In the star cluster NGC 2158, which is over a billion years old, astronomers have counted how often stars live in pairs. The Hubble telescope stared at the cluster for a long time to catch even the faintest stars: those lighter than one-third of the Sun. It turns out that pairs are not equally common: among stars with the Sun's mass, about half have a companion, but among light stars, five times less massive than the Sun, only one in ten does.

This difference matches what astronomers see in stars near us, in the galactic field. So the rules for forming pairs are probably the same everywhere. But there's a surprise: the cluster has fewer of the smallest stars than models predict. And at about one-third the Sun's mass, there is a dip in stellar properties — similar to one already seen in nearby stars. This might be a trace of how the helium-burning mode changes in the interiors of such stars.