- 4562 -
BLACKHOLE - closest ever found? Astronomers have discovered the closest
massive black hole to Earth ever seen, a cosmic titan "frozen in
time." It is an elusive
"intermediate-mass black hole," that could serve as a missing link in
understanding the connection between stellar mass and supermassive black holes.
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----------------------------------- 4562 -
BLACKHOLE - closest ever found?
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- The black hole appears to have a mass of
around 8,200 suns, which makes it considerably more massive than stellar-mass
black holes with masses between 5 and 100 times that of the sun, and much less
massive than aptly named supermassive black holes, which have mass millions to
billions that of the sun. The closest stellar-mass black hole scientists have
found is called “Gaia-BH1”, and it sits only 1,560 light-years away from us.
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- The newly found intermediate-mass black hole
dwells in a spectacular collection of about ten million stars called Omega
Centauri, which sits around 18,000 light-years from Earth. The fact that the "frozen" black
hole appears to have stunted its growth supports the idea that Omega Centauri
is the remains of an ancient galaxy cannibalized by our own galaxy.
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- This would suggest Omega Centauri is
actually the core of a small, separate galaxy whose evolution was cut short
when the Milky Way swallowed it. If this event had never happened, this
intermediate black hole may have possibly grown to supermassive status like the
Milky Way's own supermassive black hole, Sagittarius A* (Sgr A*), which has a
mass 4.3 million times that of the sun and is located is 27,000 light-years
from Earth.
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- Stellar-mass black holes are known to form
via the collapse of stars with at least eight times the mass of the sun,
supermassive black holes must have a different origin. That's because no star
is massive enough to collapse and leave a remnant millions of times as massive
as the sun.
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- Scientists propose that supermassive black
holes are born and grow due to merger chains of progressively larger and larger
black holes. This has been evidenced by the detection of ripples in spacetime,
called “gravitational waves”, emanating from black hole mergers.
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- This process of black hole mergers and
growth, combined with the vast gap in mass between stellar-mass black holes and
supermassive black holes, means there should be a population of mid-size black
holes. Yet, these intermediate-mass
black holes with masses between a few hundred and a few thousand times that of
the sun have seem to have avoided detection. That's because, like all black
holes, these mid-sized cosmic titans are marked by outer boundaries called
“event horizons”.
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- The event horizon is the point at which the
gravitational influence of a black hole becomes so immense that not even light
is fast enough to escape it. Thus, black holes are only visible in light if
they are either surrounded by matter to feed on, which glows while heating up,
or rip apart and feed on an unfortunate star in a so-called "Tidal
Disruption Event" (TDE).
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- Intermediate black holes, like the one in
Omega Centauri, aren't surrounded by a lot of matter and feeding. That means astronomers have to be a little
bit cunning when hunting for such black holes. They use the gravitational
effects these voids have on matter, like stars that orbit them or on light
passing through them.
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- The researchers wanted to find rapidly
moving stars in Omega Centauri that would prove the star cluster has a massive,
dense or compact "central engine" black hole. A similar method was
used to determine the mass and size of Sgr A* using a fast-moving population of
stars at the heart of the Milky Way.
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- They used over 500 Hubble images of this
star cluster to build a vast database of the motions of stars in Omega
Centauri, measuring the speeds of about 1.4 million stars. This ever-repeating
view of Omega Centauri, which Hubble conducted not out of scientific interest
but rather to calibrate its instruments, was the ideal data set for the team's
mission.
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- Looking for high-speed stars and documenting
their motion was the proverbial search for a needle in a haystack. The team ultimately found not one but seven
"needle-in-haystack stars," all moving at rapid velocities in a small
region at the heart of Omega Centauri.
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- The rapid speed of these stars is caused by
a concentrated mass nearby. If the team had only found one rapid star, it would
have been impossible to determine whether its speed was the result of a large
and close central mass or if that star is a runaway moving at a rapid pace in a
straight path absent of any central mass.
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- Spotting and measuring the different
velocities and directions of seven stars allowed this determination to be made.
The measurements revealed a centralized mass equivalent to 8,200 suns, while
visual inspections of the region revealed no objects that resemble stars. That
is exactly what would be expected if a black hole was located in this region,
which the team determined to be "light-months" wide.
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- The fact that our galaxy has matured enough
to have grown a supermassive black hole at its heart means it has probably
outgrown the stage of possessing many intermediate-mass black holes of its own.
This one exists in the Milky Way because the cannibalization of its original
galaxy happened to curtail its growth processes.
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- Previous studies had prompted critical
questions of 'So where are the high-speed stars?' We now have an answer to that
and the confirmation that Omega Centauri contains an intermediate-mass black
hole. At a distance of about 18,000
light-years, this is the closest known example of a massive black hole.
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- Of course, that doesn't really change the
status of Sgr A* as the closest supermassive black hole to Earth, or Gaia BH1's
status of the closest stellar-mass black hole to Earth, but it provides some reassurance that
scientists are on the right track when considering how our central black hole
became such a cosmic titan in the first place.
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September 23, 2024 BLACKHOLE -
closest ever found? 4562
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--------------------- --- Monday, September 23,
2024
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