Using computer simulations, Bo Reipurth of the University of Hawaiʻi at Mānoa’s Institute for Astronomy and Seppo Mikkola of Tuorla Observatory, University of Turku, Finland have figured out how wide binary stars—two stars that orbit each other at a distance up to a light-year—form. Their research was published on December 5 in the journal Nature.

The Sun is a single star and this puts it in a minority of stars because most stars are binaries—two stars that orbit each other and are bound together by their mutual gravity. Binaries can be very close, sometimes so close that they actually touch each other. Other pairs are extremely wide, with separations up to a light-year or so.

Astronomers have known about such wide pairs for a long time, but how they form has been a mystery. The problem is that the typical cloud cores out of which stars are born are not large enough to form the widest binaries.

Reipurth and Mikkola used computer simulations to come up with a mechanism that accounts for the formation of wide binaries. Most stars are initially formed in small compact multiple systems with two, three or even more stars at the center of a cloud core. When more than two stars are together in a small space, they gravitationally pull on each other in a chaotic dance, where the lightest body is often kicked out to the outskirts of the core for long periods of time before falling back into the fray.

Meanwhile, the remaining stars feed on the gas at the center of the cloud core and grow heftier. Eventually, the runt of the litter gets such a large kick that it may be completely ejected. But in some cases, the kick is not strong enough for the third body to fully escape, and so it is sent out into a very wide orbit. The implication is that the widest binaries really should be three stars, not just two stars.

When astronomers carefully inspect the stars in a very wide system, they often find that one of them is a tight binary. But sometimes it appears that there really are only two stars in a wide system. This means that either wide binaries with only two stars are formed in another way, or something has happened to one of the stars that was once a close binary.

What may have happened is that the stars in the close binary merged into a single larger star. 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.

Read the UH Mānoa Institute for Astronomy news releasefor more information.

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