A satellite experiment has revealed that the heaviest known neutron star is unexpectedly large, which suggests that the matter in the star’s inner core is less “squeezable” than some models predict. NICER measures the size of a neutron star by tracking the x-ray emission from “hot spots” on the surface as the star rotates. These spots occur at the magnetic poles of the star, where the field slams particles onto the star surface. Neutron stars are “cosmic zombies”—corpses of massive stars that collapsed in violent explosions after running out of fuel. By studying these ultradense objects, researchers hope to understand...