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I have a question about iron stars. I was fascinated about the theory about the inability of protons to decay, and it led me to a point where I read something about some special kind of star called an "iron star". Can anybody enlighten me a bit, if possible, on what exactly it is and why it may be significant? And how do iron stars form?

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"Iron stars" are a hypothetical kind of stellar remnant - similar to a neutron star or white dwarf - that are hypothesized to form in the far future of a Universe undergoing heat death, under the assumption that protons do not decay, which is something that we do not know if is or is not the case. Some of the more promising theoretical models for proton decay (a fair number of so-called "Grand Unified Theories" [GUTs] which one should note, despite confusion in the popular press, are not the same thing as unified quantum gravity theories like string theory) have already been experimentally rejected, but that doesn't mean it doesn't occur at a rate slower than we can currently detect (or may ever be able to).

In particular, the idea rests on the fact that, given infinite time, there is no such thing as a stable element except, perhaps, for iron. What we call "stable" elements are really just radioactive elements with a half-life too long to observe, or too long to matter, whichever you prefer. For example, up into maybe about 16 years (300 megaseconds) or so ago, the element bismuth (element 83) was widely believed to be entirely stable, but then its most stable isotope was discovered to be unstable to alpha decay with a half-life of around $10^{19}\ \mathrm{yr}$ ($10^{20}\ \mathrm{Ms}$).

Feynman (I believe) once said that "everything not forbidden is compulsory". What this means is that, according to the laws of quantum mechanics, there is a nonzero probability that an agent can register any process not forbidden by, say, a conservation law, as having occurred, and will register such as having occurred given infinite time. In the case of atomic nuclei, if there is a nucleus with less nuclear energy than a given one, there is a probability to register the latter as now being the former, i.e. as having undergone a decay to it. (It cannot go the other way, because that would imply a violation of the law of conservation of energy.)

All elements heavier than iron have more nuclear energy than iron. Thus, by this principle, ALL isotopes of them are radioactive: we just don't know the half-lives. E.g. even common copper is radioactive (unstable) and will likely undergo alpha decay to iron, two atomic numbers below it. It's just that the timescale for this decay is likely so inordinately long that we cannot ever have any hope of observing it directly.

Thus, given unlimited time, anything made entirely of such elements heavier than iron will decay until it becomes purely composed of iron.

However, now one might be wondering something: clearly, stars are not made of heavier elements, and white dwarfs are typically also made of light elements like carbon and oxygen, and iron cores collapse into neutron stars if not black holes, neither of which can reasonably be called "iron", so how can we have "iron stars"? Well, it turns out that something opposite to radioactive decay will also happen for at least aggregate concentrations of light elements too: they will undergo cold fusion. Helium has less nuclear energy than 4 (protonic) hydrogens - that's why the Sun and main-sequence stars are able to perform hot fusion to a net energy gain to begin with. Contrary to what you may have heard, cold fusion is not impossible: it's just really damned slow, as in, taking far, far longer than the elapsed age of the Universe, just like it is for the decays just mentioned. But in any case, in "eternity", cold fusion will likewise raise the atomic number of the nuclei in a white dwarf, to again, converge on iron. This is an "iron star".

But, as said, this all depends on two things:

  1. that the ultimate fate of the Universe is heat death,
  2. that there is no proton decay.

Neither of these are things we can "know" to good confidence. In particular, there are still major open questions in fundamental physics that leave the door wide open to these being things that could go differently. In the case of the ultimate fate of the Universe, it is intimately bound up with the nature and governing laws controlling the still as-yet poorly understood "dark energy". Heat death will occur if the dark energy remains constant, i.e. acts as a true cosmological constant as per Einstein. However, if the dark energy either rises or falls with time, we may end up with two other "catastrophic" scenarios respectively called the Big Rip and Big Crunch. We have no evidence to rule these out because we don't have any regarding the physics governing future evolution of dark energy. The formation of iron stars by cold fusion and radioactive decay of so-called "stable" heavy elements is estimated to take a half-life of $10^{1500}$ (years, megaseconds, pretty much doesn't matter at this scale as that exponent itself is likely approximate).

Likewise, there are potential mechanisms - such as quantum tunneling via "virtual black holes" - that could still cause the decay of protons with half-lives on the order of $10^{200}$. A detector of the type we use now to look for proton decay (basically a big tank of water) could not detect these directly, as it would require many orders of magnitude more mass than there is in the current observable Universe and, moreover, would collapse into a black hole long before one got that far.

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The basic premise of an iron star is a 'dead' star whose mass is made primarily of iron. It's a hypothetical type of compact star. Wikipedia has a bit of a blurg here: https://en.wikipedia.org/wiki/Iron_star

The process of forming an iron star would be tricky, because normally iron does not fuse and so the star would normally collapse under it's own weight - this is why the Wiki article describes a cold-fusion process via quantum tunneling processes.

This article describes the process of fusion in stars and might help you understand why this is more significant compared to regular stars: http://www.eso.org/public/usa/news/eso0129/

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  • $\begingroup$ Sounds like the liquid water zone would be located quite close to an iron star, if such a thing were even to exist. $\endgroup$ – Wayfaring Stranger Sep 3 '15 at 12:50
  • $\begingroup$ But how would they be called a "star"? $\endgroup$ – Rohit Nutalapati Sep 4 '15 at 14:18
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    $\begingroup$ This makes little sense. Iron stars are not undergoing fusion, they would be supported by electron degeneracy pressure. I also doubt that iron is the lowest energy density nucleus. It would more likely be Nickel 62, or possibly something slightly more neutron rich, depending on the mass of the compact object. $\endgroup$ – Rob Jeffries Sep 4 '15 at 14:35
  • $\begingroup$ @Rohit Nutalapati - it's a "dead" star, or more properly, a stellar remnant. $\endgroup$ – Darth Futuza Sep 4 '15 at 20:57

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