If you squeeze a handful of sand down to the size of a pinhead, it turns into a rock. But squeeze it even harder, to the density of an atomic nucleus, and the grains lose their identity. Protons and neutrons dissolve, and the stuff becomes a soup of free quarks. In the cores of neutron stars, the pressure is just right for this soup to thicken: quarks start pairing up like droplets of mercury on glass, forming a single superfluid liquid. That's color superconductivity — a state where quarks move without friction.
Physicists studied a special kind of such matter — the color-flavor-locked (CFL) phase, where all three quark flavors pair in a symmetric pattern. In ordinary nuclear matter, neutrons eventually decay, and the star holds together only by gravity. But in the CFL phase, the quark glue is so strong that the star can keep its shape without external pressure, like a water droplet in free fall. The authors calculated the strengths of attraction and repulsion between quarks for which such a droplet neither collapses nor flies apart.
It turns out that within the NJL model, a working scheme for describing quark interactions, there is a whole region of parameters where CFL matter is more stable than ordinary nuclear matter. This means compact objects could be not neutron stars but quark stars from crust to core. Such a scenario fits observational data: both the supermassive component in the gravitational-wave merger GW190814 (about 2.6 solar masses) and the ultralight object in the supernova remnant HESS J1731-347 (0.77 solar masses) get a natural explanation. The former is a heavy quark star, the latter a light one but still self-bound.
The work does not claim that such stars definitely exist, but it shows that known anomalies in the masses of compact objects cease to be anomalies if we allow that their matter has transitioned into the CFL phase. It's like finding the missing puzzle piece, and suddenly seeing the whole picture.
A quark star the size of a city (about 10 km) could weigh half a million times more than Earth, yet remain stable without gravitational compression: it's held together by the internal pressure of quark 'glue'.
In the novel 'The Quark Dream' (Japanese: クォークの夢), stars made of strange matter serve as beacons for interstellar navigation: they can't be confused with ordinary neutron stars because of their unique spectrum.