherm for nuclei below it.

4. endotherm fission

Thus, first the fission of the nuclei decelerate the contraction, then (after iron nuclei decayed), they accelerate again.

The produced energy radiates away (more exactly, it hits the upper layers of the star, causing the supernova explosion what we can see), and the contraction goes, this time with elevating temperature.

Over 60billion degrees, even the helium nuclei decay, which is still a highly endotherm process. From that point, there is no more way to get away energy from the core by nuclear processes: there is too hot to fuse the nuclei and there is no more way to decay.

The result is a mess of protons, neutrons and electrons. Essentially what the star did millions (or hundred millions) years long, is reversed in some seconds. The difference is that this mess is compressed into a size somewhere between a neutron star and a white dwarf, and it is 60billion degrees hot (normal stellar core temperature is in the order of some ten millions).

This is the point where there is no more obstacle before the further contraction.

5. proton-electron fusion

More exactly, the next obstacle will be that the beta decay of protons becomes energetically favorable. It can happen either by the fusion of a proton and an electron to a neutron and an anti-neutrino, or by the fission of a proton to a neutron, a positron and an anti-neutrino. The positron finds an electron quickly and annihilates.

Although neutrinos can permeate normal matter very quickly, their mean free path in such dense matter is only some hundred meters. They explode away the upper layers of the forming neutron star, dispersing all the elements of the periodical system in the Universe.


This all happens quicker than a minute, the steps 2-5 happen in seconds.

Thus, the answer is, that in a supernova explosion, practically all matter in the stellar core will be iron. Not even once, but twice. Although both is happening only for a very short time, they might be iron nuclei maybe some tenth of seconds long.

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