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Mid-mass Black Holes
There is strong evidence for two types of black holes: stellar black holes with
masses of a dozen or so Suns, and supermassive black holes with masses of many
millions of Suns. Stellar black holes are formed as a natural consequence of the
evolution of massive stars. The origin of supermassive black holes is a mystery.
They are found only in the centers of galaxies. It is not known whether they
formed in the initial collapse of the gas cloud that formed the galaxy, or from
the gradual growth of a stellar mass black hole, or from the merger of a
centrally located cluster of black holes, or by some other mechanism.
Radiation
Pressure Schematic
When matter is pulled toward a black hole,
it is heated and produces X-rays. These
X-rays create a radiation pressure which
pushes out on the matter. If the matter
continues to fall in,the radiation pressure
of the X-rays must be less than the pull of
the black hole's gravity. This effect, called
the Eddington limit, enables astronomers to
estimate the mass of a black hole.
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The mass of a stellar black hole can be deduced by observing
the orbital acceleration of a star as it orbits its unseen companion. Likewise,
the mass of a supermassive black hole can be determined by using the orbital
acceleration of gas clouds swirling around the central black hole. When orbital
acceleration cannot be used to establish the mass of a black hole, astronomers
can place a lower limit on its mass by measuring the X-ray luminosity due to
matter falling into a black hole. The pressure of the outflowing X-rays must be
less than the pull of the black hole's gravity on the inflowing matter.
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