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Just a soft question:

Let's say a field of stars all die within a short amount of time. Just for argument's sake they produce a debris field of iron ( or any other heavy element). Provided that there is enough time the debris will agglomerate, we know this.

My question: Given enough mass, will this agglomeration of heavy elements fuse into even heavier elements?

Edit: (After reviewing related questions)

To be plain, I am not talking about stellar fusion or whether there is enough latent energy to continue fusion at an already existing core. I want to know if there was enough collective mass in the debris field, could the field itself, coalesce to the point where the heavy elements will fuse.

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Let's say a field of stars all die within a short amount of time. Just for argument's sake they produce a debris field of iron ( or any other heavy element). Provided that there is enough time the debris will agglomerate, we know this.

My question: Given enough mass, will this agglomeration of heavy elements fuse into even heavier elements?

Short answer, no, as others have said. At least, not in the stellar fusion sense, because heavier than Iron doesn't fuse in stellar fusion. Heavier than Iron fuses in supernova explosions.

Quick Source: http://curious.astro.cornell.edu/copyright-notice/85-the-universe/supernovae/general-questions/418-how-are-elements-heavier-than-iron-formed-intermediate

More info here: Origin of elements heavier than Iron (Fe)

To be plain, I am not talking about stellar fusion or whether there is enough latent energy to continue fusion at an already existing core. I want to know if there was enough collective mass in the debris field, could the field itself, coalesce to the point where the heavy elements will fuse.

Lets examine what happens when the iron coalesces, and in reality, it's unlikely that you'd have Iron and nothing else. It's hard to imagine a share of hydrogen, helium, carbon, etc, wouldn't be present, but assuming just Iron:

First, you get something similar to a planet or a planet core. That gets bigger as more coalesces. Then something cool happens (or, well, hot more specifically), but it's kinda neat. At a certain point, the planet stops getting bigger and starts getting smaller, and as it gets smaller, it gets hotter, not from fusion, just the energy of coalescing. In time, it could glow hot like the sun, but much smaller than a sun. If I was to guess, the peak size might be around the size of Neptune. (Peak hydrogen planet size is about the size of Jupiter, peak Iron planet size would, I would think, be a fair bit smaller).

Eventually, with enough mass, you get something something similar to a white dwarf. Iron white dwarfs don't exist because stars that become white dwarfs don't get to the Iron creation stage. There's some metallicity, but essentially, white dwarfs are carbon/Oxygen, or, smaller ones can be made of Helium and sometimes, there's some Neon, Magnesium - more on that here: https://en.wikipedia.org/wiki/White_dwarf

Your scenario would essentially become an Iron white dwarf.

Then, as with any white dwarf, at a certain mass, it reaches the Chandrasekhar limit and the inside of the star would begin to degenerate into a Neutron Star and once that process begins, it moves quickly and you basically have a really really really big boom and a type 1a supernova. And during the supernova, a lot of the Iron would fuse into heavier elements, but it would kind of happen all of a sudden, in the reasonably short period of time.

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  • $\begingroup$ So short answer no. Long answer, while gravity won't do it, the explosion driven by the agglomeration would. And you would get a supernova without direct hydrogen / helium fusion but not in the way I suppose. $\endgroup$ – user33995 Jul 20 '15 at 19:26
  • $\begingroup$ Yes. The explosion would be like clockwork too. You could predict with very good accuracy at what mass it would happen. $\endgroup$ – userLTK Jul 20 '15 at 19:28
  • $\begingroup$ Iron can exist in the cores of WD stars, but an entirely iron WD seems a little far-fetched to me considering fusion couldn't occur (similarly for all that follows the iron WD). $\endgroup$ – Kyle Kanos Jul 20 '15 at 19:43
  • $\begingroup$ I imagine it is a bit far fetched but then at some mass, one would think, the composition would be all but moot. You would still have a point of critical mass were every thing goes boom. Thus you would still a have fusion of heavy into super heavys $\endgroup$ – user33995 Jul 21 '15 at 2:44
  • $\begingroup$ The Chandrasekhar is pretty much absolute. I don't see any way it wouldn't happen once a certain mass was reached. It's the fusion process of stars that prevents all that mass from forming a white dwarf. In the case of nothing but Iron, it would form a white dwarf like object fairly quickly. I don't see any way that wouldn't happen. $\endgroup$ – userLTK Jul 21 '15 at 3:49
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As I understand it regular fusion in a star takes light elements as input and the output is heavier elements and energy. There are several potential steps in the regular process, e.g:

  • Hydrogen fusing to helium and producing energy which keeps the star from collapsing.
  • After a lot of hydrygen is spent and helium has collected in the center of the star the hydrogen fusion reactions tapers off. The star shrinks. Pressure and heat increase in the core. The core starts burning helium.
  • Repeat for heavier elements. E.g. up to oxygen/carbon.

Now I stressed a few points in the regular process:

  • "and producing energy": The reaction is endothermic. If you needed to add energy instead then processes tend not to work or much slower.
  • Add to that that fusing iron does not produce energy. Lighter element than iron/tin produce energy if fused. Heavier elements produce energy when split. Iron/Ni is the most stable element.

