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Mid-mass Black Holes
This technique was used to estimate the mass of a black hole discovered in a
dense star cluster about 600 light years from the center of the galaxy M82. Astronomers found that the mass of the
black hole must be greater than 500 Suns. This is much more massive than known
stellar black holes, and much smaller than supermassive black holes, so it is
called a "mid-mass" black hole.
Astrophysicists had come to believe that galactic
centers were the only places where conditions were right for the formation
and growth of large or very large black holes. The discovery of a large, mid-mass
black hole located away from the galaxy's center, shows that somehow large black
holes can also form in dense star clusters.
Current possible explanations for the formation of mid-mass black holes include
the mergers of scores of stellar black holes, or the collapse of a superstar.
Another possibility is that the X-rays produced by infall into the black hole are
beamed toward the Earth. This would reduce the overall output of X-rays from the
source, and reduce the mass needed to overcome the pressure of the outflowing
X-rays to a value consistent with stellar mass black holes.
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