Hyper-velocity star found speeding through space at 1 million mph
Astronomers think hyper-velocity 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.
A University of Utah-led team say they have discovered a
hyper-velocity 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 hyper-velocity 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.
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. |
"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 centre. 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 and his colleagues discovered the new hyper-velocity 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 north-east of Beijing.
LAMOST boasts a 13.1-foot-wide aperture and houses 4,000 optical
fibres, 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 hyper-velocity 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."
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 centre, 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 five 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 travelling 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 centre of the galaxy - more than halfway out from the
centre of the visible disk.
By comparison, the new hypervelocity star is about 62,000 light years
or 364 quadrillion miles from the galactic centre, 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 hyper-velocity stars discovered so far," Zheng says.
- EarthSky
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