Cast out (Image: Science/AAAS)

Love triangles are rarely sustainable – even in space. New simulations show that single stars that try to come between a tight stellar pair are kicked into space at breakneck speeds, explaining the origin of “runaway” stars that have puzzled astronomers for half a century.

Most stars in the Milky Way plod around the galaxy at a relatively sedate pace of 5 kilometres per second. But some rocket along at more than 30 kilometres per second, faster than the Earth orbits the sun.

In 1961, Adriaan Blaauw of Leiden University in the Netherlands suggested that the runaways were shoved to punishing speeds when a companion star exploded as a supernova, a picture that was bolstered by the discovery of just such a pair in 1997.


But last year, two high-speed stars were spotted fleeing a cluster of stars called R136, a dense group of infant stars thought to be less than 2 million years old.

“That was odd,” says Simon Portegies Zwart of Leiden University. Because such stars don’t explode until they’re at least 3 million years old, the exiles from R136 could not have been kicked out by detonating partners.

Portegies Zwart had another idea: instead of having just one violent companion, the runaways were the unfortunate victims of a love triangle.

Tighter pair

Massive stars in clusters sometimes come close enough to a pair of stars to be caught in their orbit. But the trio is gravitationally unstable – someone has to go. The intruder typically doesn’t have enough energy to unbind the binary, so it’s the one to get the boot. The energy it receives in the process only ties the couple together more tightly.

To test this idea, Michiko Fujii, also at Leiden, and Portegies Zwart ran about 60 simulations of star clusters. They found that each cluster developed a binary “bully” that ejected about 21 stars from the cluster before leaving the cluster itself.

That can easily account for most of the runaway stars observed in the Milky Way. “Right now I would say the majority, maybe even the vast majority, of runaways are produced by dynamical ejection,” Portegies Zwart says.

Journal reference: Science, DOI: 10.1126/science.1211927