Astrobiology institute shows how wide binary stars form
Researchers studied the motions of newborn triple stars and concluded that the widest binary systems began as three stars, not just two.
December 11, 2012
Using computer simulations, scientists from the NASA Astrobiology Institute team at the University of Hawaii are shedding light on a question that has challenged astronomers for years: What causes wide binary stars?
The widest binaries and triple systems have very elongated orbits, so the stars spend most of their time far apart. But once in every orbital revolution they are at their closest approach, as depicted in this artist's impression by Karen Teramura (UH Institute for Astronomy) with background photograph by Wei-Hao Wang.
Binary stars are pairs of stars that orbit each other. Wide binary stars are separated by as much as 1 light-year in their orbits, farther apart than some stellar nurseries are wide. Astronomers have known about such distant pairs for a long time but have not understood how they form.
Researchers simulated the complex motions of newborn triple stars still embedded in their nascent cloud cores. They studied the motions 180,000 times and concluded that the widest binary systems began as three stars, not just two.
Most stars are born in small, compact systems with two or more stars at the center of a cloud core. When more than two stars share a small space, they gravitationally pull on each other in a chaotic dance. The least massive star often is kicked to the outskirts of the cloud core while the remaining stars grow larger and closer by feeding on the dense gas at the center of the cloud core.
If the force of the kick is not forceful enough, the runt star will not escape, but instead will begin a wide orbit of the other two, creating a wide binary. However, sometimes astronomers find only two stars in a wide binary. This means either the star system formed differently or something happened to one of the original binary pair.
"What may have happened is that the stars in the close binary merged into a single larger star," said Bo Reipurth of the Institute for Astronomy at the University of Hawaii at Manoa. "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 wide binary nearest to Earth is Alpha Centauri. The star itself is a close binary. Alpha Centauri has a small companion, Proxima Centauri, which orbits at a distance of about one-quarter of a light-year, or 15,000 times the distance between Earth and the Sun. All three stars were born close together several billion years ago before a powerful dynamic kick sent Proxima out into its wide path, where it has been orbiting ever since.
NASA's Kepler mission already has proven that more than one planet can form and persist in the stressful realm of a binary star, a testament to the diversity of planetary systems in our galaxy.