As I understand it fusion inside a sun can produce heavier and heavier elements until some sort of "nucleus size limit" is reached. As far as I understand, the limit is thought to be reached with the element iron.

What accounts for this "fusion element limit"? Is it the battle between the nuclear strong force and the electromagnetic force which limits the size of the nucleus that can exist in a fusion environment?

Is the limit of what elements can be created the same for all types of suns?

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    $\begingroup$ Look up binding energy. Heavier nuclei may not neccessarily be lower energy than lighter ones. The difficulty of fusion is also not fixed. Electrostatic repulsion of two nuclei runs as the product of the charges (atomic number) of the nuclei. So heavier elements require much higher temperatures and pressures to fuse. $\endgroup$ Jun 29 '11 at 4:51

There are actually several different "limits" one might encounter. The one everyone talks about is not fusing past iron. This comes from the fact that isotopes in the vicinity of ${}^{56}\mathrm{Fe}$ consist of the most tightly bound nuclei. See Wikipedia for a discussion and some binding energy curves. If you are interested in why there is a peak, it comes from balancing various energetic considerations, and you can glean some understanding from the semi-empirical mass formula and associated liquid drop model of the nucleus. Keep in mind this is an empirical fit to a somewhat justified simplification - it does not encompass all we know about nuclear forces and such. Once a star has fused nucleons into these isotopes, energy can no longer be extracted from further fusion. As a result, stable, sustainable nuclear burning has come to an end.

However, a star may never reach this point. In general, stars go through several different phases of life, characterized by what nuclear fuel they burn. The (greatly oversimplified) story is something like $$ {}^1\mathrm{H} \to {}^4\mathrm{He} \to {}^{12}\mathrm{C}, {}^{16}\mathrm{O} \to \ldots \to {}^{56}\mathrm{Fe}. $$ As one fuel runs out, the star (or at least its core) shrinks and heats up, enabling fusion of heavier elements. Our Sun won't get very far past the helium stage; it simply does not have enough mass.

On the other hand, very massive stars don't stop at iron and nickel and such things. Just because a reaction is endothermic does not mean it cannot happen. In core-collapse supernovae, their violent death causes heavier elements to be created. A good deal of this is accomplished with neutron (or alpha, if any helium is to be found) capture, so it is fusion, but only in the sense of unequal mass things colliding. This sort of process can produce just about any heavy element in the universe, with things past about uranium undergoing spontaneous fission too quickly to gain a foothold in cosmic abundances.

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    $\begingroup$ One might add that neutron capture is undeterred by Coulomb repulsion and that the s-process can produce elements heavier than iron even in stars of relatively low mass once they have a "neutron source" (usually Carbon-13) in giant stars. $\endgroup$
    – ProfRob
    Jun 10 '15 at 21:24

From a stat mech point of view one can view a star's goal as throwing off entropy over it's life span. Iron is as stable as things get from an entropy perspective. This results in the fusion limit. The entropy that comes along with enough temp and pressure to continue this process is not favorable for the continuation of fusion beyond iron.

I believe this is true for all stars until things get to the point gravitationally where Fermi pressure is important for balancing.


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