|Supermassive Black Hole in the Milky Way Galaxy
Largest Type of Black Hole in a Galaxy
From a distance, our galaxy would look like a flat
spiral, some 100,000 light years across, with pockets of gas, clouds of
dust, and about 400 billion stars rotating around the galaxys center.
Thick dust and blinding starlight have long obscured our vision into the
mysterious inner regions of the galactic center.
And yet, the clues
have been piling up, that something important, something strange is
going on in there. Astronomers tracking stars in the center of the
galaxy have found the best proof to date that black holes exist. Now,
they are shooting for the first direct image of a black hole.
A supermassive black hole is the
largest type of black hole in a galaxy, on the order of hundreds of
thousands to billions of solar masses.
Most, if not all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.
Supermassive black holes have properties which distinguish them from lower-mass classifications:
The average density of a supermassive black hole
(defined as the mass of the black hole divided by the volume within its
Schwarzschild radius) can be very low, and may actually be lower than
the density of air.
This is because the Schwarzschild radius is directly proportional to
mass, while density is inversely proportional to the volume.
Since the volume of a spherical object (such as the event horizon of a
non-rotating black hole) is directly proportional to the cube of the
radius, and mass merely increases linearly, the volume increases by a
much greater factor than the mass as a black hole grows. Thus, average
density decreases for increasingly larger radii of black holes (due to
volume increasing much faster than mass).
The tidal forces in the vicinity of the event horizon are
significantly weaker. Since the central singularity is so far away from
the horizon, a hypothetical astronaut traveling towards the black hole
center would not experience significant tidal force until very deep into
the black hole.
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are many models for the formation of black holes of this size. The most
obvious is by slow accretion of matter starting from a black hole of
Another model of supermassive black hole formation involves a large gas
cloud collapsing into a relativistic star of perhaps a hundred thousand
solar masses or larger.
The star would then become unstable to radial perturbations due to
electron-positron pair production in its core, and may collapse directly
into a black hole without a supernova explosion, which would eject most
of its mass and prevent it from leaving a supermassive black hole as a
Yet another model involves a dense stellar cluster undergoing
core-collapse as the negative heat capacity of the system drives the
velocity dispersion in the core to relativistic speeds.
Finally, primordial black holes may have
been produced directly from external pressure in the first instants
after the Big Bang. The difficulty in forming a supermassive black hole
resides in the need for enough matter to be in a small enough volume.
This matter needs to have very little angular momentum in order for this
|Black Hole Information Paradox
The black hole information paradox results from the combination of
quantum mechanics and general relativity. It suggests that physical
information could disappear in a black hole, allowing many physical
states to evolve into the same state.
This is a contentious subject since it violates a commonly assumed tenet
of science—that in principle complete information about a physical
system at one point in time should determine its state at any other
A postulate of quantum mechanics is that complete information about a
system is encoded in its wave function, an abstract concept not present
in classical physics. The evolution of the wave function is determined
by a unitary operator, and unitarity implies that information is
conserved in the quantum sense.
There are two things to keep in mind here: quantum determinism, and
reversibility. Quantum determinism means that given a present wave
function, its future changes are uniquely determined by the evolution
operator. Reversibility refers to the fact that the evolution operator
has an inverse, meaning that the past wave functions are similarly
With quantum determinism, reversibility, and a conserved Liouville
measure, the von Neumann entropy ought to be conserved, if coarse
graining is ignored.
Stephen Hawking presented rigorous theoretical arguments based on
general relativity and thermodynamics which threatened to undermine
these ideas about information conservation in the quantum realm. Several
proposals have been put forth to resolve this paradox.
Supermassive Black Holes
black hole is a black hole with a mass in the range of hundreds of
thousands to tens of billions of solar masses.
It is currently thought
that most, if not all galaxies, including the Milky Way, contain a
supermassive black hole at their galactic center.
the process of accretion involves transporting a large initial
endowment of angular momentum outwards, and this appears to be the
limiting factor in black hole growth, and explains the formation of
Currently, there appears to be a gap in the observed mass distribution of black holes.
There are stellar-mass black
holes, generated from collapsing stars, which range up to perhaps 33
solar masses. The minimal supermassive black hole is in the range of a
hundred thousand solar masses.
Between these regimes there
appears to be a dearth of intermediate-mass black holes. Such a gap
would suggest qualitatively different formation processes. However, some
models suggest that ultraluminous X-ray sources (ULXs) may be black
holes from this missing group.
It is now widely accepted that
the center of nearly every galaxy contains a supermassive black hole.
The close observational correlation between the mass of this hole and
the velocity dispersion of the host galaxy's bulge, known as the M-sigma
relation, strongly suggests a connection between the formation of the
black hole and the galaxy itself.
explanation for this correlation remains an unsolved problem in
astrophysics. It is believed that black holes and their host galaxies
coevolved between 300-800 million years after the Big Bang, passing
through a quasar phase and developing correlated characteristics, but
models differ on the causality of whether black holes triggered galaxy
formation or vice versa, and sequential formation cannot be excluded.
The unknown nature of dark
matter is a crucial variable in these models. At least one galaxy,
Galaxy 0402+379, appears to have two supermassive black holes at its
center, forming a binary system.
If they collide, the event would create strong gravitational waves.
Binary supermassive black holes are believed to be a common consequence
of galactic mergers.
As of November 2008, another
binary pair, in OJ 287, contains the most massive black hole known, with
a mass estimated at 18 billion solar masses. Currently, there is no
compelling evidence for massive black holes at the centers of globular
clusters, or smaller stellar systems.