Lets say I had a bag that when turned upside would start pouring out iron shavings and never ever stop. Viola, there's my infinite source of iron. Now, lets say I just continued to dump this iron together until I had a ball of iron the size of earth, Jupiter, the sun, and kept on going?

To my understanding, self sustaining fusion would not occur because iron takes more energy to fuse than it releases. Is that to say no fusion would occur at all? Could I just keep dumping iron until vwoop my big ol' ball of iron is now a black hole? Would some fusion occur in the center of the iron ball and just not be propagated to the exterior of the ball?

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    $\begingroup$ You would be releasing enormous amounts of gravitational energy while doing this, so your star sized heap of iron would be extremely hot and it would get even hotter while it turned into a neutron star. The core collapse of the destabilizing iron star would cause a supernova explosion, which would lead to fusion and the production of heavy elements. There may be some minor differences between this scenario and a real supernova, but there should still be plenty of nucleosynthesis. $\endgroup$
    – CuriousOne
    Commented Feb 3, 2016 at 22:03
  • $\begingroup$ Since they're being moved out of the bag by some force, I imagine you'd end up with a vast nebula of iron that eventually collapses into rogue planets. $\endgroup$
    – Asher
    Commented Feb 3, 2016 at 22:46

2 Answers 2


Your ball of iron might reach as much as 1.2 solar masses before something drastic were to happen.

Up until that point, the ball could be supported by electron degeneracy pressure. The iron ball would contract to about the size of the Earth or a little smaller, the interior would heat sufficiently to completely ionise the iron. The electrons would be so tightly packed that the Pauli exclusion principle demands that many of them occupy high momentum states and this provides the supportive pressure.

However, the more massive such a ball becomes, the smaller its size and higher its density. At around $10^{12}$ kg/m$^3$, the electron Fermi energy is high enough to induce inverse beta decay in the iron (turning a proton into a neutron). The removal of free electrons tips the ball into instability and it collapses.

From there it is the basic story of a core collapse supernova. The iron gets photodisintegrated; most of the electrons and protons combine to form neutrons and neutrinos. However, unlike a core-collapse supernova, there is no envelope above the core, so it is not clear to me, beyond the release of $10^{45}$ J of neutrino energy that there would be a supernova in the conventional sense.

The collapse takes a second or so and is halted by the repulsive nature of the strong nuclear force between nucleons brought closer than a femtometre. The hot remnant will "bounce" and then stabilise as a neutron star with a radius of about 10 km.

It is almost certain you would not get a black hole. Whilst neutron stars with masses as low as 1.1 to 1.2 solar masses are produced in supernovae, we do not see black holes (or at least not yet) with masses below about 4 solar masses.

To address CuriousOne's comment. I think there would have to be some nucleosynthesis (e.g. r-process neutron capture and the production of very neutron-rich isotopes), during the collapse. However, I think it is unlikely that much of that material would escape and it would get incorporated into the neutron star. Some heavy, neutron-rich elements would then be part of the neutron star crust.


Supernova happens when the core of a supermassive dying star starts fusing iron and heavier elements under massive gravitational pressure. The reaction is endothermic, unlike the fusion of lighter elements (iron is the peak) so the resulting outward radiation pressure stops apposing inward gravitational pull and the star collapses on itself.

The pressure will become high enough to fuse electrons and protons together to form Neutrons, which are charge neutral and do not repel each other (degeneracy pressure) and the core contracts by a factor of 10,000 in a fraction of a second and turns into a tightly packed sphere made out of neutrons only (degenerate matter).

The outer layers of the star collapse onto this incredibly dense object and bounce off in a spectacular shockwave ... a supernova. The resulting heat vaporizes the material and makes it glow. Depending on the mass of the remaining core, you would either end up with a neutron star or a black hole.

So I presume this would be the faith of your supermassive ball of iron :)

  • $\begingroup$ A few details: radiation pressure is not what supports the core pre-supernova; neutrons (crucially) do repel each other if they get close enough; the core contracts by a factor of around 100 in a fraction of a second (about a thousand km to ten km); neutron stars are not made solely of neutrons; difficult to see how you could end up with a black hole, because the iron becomes unstable at a mass that is demonstrably stable as a neutron star. $\endgroup$
    – ProfRob
    Commented Feb 4, 2016 at 7:57

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