Stellar Physics 5h: Fusion Beyond Iron

Nucleosynthesis of nuclei/elements beyond the iron peak. Nuclei beyond the iron peak (essentially iron to uranium) cannot be fused inside of stars, so they must be made another way. This video covers:
- The rp-process (rapid proton capture)
- The s-process (slow neutron capture)
- The r-process (rapid neutron capture)
- The alpha effect

[koyotekola]

I haven't watched this video yet, but I have watched the previous videos and understand them. One reason is because I'm an engineer and a mathematician. Another and very important reason is because I'm taking leisure classes at a major university, and one of them is astronomy. Surprisingly much of it dwells on stars, because they're the stepping stone between the Big Bang theory to what we are today. I'm about to find out why.

[phdnk]

Why is it that fusion reactions are impossible beyond iron peak ? What is the real immediate reason ?
The endothermicity of those fusion reactions is not enough to suppress them: the star still has plenty of gravitational potential energy to spend on endothermic fusion the star can afford to shrink and fuse on.
So the alleged beyond iron fusion either occurs or has event better mechanism of suppression than endothermicity alone can provide.
Candidates:
(1) the columb barier becomes insurmountably high,
(2) photodisintegration kicks in.

Analogy: During core collapse, the endothermicity of p + e -> n + v process does not prevent neutronization from happening, because gravitational potential is being spent.

[alexanderhuang]

could you provide more detail about the alpha-process, maybe suggesting a paper questioning the core-collapse sn as the candidate for r-process

[phdnk]

Consider tidal disruption of a neutron star.
Neutronium that gets flung away will be unstable to beta decay. On the diagram it will start at low Z and high N, from the far right side.
Yet this beta decay of neutronium is not an r-process, since neutrons are not being captured but are already in the nuclei from the beginning.
What governs the element production it the neutron decay rate and the the decaying neutronium fission-fragmentation (boil-off ? pulverization ?) under columbic repulsion of protons being generated in it.

[Chromazene]

I thought you can get nickel thermodynamically favorably through the alpha process? Obviously that is not as common as fusion into Fe.

[guill]

The theory of nucleosynthesis is not based on facts, but on assumptions.