In 1983, three American physicists hypothesised that a type of degenerate matter referred to as “nuclear pasta” exists between the crust and core of a neutron star. Another hypothesis, perhaps more substantially evidenced, is that Ravenhall, Pethick and Wilson were particularly hungry during their research, likening the structures of different phases to Italian foods including spaghetti, penne and lasagna.
Formed in the final stage of stellar evolution, neutron stars are the collapsed cores of stars with initial mass of approximately 8 to 30 solar masses (M☉). A star of initial mass significantly below this range will form a white dwarf, while one above will become a black hole. In an eligible star, hydrogen fusion followed by fusion reactions involving heavier elements yields an iron core. Fusion releases energy, providing the electron degeneracy pressure necessary to counter gravity and keep the star from collapsing. No more fusion can occur once the iron core is formed. This is due to the fact that at low pressures Iron-56 has the highest of any binding energy per nucleon, so further fusion would require an energy input.
The core accumulates until it reaches the maximum mass which can be supported against gravity by electron degeneracy pressure: the Chandrasekhar mass. After this point the core collapses under gravity, producing pressures high enough for protons and electrons to form neutrons and a flood of neutrinos. The collapse is stopped when the nuclear density of 4×1017 km/m3 is reached – exceeding that of an atomic nucleus. At this point mutual electromagnetic repulsion and neutron degeneracy pressure combat gravity. Neutron degeneracy creates a pressure as no two neutrons can occupy the exact same state. The falling outer layers of the star are flung outward due to a rapid release of gravitational energy carried mostly by neutrinos, causing an explosion transferring energy outwards with a shock wave, in a Type II supernova. What remains after only seconds is a neutron star.
Neutron stars are not large in size, with diameters falling within the range of 15-30 kilometres. On average, they have a mass of roughly 1.4 M☉. Such great mass accompanied by small volume results in density within an order of magnitude of that of an atomic nucleus. If a radius of 15km and mass of 1.4 are used, an estimate for density is returned as 2.0×1026km/km3, in comparison to the Earth’s meagre density of 5.0×1022 kg/km3. A 6ml imperial teaspoon of neutron star material would have a mass of around 900 billion kg. This awards neutron stars the title of the densest observed objects in the universe.
Towards the centre of a neutron star, density increases. Between the crust and core, it is postulated that most protons merge with electrons to form neutrons, as unimaginable density squeezes nuclei closer together. Competing nuclear attraction between protons and neutrons, electric repulsion of the protons, and the pressure at this depth in the star is thought to trigger the formation of a variety of structures made up of neutrons and protons. The name “nuclear pasta” refers to the geometry of these theoretical structures. The incredibly magnitudinous density of this material means that it would be the strongest material in the universe. To eat a plate of nuclear pasta would require about 10 billion times the force needed to shatter steel – al dente is perhaps an understatement.
From the outer layers of nuclear pasta, progressing inwards, the increasing density gives rise to a series of layers – or “phases” – characterised by different configurations of distorted neutron matter.
Nearer the surface is the gnocchi phase – round, bubble-like neutrons alternatively referred to as meatballs. Travelling deeper in, the pressure manipulates the neutrons into long tubes – the spaghetti phase. Next are cross-rods sometimes described as waffles. This is followed by sheets of neutrons called lasagna, then tubes called penne, bucatini or antispaghetti, and finally bubbles called Swiss cheese or antignocchi.
There is a surprising similarity between the shapes of nuclear pasta phases and lipid polymers, found in fats. These molecules are made up of closely packed water-attracting and repelling layers, which, in a liquid environment, form endoplasmic reticulum structures found in eukaryotic cells. While the two environments could not differ more, and the structures’ building blocks differ drastically between protons and neutrons and long chains of molecules, nuclear pasta does share a common aim with these formations. They both work to minimise surface energy. Surface forces draw structures into spheres initially because this shape involves the least energy on the surface. This results in bubbles and folded layers in the nuclear pasta layer of the neutron star, or the watery interior of a cell. The forces in a neutron star are strong electromagnetism and nuclear, while cells abide by weaker molecular electric forces and the properties of water. Remarkably similar shapes, in environments on opposite ends of the scale of extremity.
The characteristics and phases of nuclear pasta can only be investigated using computer models, squeezing and stretching the pasta to determine its strength. One simulation involved 3 million protons and neutrons, requiring a supercomputer and hundreds of processors running simultaneously for about a year to complete the 2 million processor hours.
While there is a lack of observational evidence to confirm the existence of nuclear pasta, recent research has provided some optimism. Pulsars are highly magnetised rotating neutron stars, which, at middle-age, should spin down – slow in rotational velocity at a very small constant rate – from their 10 second rotations to much longer periods of up to 100 seconds. These longer periods have not yet been observed. Physicists Pons, Vigano and Rea have shown that this lack of observation may be attributable to the pasta limiting the maximum spin period of pulsars. This link emerges from the possibility that nuclear pasta could have irregular topological defects, which could decrease the electrical and thermal conductivity of neutron stars. This would, in turn, lower the temperature of the crust of the star, with greater resistance causing the magnetic field of the neutron star to decay over 0.1-1 Myr. Decay of the magnetic field decreases the torque, reducing the spin down process and establishing a low maximum rotation period. Perhaps proof that this menu of peculiar geometrical structures really could lie below the surface of neutron stars.