### Which atom is a Neutron star made of?

• I understood that everything was made of Atoms.

An atom is the smallest constituent unit of ordinary matter that has
the properties of a chemical element. Every solid, liquid, gas, and
plasma is composed of neutral or ionized atoms

However with Neutron Stars;

the basic models for these objects imply that neutron stars
are composed almost entirely of neutrons, which are subatomic
particles

Does this mean neutrons can exist outside of an atom, and that a Neutron star isn’t comprised of an element that is recognisable on the periodic table?

Not everything is made of atoms. The key here is that neutron stars are not made of the "ordinary matter" mentioned in the first definition. It's a very different and "special" kind of matter, which is made mostly of neutrons with a bunch of other things packed in. Yes, neutrons can exist outside of an atom; they are unstable if they are free; they are stable within the neutron star. A neutron star is not made of any "element" - that falls under the "ordinary matter" category, and this is not it. It's a different kind of stuff.

@florin-andrei Would a "100% minus one neutron" neutron star with a single proton be an isotope of hydrogen?

The notion that "everything is made of atoms" is something of a lie-to-children ( https://en.wikipedia.org/wiki/Lie-to-children ). Every "thing" which is familiar is made of atoms, but we can name all kinds of "things" and "stuff" which isn't made of atoms. Even matter. What atom is the stuff whirling around inside a cyclotron made of? The stuff your cloud chamber tracks reveal?

@PhilippeGoulet - only if you over-extend the definition of the hydrogen atom to the point where it's meaningless. Atoms and neutron stars are very different from each other. Atoms are essentially quantum objects, have a cloud of electrons around them, and interact mostly via electromagnetism. Neutron stars are macroscopic objects, do not have an electron cloud, and interact mostly via gravity. Almost none of the rules that apply to one could be applied to the other.

@PhilippeGoulet I suppose that it could only be stable in its ionized form ...

@Beanluc - I've used that "lie-to-children" concept before, but I hate that phrase for it. We essentially live in a fractal universe, as any explanation that describes it when viewed at a certain level is completely "wrong" when you dive down to a deeper level of detail. Its yet to be shown that there's any final level of detail though, so why imply lower levels are better? That's to imply we are perhaps all "children", and everything is a "lie". Perhaps true, but too depressing.

@T.E.D.: In computing we have the idea of "abstraction," where you organize your domain of study into layers. There are almost always layers above and below whatever you're talking about right now, and these are ignored, handwaved, or black-boxed as appropriate. But we never forget that they are there. The OSI model is one of the more formalized examples of this approach.

@Kevin - Having implemented a UDP stack, top to hardware driver, I'm aware of that paradigm. I don't think its really relevant here though, because the stack layers are completely separate things that interface with each other, not the exact same thing, viewed with varying levels of precision.

5 years ago

Yes neutrons can exist outside the atom (or nucleus). In free space a neutron will beta decay into a proton, and electron and an anti-neutrino on a timescale of 10 minutes. However, in the dense interiors of a neutron star, the electrons form a degenerate gas, with all possible energy levels filled up to something called the Fermi energy.

Once the Fermi energy of the electrons exceeds the maximum energy of any possible beta-decay electron, then beta decay is blocked and free neutrons become stable. This is what happens inside a neutron star and you end up with mostly neutrons with a small fraction perhaps a few per cent electrons and protons.

In the outer parts of the neutron star, the protons and neutrons can still arrange themselves into nuclei (but not atoms), but these nuclei are extremely neutron-rich (they would not normally exist in nature) and are only stabilised against beta decay by the process I described above. The very outer envelope may consist of completely ionised iron-peak element nuclei and there may be an ultrathin (few cm) layer of recognisable ionised hydrogen, helium and carbon (e.g. Wynn & Heinke 2009).

Once the density reaches about $3 \times 10^{16}$ kg/m$^3$ it becomes more favourable for the neutrons and protons to organise themselves into "macro-nuclei" - long strings and sheets of nuclear material, known colloquially as nuclear pasta.

At higher densities still, the pasta dissolves into a soup of mostly neutrons with about 1 per cent protons and electrons.

The diagram below (from Watanabe et al. 2012) shows roughly how these layers are arranged. It should be stressed that this is based on theoretical modelling, with the theory becoming less certain the further into the neutron star you go. Testing these ideas involves nuclear and particle experiments, observations of pulsars, of neutron star cooling, of X-ray bursts, mass and radius estimates in binary systems, pulsar glitches, etc., etc. None of the details have been observationally confirmed beyond dispute, but the basic picture below fits what we know. In particular, the the crust and the n,p,e fluid regions are well understood in theory. The details of the nuclear pasta phases are still the subject of a lot of theoretical work, as are the details of superfluidity in the interior, and what happens in the very central regions (solid neutron core, extra hadronic phases, boson condensation, quark matter) is still theoretically difficult and observationally untested except perhaps to say that the softest equations of state have been ruled out by the existence of $2M_{\odot}$ neutron stars.

"Once the Fermi energy of the electrons exceeds the maximum energy of any possible beta-decay electron" -> I think you mean "Once the Fermi energy of the **neutrons** exceeds the maximum energy of any possible beta-decay electron"

@NickEdwards I most certainly don't. Degenerate neutrons can't block the creation of a beta decay electron!

What's the composition of the envelope? Nuclei that aren't neutron-rich?

Does this mean that the outer crust consists of things that a chemist would recognize, more or less, as atoms? If so, or if not, I think that a mention of that would improve the answer.

@Qsigma Chemists deal with substances that have attached electrons, not bare nuclei. But even these exotic nuclei would not be stable outside the neutron star.

Thanks. A Coulomb crystal must be quite different from a terrestrial crystal or metal. I just read this abstract: http://iopscience.iop.org/article/10.1088/1742-6596/496/1/012010/meta I guess in first read I missed your phrase: "the protons and neutrons can still arrange themselves into nuclei (but not atoms)".

I think it's good for people new to physics to try and imagine some of the huge numbers. In this case $3 \times 10^{16}$ kg/m$^3$ is about the same as the mass of all humans in the world compressed into one cubic inch

@RobJeffries: would you mind briefly discussing in your answer what is theory and what is observation?

@martinargerami It is not possible to do that briefly. It involves nuclear and particle experiments, observations of pulsars, of neutron star cooling, X-ray bursts, mass and radius estimates, glitches, etc., etc. None of the *details* have been observationally confirmed beyond dispute, but the basic picture above fits what we know and is generally accepted bar what goes on in the centre. In particular, the basics of the crust and n,p,e fluid regions are pretty well understood theoretically.

@Qsigma Coulomb crystal just means that inter-nuclear potential energy (due to protons in nuclei and ~uniformly distributed electrons) is much greater than the thermal energy.

@bendl That's unimaginable - because of imperial unit mix :)

@RobJeffries that sounds like a good brief discussion of theory vs observation.