Astromaterial Science and Nuclear Pasta

The heavens contain a variety of materials that range from conventional to extraordinary and extreme. For this colloquium, we define Astromaterial Science as the study of materials, in astronomical objects, that are qualitatively denser than materials on earth. Astromaterials can have unique properties, related to their density, such as extraordinary mechanical strength, or alternatively be organized in ways similar to more conventional materials. The study of astromaterials may suggest ways to improve terrestrial materials. Likewise, advances in the science of conventional materials may allow new insights into astromaterials. We discuss Coulomb crystals in the interior of cold white dwarfs and in the crust of neutron stars and review the limited observations of how stars freeze. We apply astromaterial science to the generation of gravitational waves. According to Einstein's Theory of General Relativity accelerating masses radiate gravitational waves. However, very strong materials may be needed to vigorously accelerate large masses in order to produce continuous gravitational waves that are observable in present detectors. We review large-scale molecular dynamics simulations of the breaking stress of neutron star crust that suggest it is the strongest material known, some ten billion times stronger than steel. Nuclear pasta is an example of a soft astromaterial. It is expected near the base of the neutron star crust at densities of ten to the fourteen grams per cubic centimeter. Competition between nuclear attraction and Coulomb repulsion rearrange neutrons and protons into complex non-spherical shapes such as flat plates (lasagna) or thin rods (spaghetti). We review semi-classical molecular dynamics simulations of nuclear pasta. We illustrate some of the shapes that are possible and discuss transport properties including shear viscosity and thermal and electrical conductivities.