by MICHELLE WALTON
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.
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?
Binary stars are pairs of stars that orbit each
other. Wide binary stars are separated by as much as one 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 the widest binary systems began as
three stars, not just two. This research appears in a paper to be published in
the Dec. 13 issue of the journal Nature and was released last week online.
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 forecful enough,
the runt star will not escape, but instead begin a very 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 the paper's
lead author, 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.
NASA supported the University of Hawaii work
through a cooperative agreement with NASA's Ames Research Center, Moffett
Field, Calif., and the NASA Astrobiology Institute, which is a partnership
between NASA, 15 U.S. teams, and 10 international consortia. The research on
wide binary stars included the University of Turku in Finland.
Original Source: NASA Astrobiology Institute
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