Two figure skaters glide on the ice. One is light and cautious, barely rotating. The other is massive and fast, spinning with all its might, and in different directions at that. That's roughly the picture painted by gravitational waves from black hole mergers.

Scientists analyzed data from the fourth LIGO–Virgo–KAGRA catalog and found that the spin of black holes in a pair strongly depends on their mass. Light holes, up to 35–50 solar masses, barely spin: their spin (a measure of rotation) does not exceed 0.2 of the maximum possible. Heavy ones, from 70 solar masses and up, spin at full tilt — on average about 0.7 of the limit, and often lopsidedly: one hole in a pair may spin fast while the other is noticeably slower.

The slow spin of light holes fits a scenario where both stars were born together and lived quiet lives, not picking up extra spin. The fast and asymmetric spin of heavy holes looks more like the result of repeated mergers in dense stellar clusters or chaotic captures. However, the expected clear signature of "hierarchical" mergers, where one hole always spins faster than the other, did not show up in the data. Nature, once again, proved more complex.

This spin portrait is a new target for theorists. Now, models of binary black hole formation must explain not only masses but also how those masses relate to the spin of each individual hole.