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Since a neutron star is basically just a vast amount of densely packed neutrons, I was wondering whether those neutrons form a single "atom" (of atomic number zero) or whether they are further apart and in a QFT-sense isolated?

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    $\begingroup$ The complex shell structure of neutron stars is driven by gravity and thermodynamics while the nuclear shell structure is a consequence of quantum mechanics. Unless you are also prepared to make a statement like "a planet is just a large molecule", then the answer is negative. $\endgroup$ – CuriousOne Sep 13 '15 at 19:38
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    $\begingroup$ Please don't confound an atom with an isotope. They are related concepts but not the same. $\endgroup$ – Gert Sep 13 '15 at 19:57
  • $\begingroup$ @Gert I'm well aware of that, though my question's formulation probably doesn't convey that very well... $\endgroup$ – Tobias Kienzler Sep 13 '15 at 19:59
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    $\begingroup$ Related miniature neutron stars $\endgroup$ – rmhleo Sep 13 '15 at 21:45
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It is a misnomer (at best) to characterize a neutron star as all neutrons. There are protons and electrons too.

Imagine compressing a bunch of regular matter at some point it requires less energy for a proton and electron to combine to form another neutron rather than for the electron to try to fill a very high energy state. That means there are so many electrons so as to fill all the states with energy up to the difference in energy of a neutron compared to a proton plus an electron. So those electrons stay around, as do an equal number of protons.

So if you want to think of it as an isotope, it wouldn't be a Z=0 isotope. And if it is so large you could even have protons entering one side and leaving another in a spacelike separated way, there is no clean sense where it is a single object with a certain Z. It is a system of many interacting parts.

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    $\begingroup$ Not to mention a crust of neutron-rich nuclei and a core that could contain something completely different. $\endgroup$ – Rob Jeffries Sep 13 '15 at 20:50
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    $\begingroup$ And the crust that is made of a strongly ionized iron sheet. $\endgroup$ – Youstay Igo Sep 14 '15 at 7:39
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Even if it is pure neutrons, I doubt that it can be called one single atom. Strong forces are short ranged and I don't think a macroscopic number of neutrons can form one single bound state. Instead they will form into many bound states each with a few neutrons.

Calling a neutron star a giant $Z=0$ atom would be like calling the earth, which is made of electrons, neutrons and protons, a giant atom with a giant $Z\neq 0$.

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