  • "Pressure and heat increase in the core": There is no fusion to produce heat, but you can increase pressure by just adding a lot of mass. If you add enough mass then you will get very dense iron. At some point the mass gets big enough to that it is not merely resisted by the electric charges of the charged iron aton cores, but also by the pressure of electrons. (See Degenerate matter).

    Add enough and you might get neutronium rather than heavier elements.

To be plain, I am not talking about stellar fusion or whether there is enough latent energy to continue fusion at an already existing core. I want to know if there was enough collective mass in the debris field, could the field itself, coalesce to the point where the heavy elements will fuse.

Well, yes and no. Add enough and it will change to a neutron star or beyond. I guess you could consider that a technical yes with a single star sized atom. It also feels as if I am playing word games.

Everything I have read (but I am not a physic person) seems to say that fusion stops. You will get very many iron atoms close to each other. Statistically I guess some might fuse and thereby cool the coalicing mass of iron. But intuition tells me that most will stay iron. Add more and it just gets denser and denser until it become energytically advantageous to capture elctrons and change to neutronium. (which, though very heavy/dense) is the opposite. It goes to element 0, not to classic 'heavier' elements.

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  • $\begingroup$ So then you believe the iron will change states to a different phase of matter rather than fuse to a heavier element? $\endgroup$ – user33995 Jul 20 '15 at 18:47
  • $\begingroup$ Yes. Without anything more than intuition. I wish I had a source to point to though. $\endgroup$ – Hennes Jul 20 '15 at 19:01
  • $\begingroup$ To form a neutron star, you need to have nuclear densities, $\sim10^{15}\,\rm g/cm^3$; for a solar-mass object, this turns out to be $\sim17\,\rm km$ (incredibly tiny for such a huge amount of matter). $\endgroup$ – Kyle Kanos Jul 20 '15 at 19:35
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The answer is a qualified yes, depending on your definition of fusion.

If you collect together a mass of iron less than about $1.2M_{\odot}$ it is possible for that to form a stable configuration, a little smaller than the size of the Earth, supported by electron degeneracy pressure. Such a star would just sit there and cool forever and all that would happen is that the iron would crystallise.

However, if you added more mass and tipped it over about $1.2M_{\odot}$ then the degenerate electrons have enough energy to begin inverse beta (electron capture) reactions onto iron nuclei. This raises the number of mass units per electron, takes free electrons out of the gas and causes a catastrophic softening of the equation of state. The result would perhaps be some sort of weird supernova explosion, perhaps leaving behind a neutron star remnant (or the whole thing might explosively deflagrate). Many elements heavier than iron could be produced by the r-process in such an explosion.

If a neutron star remains, then the neutron star will have a crust above its mostly neutron interior. The crust composition will be determined by minimising its energy density subject to the constraints of charge and baryon conservation. Such calculations routinely show that a density-dependent equilibrium composition is reached that features increasingly neutron-rich nuclei with depth, that can have atomic numbers much higher than iron and atomic masses of up to 200 or more.

So, if you count that as fusion (I wouldn't) then yes, you could ultimately produce nuclei that were much heavier than iron.

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As I understand it, iron will not be able to fuse with iron no matter how much of it you have gravitationally bound. Instead to fuse iron atoms together requires a supernova.

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  • $\begingroup$ So you would just get a VERY large ball of iron ( or whatever ) and there is no critical mass where gravity alone would cause the heavy elements to fuse into even heavier ones? Maybe my imagination is a bit off, but I imagine this ball growing and fusing ( assuming it were possible) until we reach the transuranics at which point this ball would be come super-critical and explode. $\endgroup$ – user33995 Jul 20 '15 at 17:43
  • $\begingroup$ Producing a super-nova without actually going through direct hydrogen/helium conversion. Could be a different avenue for elements like gold and silver. It is supposition of course but that is why I asked it a as soft question. $\endgroup$ – user33995 Jul 20 '15 at 17:43
  • $\begingroup$ I don't wish to pour cold water on any imaginative idea. I like where your thoughts are going. New ideas about star formation of other kinds are fascinating. $\endgroup$ – Alex Jul 20 '15 at 17:45
  • $\begingroup$ The problem with iron though, is that it requires an enormous amount of energy to constantly fuse it. I don't think accumulated iron gravitationally bound would be enough $\endgroup$ – Alex Jul 20 '15 at 17:48
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    $\begingroup$ Well it's a thought anyway. $\endgroup$ – user33995 Jul 20 '15 at 18:02
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Iron won't fuse into heavier elements. It's a question of nuclear physics.

Iron is the most stable form of nuclear matter. In other words, iron has the lowest energy configuration of all nuclear matter. Fusion can appear in the cores of stars because it's an exothermic process, that is, fusing nuclei lighter than iron can lower the nuclear matter's energy state and release heat in the process. Once the nuclei become made of iron, fusion will not be energetically favorable.

Nuclei heavier than iron are created by neutron capture processes, which is not the same as fusion. So you can't just "clump" iron nuclei together to form heavier elements, you need to bombard them with neutrons somehow.

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