Scientists with NASA‘s Astrobiology Institute team at the University of Hawaii have used computer simulations to reveal what causes binary stars to form.

The researchers have concluded through their research that the widest binary star systems began as three stars, rather than two. To determine this, the scientists ran 180,000 simulations of the motions of newborn triple stars still shrouded in their cloud cores. The research appears in a paper to be published this week in the journal Nature.

Binary stars are pairs of stars that orbit each other. “Wide” binary stars refers to pairs of stars that orbit each other while separated by as much as one light-year. Until now, scientists have not been able to describe how these pairs were able to form, since most stars are born in compact systems with two or more stars at the center of a cloud core. Less massive stars are often flung to the outskirts of cloud cores, and can be found orbiting a pair of larger stars near the center. What the new paper describes is how these systems may have evolved into a system with only two wide-orbiting stars instead of three.

“What may have happened is that the stars in the close binary merged into a single larger star,” said Bo Reipurth, lead author of the paper and an astronomer with the Institute for Astronomy at UH. “This can happen if there is enough gas in the cloud core to provide resistance to their motion. As the two stars in the close binary move around each other surrounded by gas, they lose energy and spiral toward each other. Sometimes there is so much gas in the core that the two close stars spiral all the way in and collide with each other in a spectacular merging explosion.”

The nearest wide binary system to Earth is Alpha Centauri. Proxima Centauri is a smaller star that orbits Alpha Centauri at a distance of around one-quarter of a light year (15,000 astronomical units).

(Image courtesy Karen Teramura/Wei-Hao Wang/UH Institute for Astronomy)