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by
Zheng Zheng
and
Lee J. Siegel
May 07, 2014
from
UniversityOfUtahNews Website
Astronomers think
hypervelocity stars happen
when the galaxy's central back
hole captures
one of two stars in a binary
system
and slingshots the other out of
the galaxy.
An astrophysicist-artist's conception of a
hypervelocity star
speeding away from the visible part of a spiral galaxy
like our Milky Way
and into the invisible halo of mysterious "dark matter"
that surrounds the
galaxy's visible portions.
Image credit: Ben
Bromley, University of Utah
A University of Utah-led team say they
have discovered a hypervelocity star that is the closest,
second-brightest and among the largest of 20 such stars found so
far.
Speeding at more than 1 million mph,
this star might provide clues about the supermassive black hole at
the center of our Milky Way and the halo of mysterious dark matter
surrounding the galaxy, astronomers say.
"The hypervelocity star tells us a
lot about our galaxy - especially its center and the dark matter
halo," says Zheng Zheng, an assistant professor of physics and
astronomy and lead author of the study published recently in
Astrophysical Journal Letters by a team of U.S. and Chinese
astronomers.
"We can't see the dark matter halo, but its gravity acts on the
star," Zheng says. "We gain insight from the star's trajectory
and velocity, which are affected by gravity from different parts
of our galaxy."
In the past decade, astronomers have
found about 20 of these odd stars.
Hypervelocity stars appear to be
remaining pairs of binary stars that once orbited each other and got
too close to the supermassive black hole at the galaxy's center.
Intense gravity from the black hole - which has the mass of 4
million stars like our sun - captures one star so it orbits the hole
closely, and slingshots the other on a trajectory headed beyond the
galaxy.
Zheng Zheng and his colleagues discovered the new
hypervelocity star while conducting other research into stars with
the Large Sky Area Multi-Object Fiber Spectroscopic Telescope,
or
LAMOST, located at the Xinglong
Observing Station of the National Astronomical Observatories of
China, about 110 miles northeast of Beijing.
Zheng Zheng, an
assistant professor of physics and astronomy at the University of
Utah,
led a team of
American and Chinese scientists who discovered
the closest bright
hypervelocity star of 20 yet found.
Scientists believe
each hypervelocity star began as part of a binary pair of stars
near the center of
our Milky Way galaxy, where extreme gravity from a supermassive
black hole
sucked in one star in
the pair and, like a bolo,
simultaneously hurled
the other star - a new hypervelocity star - toward the edge of the
galaxy.
Photo Credit: Lee J. Siegel, University of Utah
LAMOST boasts a 13.1-foot-wide aperture and houses 4,000 optical
fibers, which capture "spectra" or light-wavelength readings from as
many as 4,000 stars at once. A star's spectrum reveals information
about its velocity, temperature, luminosity and size.
LAMOST's main purpose is to study the distribution of stars in the
Milky Way, and thus the galaxy's structure.
The new hypervelocity star - named
LAMOST-HVS1 - stood out because its speed is almost three times the
usual star's 500,000-mph pace through space: 1.4 million mph
relative to our solar system. Its speed is about 1.1 million mph
relative to the speed of the center of the Milky Way.
Despite being the closest hypervelocity star, it nonetheless is 249
quadrillion miles from Earth (in U.S. usage, a quadrillion is
1,000,000,000,000,000 miles or 10 to the 15th
power, or 1 million billion).
"If you're looking at a herd of
cows, and one starts going 60 mph, that's telling you something
important," says Ben Bromley, a University of Utah physics and
astronomy professor who was not involved with Zheng's study.
"You may not know at first what that
is. But for hypervelocity stars, one of their mysteries is where
they come from - and the massive black hole in our galaxy is
implicated."
The down-low
on a fast and loose star
A cluster of known hypervelocity stars, including the new one, is
located above the disk of our Milky Way galaxy, and their
distribution in the sky suggests they originated near the galaxy's
center, Zheng says.
The diameter of the visible part of our spiral-shaped galaxy is at
least 100,000 light years, or 588 quadrillion miles. Zheng says that
when the halo of
dark matter is added, the estimated
diameter is roughly 1 million light years, or 5,880 quadrillion
miles.
Scientists know dark matter halos surround galaxies because the way
their gravity affects the motion of a galaxy's visible stars and gas
clouds.
Researchers say only about 5 percent of
the universe is made of visible matter, 27 percent is invisible and
yet-unidentified dark matter and 68 percent is even more mysterious
dark energy, responsible for accelerating the expansion of the
universe. By traveling through the dark matter halo, the new
hypervelocity star's speed and trajectory can reveal something about
the mysterious halo.
Our solar system is roughly 26,000 light years or 153 quadrillion
miles from the center of the galaxy - more than halfway out from the
center of the visible disk.
By comparison, the new hypervelocity star is about 62,000 light
years or 364 quadrillion miles from the galactic center, beyond as
well as above the galaxy's visible disk. It is about 42,400 light
years from Earth, or about 249 quadrillion miles away.
As far as that is - the star has a magnitude of about 13, or 630
times fainter than stars that barely can be seen with the naked eye
- it nevertheless,
"is the nearest, second-brightest,
and one of the three most massive hypervelocity stars discovered
so far," Zheng says.
It is nine times more massive than our
sun, which makes it very similar to another hypervelocity star known
as
HE 0437-5439, discovered in 2005,
and both are smaller than HD 271791, which was discovered in 2008
and is 11 times more massive than the sun.
As seen from Earth, only HD 271791 is
brighter than LAMOST-HVS1, Zheng says.
The newly discovered hypervelocity star also outshines our own sun:
It is four times hotter and about 3,400 times brighter (if viewed
from the same distance). But compared with our 4.6-billion-year-old
sun, the newly discovered LAMOST–HVS1 is a youngster born only 32
million years ago, based on its speed and position, Zheng says.
Is there any chance that the supermassive black hole might hurl a
hypervelocity star in Earth's direction one day?
Not really, Zheng says.
-
First, astrophysicists estimate
only one hypervelocity star is launched every 100,000 years.
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Second, possible trajectories of
stars near the supermassive black hole don't forebode any
danger, should any of them become a hypervelocity star in
the future.
Collaborating
Institutions and Funding
Zheng conducted the study with researchers from,
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Rensselaer Polytechnic
Institute, Troy, N.Y.
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National Optical Astronomy
Observatory, Tucson, Ariz.
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National Astronomical
Observatories, Chinese Academy of Sciences, Beijing and
Nanjing
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Spitzer Science Center,
Pasadena, Calif.
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University of California
Observatories-Lick Observatory, University of California,
Santa Cruz
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Georgia State University,
Atlanta
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Fermi National Accelerator
Laboratory, Batavia, Ill.
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University of Science and
Technology of China, Hefei
The study was funded by,
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