Thermal fission of gold?

Question

Here's a slightly silly idea that arose from this question "What elements would be created in a star composed entirely of gold?":

If we accumulate a lot of gold (really a lot) fast enough, it will heat up as a result of the gravitational energy being released, loosely speaking. I think, if the temperature gets high enough, gold atoms will start colliding with enough energy to break apart, releasing more energy in a sort of run-away, thermal fission process. But am I right?

Answer 1

This is a hypothetical question.

If somehow you could assemble a stellar-sized ball of atoms and allow them to radiate their gravitational potential energy so that they can contract to become a compact object, Then one of two things can happen.

1. The mass is low enough that the object can be supported by electron degeneracy pressure. The threshold mass is given approximately by $1.44 (\mu_e/2)^{-2}$ in solar masses and is set by the electrons all becoming ultra-relativistic. If we are dealing with stable gold-197, then $\mu_e$, the number of mass units per electron is $197/79$ and the threshold mass is $0.93 M_{\odot}$. For a ball of lower mass than this, you get a stable "gold white dwarf".

This limit will be reduced slightly by doing the calculation in General Relativity. One may also have to account for inverse beta decay, which can turn the gold into Platinum and destabilise the white dwarf - leading to the second possibility.

2. If the "star" is more massive than the threshold above it, then electron degeneracy cannot support it. It will collapse further and then two things will happen. First, the photons in the interior will become energetic enough to photodisintegrate the gold nuclei. Second, the electrons in the interior will become energetic enough to neutronise the protons.

The resultant collapse will produce some sort of supernova. If the original star was less massive than about 2.5 solar masses, then it is almost certain that a neutron star would form. If more massive than this, then there is a possibility that a neutron star or a black hole will result, depending on whether mass is lost in the supernova.

I suppose there is the possibility that the photodisintegration process will initially be exothermic as the gold nuclei split to yield nuclei closer to the peak of the binding energy curve. However, given that the final pre-supernova stages of nuclear fusion, in far less energetic conditions, last only about a day, and the available energy per nucleus from fission is less than in fusion, then I think there could only be a brief pause in the collapse. Certainly no stable star could form.