B Question about the Silicon-burning Process beyond iron

[DrLich]

I came across an article (https://en.wikipedia.org/wiki/Silicon-burning_process) that claims the silicon-burning process remains exothermic all the way to tin-100. Although iron is the endpoint for energy release in stellar fusion, how does the process continue to be exothermic beyond iron?

 

[Drakkith]

The silicon burning process adds alpha particles to successively more massive nuclei, building up sulfur, argon, calcium, titanium, chromium, iron, and then nickel. However, this process also results in less and less energy being released each time. Combine this with increasing amounts of photodisintegration as the core grows hotter and you reach a point where no significant amount of elements beyond nickel-56 can be created from fusion anymore. At least in this specific chain. There are probably a small amount of heavier elements created, but the star won't generate huge quantities of it compared to the amount of nickel and iron created.

how does the process continue to be exothermic beyond iron?

The details are beyond me, but the fact is that fusing an alpha particle to a heavier nucleus continues to release energy past iron and nickel.

 

[Bandersnatch]

So, I can't see anything relevant after (admittedly only) skimming the sources given for the claim in the Wikipedia article.
The best justification for the statement I can think of is that the binding energy of silicon and tin are about on the same level, so if you sort of start with silicon and end with tin, treating everything in-between as intermediate steps in the reaction, you still end up with a nett release in energy by the time you get to tin.
I'm not sure that's physically relevant, though.

 

[phyzguy]

The answer is that while the binding energy per nucleon peaks at Fe (8.79032 MeV), the binding energy per nucleon of an alpha particle is much lower at 7.074 MeV. So the average binding energy of Fe + He is (4 * He + 56 * Fe) / 60.0 = 8.676 MeV, which is lower than the binding energy per nucleon of Ni, which is 8.76502 MeV. So the reaction 56Fe + 4He -> 60Ni generates energy, even though the product of 60Ni has a lower binding energy per nucleon than 56Fe. This continue to be true up to about 116Sn. In essence the energy is coming from the more loosely bound alpha particle.

 

[phyzguy]

At tin, (4 * He + 116 * Sn) / 120.0 = 8.4748 MeV, which is basically equal to 120Te at 8.47814 MeV.