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| Artist's conception of black hole in globular cluster CREDIT: Benjamin de Bivort; Strader, et al.; NRAO/AUI/NSF |
Charlottesville, Virginia: In an unexpected finding,
astronomers recently discovered two black holes while searching for a unique black hole in a tight cluster of stars 10,000 light years away from Earth. Instead of finding one
black hole, scientists with the National Radio Astronomy Observatory (NRAO) found two
– twins – something that surprised them because, according to modern theory,
there should only be one black hole in a cluster.
The discovery could
make scientists reconsider their current understanding of the environment in globular star clusters.
The astronomers made
the find while observing Messier
22 (M22), a globular star cluster containing hundreds of thousands of
stars, with the Very
Large Array (VLA) radio telescope located in New Mexico. They were
searching for proof of a rare black hole known as an intermediate-mass black hole.
Unlike the supermassive
black holes found at the center of galaxies, an intermediate-mass black hole has 10- to
several tens of times more mass than the Sun, but is comparatively smaller in size.
“We didn’t find what
we were looking for, but instead found something very surprising – two smaller
black holes,” said Laura Chomiuk of Michigan
State University and the National Radio Astronomy Observatory.
Black holes are
concentrations of mass so dense that nothing, not even light, can escape
them. The NRAO astronomers think most black holes found within the
globular cluster were likely produced early in the cluster’s 12-billion-year
history after massive
stars exploded
as supernovae.
Past scientific
simulations suggest black holes fall toward the center of the cluster,
beginning a violent gravitational competition with each other. In the
end, only a single black hole remains in the cluster.
“There is supposed to
be only one survivor possible,” said Jay Strader of Michigan State University
and the Harvard-Smithsonian
Center for Astrophysics. “Finding two black holes, instead of one in this
globular cluster, definitely changes the picture.”One possible explanation is
that the black holes may still be working to enlarge the center of the star
cluster, reducing its density and slowing down the rate at which the black
holes eject each other as they compete to become the cluster’s lone entity.
On the other hand, the
astronomers think the reduction of the black hole’s density and ejection rate
could also be due to the fact that the star cluster is not as far along in its
process of contracting as previously thought.
“Future VLA
observations will help us learn about the ultimate fate of black holes in
globular clusters,” Chomiuk said.
A paper on this finding was just published in the
journal “Nature”.
According the NARO,
these twin black holes are also the first stellar-mass black holes to be found in any globular cluster in our Milky Way Galaxy. A stellar-mass black hole is formed by
the collapse of a star that has a mass that is generally 10 to 24 times that of
the Sun, as compared to the giant “supermassive” black holes, which are
millions, if not billions, of times as massive as the Sun.
Astronomers believe
supermassive black holes lie at the center of virtually all large galaxies,
including ours.