For a long time, a void yawned in the family of ultra-dense remnants of dead stars. Neutron stars, swift city-sized tops, rarely weigh more than two Suns. Black holes start at about five solar masses. Between them lies a 'gap' where for decades no object could be found. Recent observations of gravitational waves and radio telescopes have begun to fill this gap: compact bodies of unknown nature have been discovered there.

The authors of the paper tried on the role of these wanderers a hybrid star — a neutron star with a quark core. At its center, the pressure is so great that neutrons 'break apart' into constituent quarks, like an eggshell under a press. This transition is sharp, as if water turns into ice at the same temperature. For a hybrid star to reach the mass of the gap (2.5–5 solar masses), the quark core would have to appear suspiciously early, just as the star gains a mass of less than one solar mass, and the resulting quark matter would have to be unusually stiff, almost incompressible.

Then the scientists compared this exotic picture with what real measurements of neutron star masses and radii say. A Bayesian analysis (a method that weighs the probability of different scenarios given the data) showed that nature prefers a more modest option. The quark core most likely forms at a mass of about 1.4 solar masses — typical for neutron stars. This gives rise to 'twin stars': two stars with the same mass but different radii — one purely neutron, the other with a quark core. If such twins are found, hybrid stars in the mass gap will be ruled out. For now, the main candidates in the gap are light black holes.