Two dim stars whirl so close together that one siphons material off the other, like a spoon delicately skimming foam from broth. Astronomers have for the first time calculated the fate of such pairs — ultracool and brown dwarfs, the coldest and dimmest inhabitants of the galaxy. Previously, it was thought that mass exchange between them was impossible: they were too fragile. But simulations showed that if the masses are nearly equal, the system is unstable, while with a slight difference, it can stably exist for a very long time.

When matter flows over, it does not form a disk but crashes directly onto the surface of the second star. At the impact point, a bright spot appears — in ultraviolet or even visible light. Such a pair completes a full orbit in one to three and a half hours, and both stars always face each other with the same side, like dancers in a slow waltz. The system is destined to live for about one hundred million years — short by cosmic standards, but for observers on Earth, it is an eternity.

Thus, the coldest stars in the galaxy turn out to be capable of turbulent relationships. By studying them, we better understand how objects on the boundary between planets and stars are born and die.