A neutron star resembles a giant atomic nucleus, with 1–2 times the Sun’s mass packed into a ball about 20 kilometres across. Its gravity is so strong that a projectile would need to be launched at about half the speed of light to escape from its surface. Extreme density, pressure, temperature, magnetism and relativistic gravity make these objects fascinating but challenging to study. Surprising observations of spin-down irregularities in one intensely magnetized neutron star, reported by Archibald et al.1 on page 591 of this issue, offer clues about exotic processes occurring deep inside these objects.
The basic structure of a neutron star is generally agreed on. It has a crust about 1 km thick, in which nuclei are arranged in a crystal lattice immersed in a ‘sea’ of electrons. Near the surface, the nuclei are plain iron, but the pressure and density increase rapidly with depth, so that the nuclei become increasingly bloated and neutron-rich. At moderate depth, neutrons ‘drip’ out of the nuclei, forming a neutral liquid between the lattice nuclei. At the base of the crust, the bloated nuclei merge. Below this lies pure nuclear fluid, more than 200 trillion times denser than liquid water.