Waves sort pebbles on a sandy beach. Small stones lie close together, nearly identical in size. Farther from shore, there's a scatter of larger and more varied ones. And right at the waterline, where the surf is strongest, the biggest and most rounded fragments pile up. Nature sorts black holes by mass in much the same way when they merge in pairs.
In the new gravitational wave catalog GWTC-5, astronomers noticed that black hole pairs aren't random. Light holes (up to 15 solar masses) almost always find a partner of similar mass and spin in the same direction as their orbital motion, like two figure skaters holding hands and spinning in sync. In the 18–30 solar mass range, the picture changes dramatically: partner masses can differ widely, and spins become chaotic — the skaters let go and twirl every which way.
Above 45 solar masses, it's a real mixed bag. Masses and spins are scattered broadly, which looks just like what we'd expect from black holes assembled from remnants of previous mergers, as if skaters collided, fell, then got up and continued the dance in threes or fours.
Such clear mass sorting with sharp transitions suggests that light and heavy pairs are born in different "nurseries." Light ones likely come from binary stars that spent their whole lives together. Heavy ones assemble in dense star clusters, where black holes constantly collide and swap partners. So mass isn't just a number: it's a birthmark, like a stamp on a pebble from a particular stretch of beach.
The heaviest black hole in the GWTC-5 catalog weighs as much as 150 Suns, but it couldn't have been born from a single star, because such stars explode without leaving a black hole. So it must have been assembled from several smaller holes, like a snowball.
In Robert Wilson's novel "Spin," astronomers notice stars going out on schedule, as if someone were sorting them by some unknown principle. Here, nature itself has sorted black holes by mass, and this schedule tells the story of their lives.