A neutron star is what remains after a massive star explodes. Its matter is squeezed so tightly that a teaspoon would weigh as much as a mountain. But what exactly happens in the deepest interior, where protons and neutrons may break apart into quarks, remains a mystery.
The authors took data from the X-ray telescope NICER, which measured radiation from two pulsars — rapidly rotating neutron stars that send us strictly periodic signals. For one of them, PSR J0030+0451, the surface temperature strongly depends on the chosen heating model: it differs in scenarios A, B, or C. This leads to different estimates of the star's radius, and thus different conclusions about its internal structure.
The scientists applied a model with a constant sound speed in quark matter (CSS) and checked which parameters do not contradict observations. It turned out that if the density jump at the boundary between ordinary nuclear matter and the quark phase is small, many equations of state work acceptably. But if the jump is large, the model fails to produce sufficiently massive stars, yet astronomers have already found neutron stars with masses around two solar masses.
Additionally, the researchers checked their conclusions against data from another heavyweight pulsar, PSR J0740+6620, and the spin record-holder PSR J0952-0607. It turned out that a high sound speed in the quark core helps keep massive stars from collapsing. So, by comparing different 'recipes' for matter with real cosmic objects, we can gradually figure out what goes on in the very heart of a neutron star.
A teaspoon of neutron star matter would weigh about as much as all the people on Earth combined.
In Robert Forward's novel 'Dragon's Egg', life is described on the surface of a neutron star, where due to the immense gravity and density, time flows a million times faster.