Source that got me curious (page 5): http://astro1.physics.utoledo.edu/~megeath/ph6820/lecture27_ph6820.pdf

High energy photons can cause larger, less stable elements to undergo fission.

Uranium for example can photo-disintegrate, or, I think the correct term is undergo photo-fission, by photons of a few 10MeV. Source: http://en.wikipedia.org/wiki/Photofission

I believe the gamma rays created in the sun by hydrogen fusion are more energetic than that - though I had some trouble looking that up.

I'm just curious if Uranium wouldn't last long in the center of the sun because of the high energy photons, and what other elements might undergo photo-fission at the core of the sun. I'm guessing the pressure wouldn't matter cause it likely has a higher coulomb barrier than hydrogen, but the gamma rays might matter.



1 Answer 1


Not a definitive answer, but I note the following.

The solar photospheric abundance of Thorium is indistinguishable from the abundances in the protosolar nebula deduced from meteorites (to within 0.1 dex, Asplund et al. 2009). Of course it might be thought possible that Th-depleted material in the centre of the Sun never makes it to the photosphere. However, this is not the case for other elements such as Li, which really are depleted in the solar interior and this is reflected in the solar photospheric Li abundance. So there are deep mixing processes at work, extending well into the solar radiative core (and whatever they are, they happen during the main sequence lifetime of the Sun, since pre-main sequence Li depletion is almost non-existent in solar mass stars - e.g. Jeffries 2014), that would be capable of bringing any Th-depleted material to the surface. Asplund et al. also comment that it is thought that radiative diffusion only reduces the photospheric abundance of heavy elements by a few hundredth of a dex. I would conclude therefore that Th survives intact in the solar interior.

I have not been able to find a reliable source for the photospheric Uranium abundance - the abundance in meteorites is very low and so the lack of good measurable lines is consistent with this. Grevesse (1969) outline the problems and derive an upper limit considerably larger than the meteoritic abundance.

Thus, from an empirical point of view, there is reasonable evidence that Thorium survives on 4.5 Gyr timescales inside the Sun.

Why would this be? The pp-chain does result in gamma rays with energies of a few to tens of MeV. But the reaction rate is not that high and the abundance of very heavy elements in the Sun is very low. It is overwhelmingly more likely for the gamma rays to interact with electrons and protons first. Maybe Chris White has this calculation in mind? The other issue to explore is whether photodisintegration/photofission is actually likely with $\sim 10$ MeV photons. This paper suggests it is possible, as does the paper by Findlay et al. (1986) who measure the cross-section of Th to photofission at about 6.5 MeV.

  • $\begingroup$ I think lithium is a bit of a red herring here. Lithium burning occurs when a star is very young and fully convective. The photofission being discussed here occurs after a star has started fusing hydrogen. $\endgroup$ Jun 10, 2015 at 19:18
  • $\begingroup$ @DavidHammen Not so. The pre-main sequence Li burning in stars of a solar mass is extremely limited (less than a factor of two). The majority of Li depletion in a star like the Sun (which has depleted its Li by a factor of 200) occurs during its main sequence lifetime while it has a radiative core. See my reviews on the subject! arxiv.org/abs/astro-ph/0411111 arxiv.org/abs/1404.7156 $\endgroup$
    – ProfRob
    Jun 10, 2015 at 19:50
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    $\begingroup$ Nice when someone asks a question that run head on into one of your own publications. $\endgroup$ Jun 10, 2015 at 21:22

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