We begin at the "atmosphere" of a neutron star. It has thickness on the order of a human hair, courtesy of the insane gravitational fields at play. While normal atmospheres are governed by factors like heat, pressure, and the like, the atmospheres that neutron stars have is quite different. The flow of these atmospheres is actually determined primarily by the tremendous magnetic fields generated by neutron stars.
A hair in, we encounter the outer crust. Scientists aren't quite sure what makes up this portion of the crust, but many think that it's comprised of iron atoms crushed together into a giant, neutron-star-sized shell. Diving into the inner crust, we encounter the first type of nuclear pasta - the gnocchi phase.
The gnocchi phase is, as the name would suggest, comprised of blobs. These blobs are actually nuclei that have been squished so close together that they begin to physically touch, and merge with each other. Here, the normal rules of nuclear physics, with atomic numbers and stable neutron numbers, stop applying.
Go down farther, and we reach the spaghetti phase, where these blobs themselves merge with each other, forming long, tangled strands of super-nuclei. By this point, the notion of a single nucleus doesn't really apply. Past spaghetti, naturally, comes lasagna, where these strands form long, planar sheets of protons and neutrons. These sheets are massive (by molecular standards), and it is hypothesised that the largest among them reach the millimetre scale. That may not sound like much, but consider; a single neutron is about one billionth of one millionth of a meter across.
But what after lasagna? The answer, it turns out, is anti-pasta.
Lasagna turns into anti-spaghetti, sheets forming hollow tubes spitting out free neutrons. Free neutrons are neutrons which are, well, free, to move through the matter that they exist as part of. Compare this to bound neutrons, which are attached to other neutrons and protons, and make up the vast majority of all neutrons in existence.
Deeper still in, and anti-spaghetti forms anti-gnocchi. In this anti-gnocchi phase, our super-nuclear tubes begin to break up, parts flying off and forming hollow bubbles.
Having exhausted the culinary delights, we reach the core of neutron stars. By definition, at this point, the vast majority of particles are neutrons. Neutron star cores bear similarities to the conditions the universe found itself in shortly after the big bang. Because of this, they are of great interest to researchers, as they provide a glimpse into the far past. By studying the conditions that may exist inside the cores of neutron stars, researchers are able to glean knowledge about what the early days of the universe may have been like, when everything was hot, and nothing was formed.
Other than that though, we don't know for sure what the core looks like. Researchers have put forth many theories, from a neutron-degenerate superfluid (a fluid with no thickness), to a mix of neutrons and other rarer, heavier particles, such as pions and kaons. One of the most interesting, however, and possibly the weirdest, involves hyper-dense matter supported by quark-degeneracy, and a little something known as... strange matter.
References
- M. E. Caplan, A. S. Schneider, C. J. Horowitz, D. K. Berry, "Pasta Nucleosynthesis: Molecular dynamics simulations of nuclear statistical equilibrium", American Physical Society, vol. 91, 065802, June 2015, doi: https://doi.org/10.1103/PhysRevC.91.065802
- A. Heger, C. L. Fryer, S. E. Woosley, N. Langer, and D. H. Hartmann, How massive single stars end their life, The Astrophysical Journal, vol. 591, 288, 2003, doi: http://dx.doi.org/10.1086/375341
Images
[1] "Neutron Stars: The Most Extreme Objects in the Universe", PBS Spacetime
[2] M. E. Caplan, A. S. Schneider, C. J. Horowitz, D. K. Berry, "Pasta Nucleosynthesis: Molecular dynamics simulations of nuclear statistical equilibrium", American Physical Society, vol. 91, 065802, June 2015, https://doi.org/10.1103/PhysRevC.91.065802