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When neutron stars collide, they crash into and kill each other in a kilonova explosion, which blasts a lot of their material into space. A huge amount of neutronium, neutron star stuff, is suddenly no longer in the high pressure of the star, and instantly expands explosively into normal matter. In other words, thou shalt not have isolated teaspoons of neutronium. But is the same true for the crust, made of a form of highly compressed iron called nuclear pasta, 10 billion times stronger than steel? Is the crust material of neutron stars stable outside of neutron stars?

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"made of a form of highly compressed iron called nuclear pasta,"

This isn't the case. The crust of a neutron star consists of (starting at the surface going down).

(Picture from Watanabe & Maruyama 2012) enter image description here

  • Iron-peak nuclei plus degenerate electrons.

  • Increasingly neutron-rich elements with higher atomic numbers (i.e. not iron) plus degenerate electrons.

  • Very, very neutron rich heavy nuclei (mass numbers of several hundred) plus degenerate electrons and free neutrons.

  • Nuclear pasta, where the identity of individual nuclei disappears and the nucleons (mainly neutrons) form long chains, or sheets, surrounded by degenerate free neutrons and electrons.

None of these compositions is stable at low pressures. Specifically, you need to have them surrounded by a high density gas of degenerate electrons to suppress the beta decay of neutrons and neutron-rich nuclei. Putting this stuff into a low density environment would be explosive (but somewhat less) just as in the case of "neutronium" (neutron matter inside neutron stars contains protons and electrons too). This is not because of the beta decay, which is a slow process, but because the constituents have a huge amount of kinetic energy (that is what pressure is).

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    $\begingroup$ I thought the "Very very neutron-rich..." layer was considered part of the pasta too. The "gnocchi" layer, found above the "spaghetti" and "lasagna" layers :-) $\endgroup$ – Steve Linton Nov 26 '20 at 9:52
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    $\begingroup$ @SteveLinton The two are considered separately. There is an extensive layer in which there are free neutrons but the nuclei maintain their identity. The "pasta" is usually considered to be at higher densities than that when the structures become non-spherical. Someone ovbviously thought it clever to extend the pasta analogy to cover the isolated nuclei - or perhaps I am just a bit out of date on the current terminology. $\endgroup$ – ProfRob Nov 26 '20 at 10:33
  • $\begingroup$ This description sounds like if there is an island of stability, then kilonova remnants could be the place to look for them(?!) How wrong am I? $\endgroup$ – kutschkem Nov 26 '20 at 16:19

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