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Along with black holes, neutron stars are the result of stars collapsing under gravity once their fuel burns out, until their density is about the same as that of the nucleus of an atom, at which point the protons and electrons "melt" into pure neutrons. Just how dense are we talking? If you had a piece of neutron star the size of a mere sugar cube, it would weigh a whopping 100 million tons. Only black holes are denser.
Scientists suspect that something akin to a quark-gluon plasma may lie at the innermost core, while more conventional matter comprises the outermost edges of the star. In between, making up the crust, is a rare state of degenerate matter known as "nuclear pasta" (made up not just of neutrons, but also protons and electrons). And the various shapes such "pasta" takes represent a series of phase transitions as this boundary layer goes from lumpy to smooth.
At a press conference of the APS April Meeting this past weekend in Baltimore, Maryland, Indiana University graduate student Matt Caplan gave an overview of the ongoing work in Charles Horowitz's lab, running intricate computer simulations of various types of nuclear pasta to calculate the respective properties, in hopes of learning more about the evolution of neutron stars. It has to be simulated, because the rare conditions necessary for the pasta to form can really only be found in the cores of neutron stars.
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The notion of nuclear pasta emerged a few years ago as a possible explanation for the unusual behavior of certain kinds of neutron stars. They come into existence spinning rapidly -- as fast as several rotations per second -- and gradually slow down over time as energy dissipates. But the x-ray pulsar variety (the brightest and hence easiest to spot) eventually hit a steady state, completing a rotation once every 12 seconds or so. There should, in theory, be some x-ray pulsars with slower spin rates -- or a good reason why this is clearly not the case.
In 2013, a team of scientists at the University of Alicante in Spain proposed that the difference had to do with whether the crust at a star's surface was lumpy or smooth, based on a series of computer simulations investigating different crust configurations. Stars with lumpy crusts would slow down as usual, but in just 100,000 years -- a blink of an eye in a stellar lifetime -- they would settle into a steady state of one rotation every 10-20 seconds for the next million years. This didn't happen in those stars with smoother crusts; they just kept spinning more and more slowly -- as slow as one rotation every 100 seconds.
And just what makes a crust lumpy rather than smooth? The Spanish scientists proposed lumpy crusts are filled with nuclear pasta. Per New Scientist: "Because neutron stars are so dense, atomic nuclei are packed together tightly in the crust. The particles in these compacted nuclei could be forced into exotic groupings that resemble spaghetti, macaroni, and layers of lasagna. Mixing these shapes together would make it bumpier than one that only contains regular nuclei arranged in orderly crystals."
The various shapes of pasta represent different stages in a phase transition process. To wit:
Gnocchi Phase: This is a semi-spherical phase resembling those yummy potato-based dumplings that perhaps aren't technically pasta, but heck, close enough.
Spaghetti Phase: At some point, as compression continues to intensity, there just isn't sufficient electric repulsion in protons to maintain a spherical shape, so the gnocchi phase gets crushed into long spaghetti-like strands.
Lasagna Phase: Eventually the compression will become so strong that the rods of spaghetti fuse together forming sheets -- ergo, this is dubbed the lasagna phase.
Penne (or Anti-Spaghetti) Phase: At this point the outer core of the star is pretty far along in the phase transition process, looking more uniform as compression continues, with intermittent cylindrical voids. This is the penultimate stage. As the process continues, the voids become more spherical, before smoothing out the remaining lumps.
The latest paper on the spin rates of neutron stars and how they might be affected by nuclear pasta appeared in Physical Review Letters in February of this year, adding yet another a new wrinkle to this fascinating form of matter. Specifically, the latest computer simulations revealed possible topological defects within the various pasta shapes, which may reduce the thermal and electrical conductivity of the star, thereby causing the magnetic fields to dissipate much more quickly than expected. Such stars radiate less energy into space, which stabilizes their rotation rate.
For instance, that lasagna phase of nuclear pasta could form corkscrew-shaped defects connecting the layers of "lasagna." "I have been trying for years to imagine neutron stars as geologic worlds with different kinds of nuclear rocks, faults, mountains," Horowitz told Space.com. "Then, one molecular dynamics simulation found a mistake -- a defect in otherwise regularly perfect pasta shapes that persisted for a very long time."
Further analysis revealed that the simulations' prediction of evidence for crust cooling -- an indicator of decreased conductivity -- for a particular neutron star match quite well with observational data gathered by the Chandra X-ray telescope. Chances are, such topological defects may play a role in other unusual aspects of neutron stars -- all part of the ongoing quest to understand how matter behaves when compressed to its densest form.
References:
Fowler, R.H. (1926) "On Dense Matter," Monthly Notices of the Royal Astronomical Society 87.
Horowitz, C.J. et al. (2015) "Disordered nuclear pasta, magnetic field decay, and crust cooling in neutron stars," Physical Review Letters 114: 031102.
Pais, Helena and Stone, Jirina R. (2012) "Exploring the nuclear pasta phase in core-collapse supernova matter," Physical Review Letters 109: 151101.
Pons, Jose A.; Vigano, Daniele; Rea, Nanda. (2013) "Too much 'pasta' for pulsars to spin down," Nature Physics 9(7): 431-434.