Two stars orbit so closely that their outer layers merge into a shared envelope. Matter flows from one to the other, like water spilling from an overfull cup into its neighbor. In this cosmic dance, where stars literally touch each other, astronomers have for the first time detected magnetic fields in large numbers.

The team pointed a telescope with the HARPSpol polarimeter at 14 systems where stars exchange matter. Out of 53 observations, 13 systems showed clear magnetic fields, and only the dim SV Cen showed a barely noticeable signal. The field strengths range from a few hundred gauss (like a fridge magnet) to 4–5 thousand gauss in the triple system MY Ser. For comparison, such fields are rare among single hot stars.

Previously, it was thought that massive stars either inherit their magnetic fields at birth or acquire them in supernova explosions. Now a third path emerges: when two stars exchange mass, their material gets mixed and spun up. This triggers a dynamo — the same mechanism that generates Earth's magnetic field, but on a gigantic scale.

Among the findings are record-breakers. In the MY Ser system, both stars of the contact pair carry fields of 4–5 kilogauss. V1294 Sco and V606 Cen also have fields reaching thousands of gauss. It seems that the field strength depends on how much the stars fill their shared "cavity" and how different their masses are.

This large-scale "catch" of magnetic fields in interacting stars is the first estimate of how common the phenomenon is. Previously, the magnetism of close binaries was judged from isolated examples. Now it's clear: if stars touch, a magnetic field is almost guaranteed. So it's the dance of stars, not their solitary lives, that may be the main source of cosmic magnetism.