Che cos'è un Quark Star?
A quark star is a proposed stellar remnant one step beyond a neutron star: so dense that the neutrons themselves break down and their constituent quarks roam free through the whole object. Nobody has confirmed one exists, and several candidates look suspiciously like they might.
A neutron star is already matter at its structural limit — a body heavier than the Sun packed into something the size of a city, held up by the refusal of neutrons to occupy the same quantum state. But a neutron is not fundamental. It is three quarks bound by the strong force, and if you keep squeezing, theory says those bonds should give way and the quarks should deconfine into a single continuous fluid. Whether the cores of the heaviest neutron stars have already crossed that line is one of the genuinely open questions in physics, and it is being attacked from an unexpected direction: gravitational-wave detectors can measure how much a neutron star deforms in the last seconds before a merger, and how easily it deforms depends on what it is made of.
The twist is that the deconfined state may not just be reachable — it may be preferred. The strange matter hypothesis proposes that a quark fluid containing up, down and strange quarks in roughly equal numbers is the true ground state of matter, more stable than iron. If that is right, a quark star is not held together by gravity at all. It is self-bound by the strong force, and gravity is merely holding it in place.
That has a consequence you can see, and it is what this model is built around. Self-bound matter needs no crust and holds no atmosphere: the surface is a bare, essentially featureless boundary where density falls fourteen orders of magnitude across about one femtometre. Where a neutron star has a stressed, veined iron crust, this has a knife-edge limb and nothing else. It would also be smaller than a neutron star of the same mass, which is the other measurement astronomers are hunting.