Neutron stars are cosmic 'ice cubes' of unimaginable density: a tablespoon of their material would outweigh Mount Everest. It was a mystery what lay in their cores: ordinary neutron matter (a giant atomic nucleus) or exotic quark matter, where particles are not bound into neutrons. New analysis of gravitational waves and pulsar data from the NICER telescope showed: the abrupt transformation into quarks begins only at pressures higher than in the center of the most massive neutron stars. All known neutron stars are solid nuclear spheres teetering on the edge of a liquid state.

This discovery resolves the contradiction: neutron stars must be stiff enough to withstand twice the Sun's mass, yet theory demands that under extreme compression, matter becomes soft, like a liquid. Were the transition to happen inside stars, they'd become too flimsy for such heft. Nature 'protected' itself: the critical density is just out of reach, so stars stay elastic and massive. Like ice that only melts under the pressure of a skate blade, neutron stars never 'melt' – that's the secret of their stability. By the way, the first pulsar was discovered by graduate student Jocelyn Bell Burnell, giving science unique laboratories for studying ultra-dense matter.