Two watermelons of equal weight sit on a counter. One is ripe, juicy, with a thick rind. The other looks as if carved from oak: inside it is completely different, and squeezing it is nearly impossible. They look like twins, but tap them, and the difference is obvious. That's roughly how astrophysicists describe 'twin stars'—neutron stars of the same mass but with fundamentally different interiors.

In an ordinary neutron star, matter is compressed so intensely that atoms are crushed, and protons and electrons merge into neutrons. It's like a giant atomic nucleus the size of a city. But what if, at the very center, pressure is so immense that neutrons break apart into quarks — giving rise to a quark core? It's exactly this kind of phase transition (a jump in density, like water turning to ice) that the authors explored. They took mass and radius data for several neutron stars measured by the NICER telescope and added two mysterious objects — HESS J1731-347 and XTE J1814-338. Their radii are suspiciously small, around 6–7 kilometers.

It turns out that explaining such tiny stars requires a very sharp energy jump — about 600–700 MeV per cubic femtometer (hundreds of times more energy than released in uranium fission). After the transition, matter becomes nearly incompressible: the speed of sound in it approaches the speed of light. This gives rise to a second, 'hybrid' branch of stars: at the same mass, the radius drops by almost half compared to a purely neutron star. The most intriguing part is that such twin stars are far less deformable under tidal forces. If one merges with another in a pair, the gravitational waves from the event would differ noticeably from the usual ones. Next-generation detectors could pick up this difference.