Neutron stars are presumed to contain the densest matter in the cosmos. These remnants of core-collapse supernovae pack more than the Sun’s mass (M ⊙) into a sphere less than 30 km across. There is considerable uncertainty about the character of matter squeezed to such ultrahigh densities, which cannot be reproduced in the laboratory.
As the name implies, much of a neutron star’s interior is probably just neutrons packed together at a density of about 1015 g/cm3—a few times that of a typical nucleus. The protons and electrons of the progenitor material have mostly merged into neutrons by inverse beta decay.
Theorists speculate, however, that at the highest densities near the cores of the most massive neutron stars there may be phase boundaries that enclose more exotic states: free-quark matter rich in strange quarks, Bose–Einstein condensates of K mesons, or simply nuclear matter in which...