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| [CREDIT: Artist’s impression by Karen Teramura (UH Institute for Astronomy) with background photograph by Wei-Hao Wang] |
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 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.