NASA ASTROBIOLOGY INSTITUTE SHOWS HOW WIDE BINARY STARS FORM
WASHINGTON -- 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
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.