The third star is key to the ejection, Reipurth explained.
"It's a fact of nature that, if you have two bodies alone, then they move in a completely deterministic way — it's possible to say exactly where they will be later on in their orbits," he said. "As soon as you put a third body in there, the system becomes completely chaotic."
Two bodies together will simply orbit one another, if not otherwise interrupted. But the third body creates a "kick" that eventually results in the ejection of one of the stars to a distant orbit.
Over the course of several months, Reipurth and Mikkola ran over 180,000 simulations of triple systems that would evolve into wide binaries. They found that more than ten percent of the triple systems ended up with stars spread thousands or tens of thousands of astronomical units from each other, a number that agrees with observations.
"We were surprised to see how well the results agreed with the observations," Mikkola wrote in an email. "Getting the 'right' answer did not require any adjustments of the model."
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