What do calcium and tin isotopes reveal about the invisible matter inside neutron stars?
Astronomers can weigh the heaviest pulsars, but they cannot touch the matter inside them. To bridge that gap, nuclear physicists tune mathematical descriptions of atomic nuclei using measured masses, sizes and excitations, then ask what those same rules imply for a star made mostly of neutrons. A fresh Bayesian calibration adds calcium and tin isotopes whose protons and neutrons do not all pair off neatly, along with an updated giant-monopole energy in zirconium-90. The result is a probability map, not a single answer, for the softness of neutron-rich matter and the composition of a neutron star's solid outer layer.