Neutron Stars: Frozen on the Verge of a Phase Change express
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.
Original abstract
as written by the authors arXiv:2606.06378
Whether cold dense QCD matter undergoes a strong first-order phase transition remains an open question. In nature, neutron stars provide the most direct probe of cold dense QCD matter. Theoretically, chiral effective field theory constrains the equation of state of dense matter near nuclear saturation density, while perturbative QCD calculations constrain it at densities well beyond stable neutron-star interiors. We perform Bayesian inference with non-parametric Gaussian-process equation of state for $$\beta$$-equilibrated neutron-star matter under the assumption with and without a strong first-order phase transition, using the tidal deformability from GW170817, the NICER mass--radius measurements of PSR~J0740$$+$$6620, PSR~J0030$$+$$0451, PSR~J0437$$-$$4715, PSR~J0614$$-$$3329, chiral effective field theory, and perturbative QCD. Within this Bayesian inference framework, the model comparison mildly to moderately favors a strong first-order phase transition, with its onset most likely lying \emph{above} the central density of the most massive neutron star. Such an onset reconciles the stiffness required to support massive neutron stars with the softening favored by perturbative QCD from asymptotically high density.
If a person could stand on the surface of a neutron star, their weight would be about one hundred billion tons – roughly the weight of Mount Everest compressed to the size of a sugar cube.
In Robert Forward's novel 'Dragon's Egg', a neutron star is the cradle of intelligent microscopic life, and in the series 'The Expanse', the ultra-dense matter of such stars is turned into fuel for faster-than-light ships.