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| Black hole with
corona, X-ray source (artist's concept). |
Black Holes
A black hole is a region of space from which nothing can
escape, according to the general theory of relativity,
it is the result of the curving of spacetime caused by a
huge mass. Around a black hole there is a position of no
return, called the event horizon. It is called "black"
because it absorbs all the light that hits it,
reflecting nothing, just like a perfect black body in
thermodynamics.
Under the theory of quantum mechanics black holes have a
temperature and emit Hawking radiation, which makes them
slowly get smaller.
A black hole is found by its interaction with matter.
The presence of a black hole can be inferred by tracking
the movement of a group of stars that orbit a region in
space. Alternatively, when gas falls into a black hole
caused by a companion star or nebula, the gas spirals
inward, heating to very high temperatures and emitting
large amounts of radiation. This radiation can be
detected from earthbound and Earth-orbiting telescopes.
Astronomers have also found evidence of supermassive
black holes at the center of almost all galaxies. After
observing the motion of nearby stars for 16 years, in
2008 astronomers found compelling evidence that a
supermassive black hole of more than 4 million solar
masses is near the Sagittarius A* region in the center
of the Milky Way galaxy. Inside a black hole the rules
of physics are very different. |
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History
In 1783, an English clergyman called John Michell wrote
that it might be possible for something to be so heavy
you would have to go at the speed of light to get away
from its gravity. Gravity gets stronger as something
gets bigger or more massive. For a small thing, like a
rocket, to escape from a larger thing, like Earth, it
has to escape the pull of our gravity or it will fall
back. The speed that it must travel upward to get away
from Earth's gravity is called escape velocity. Bigger
planets (like Jupiter) and stars have more mass, and
have stronger gravity than Earth. Therefore, the escape
velocity is much faster. John Michell thought it was
possible for something to be so big that the escape
velocity would be faster than the speed of light, so
even light could not escape. In 1796, Pierre-Simon
Laplace promoted the same idea in the first and second
editions of his book Exposition du système du Monde (it
was removed from later editions).
Some scientists thought Michell might be right, but
others thought that light had no mass and would not be
pulled by gravity. His theory was forgotten.
In 1916 Albert Einstein wrote an explanation of gravity
called general relativity. |
- Mass causes space (and spacetime)
to bend, or curve. Moving things "fall along" or
follow the curves in space. This is what we call
gravity.
- Light always travels at the same
speed, and is affected by gravity. If it seems to
change speed, it is really traveling along a curve
in spacetime.
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A few months later, while serving in World War I, the
German physicist Karl Schwarzschild used Einstein's
equations to show that a black hole could exist. In
1930, Subrahmanyan Chandrasekhar predicted that stars
heavier than the sun could collapse when they ran out of
hydrogen or other nuclear fuels to burn. In 1939, Robert
Oppenheimer and H. Snyder calculated that a star would
have to be at least three times as massive as the Sun to
form a black hole. In 1967, John Wheeler invented the
name "black hole" for the first time. Before that, they
were called "dark stars".
In 1970, Stephen Hawking and Roger Penrose showed that
black holes must exist. Although the black holes are
invisible (they cannot be seen), some of the matter that
is falling into them is very bright.
As of spring 2019, there is an image of a black hole,
rather, the things orbiting the black hole, that was
produced by a large team. The image require many photos
from many different locations. One of the members being
Katie Bouman who made the compilation of all the images
into one singular image possible with her computer
science background. She claims minimal knowledge of the
subject matter prior her involvement in the team and she
is now a prominent figure in the progression of
understanding black holes. |
|
 |
| Simulation of
gravitational lensing by a black hole, which
distorts the image of a galaxy in the
background. |
Formation of black holes
Gravitational collapse
The gravitational collapse of huge (high-mass) stars
cause "stellar mass" black holes. Star formation in the
early universe may have resulted in very massive stars,
which on collapse would produce black holes of up to 103
solar masses. These black holes may be the seeds of the
supermassive black holes found in the centers of most
galaxies.
Most of the energy released in gravitational collapse is
emitted very quickly. A distant observer sees the
infalling material slow and halt just above the event
horizon, due to gravitational time dilation. The light
emitted just before the event horizon is delayed an
infinite amount of time. So the observer never sees the
formation of the event horizon. Instead, the collapsing
material seems to become dimmer and increasingly
red-shifted, eventually fading away.
Supermassive black holes
Black holes have also been found in the middle of almost
every galaxy in the known universe. These are called
supermassive black holes (SBH), and are the biggest
black holes of all. They formed when the Universe was
very young, and also helped to form all the galaxies.
Quasars are believed to be powered by gravity collecting
material into SBHs in the centers of distant galaxies.
Light cannot escape the SBHs at the center of quasars,
so the escaping energy is made outside the event horizon
by gravitational stresses and immense friction on the
incoming material.
Huge central masses (106 to 109 solar masses) have been
measured in quasars. Several dozen nearby large
galaxies, with no sign of a quasar nucleus, contain a
similar central black hole in their nuclei. Therefore,
it is thought that all large galaxies have one, but only
a small fraction are active (with enough accretion to
power radiation) and so are seen as quasars.
Effect on light
At the middle of a black hole, there is a gravitational
center called a singularity. It is impossible to see
into it because the gravity prevents any light escaping.
Around the tiny singularity, there is a large area where
light which would normally pass by gets sucked in as
well. The edge of this area is called the event horizon.
The area beyond the event horizon is the black hole. The
gravity of the black hole gets weaker at a distance. The
event horizon is the place farthest away from the middle
where the gravity is still strong enough to trap light.
Outside the event horizon, light and matter will still
be pulled toward the black hole. If a black hole is
surrounded by matter, the matter will form an "accretion
disk" (accretion means "gathering") around the black
hole. An accretion disk looks something like the rings
of Saturn. As it gets sucked in, the matter gets very
hot and shoots x-ray radiation into space. Think of this
as the water spinning around the hole before it falls
in.
Most black holes are too far away for us to see the
accretion disk and jet. The only way to know a black
hole is there is by seeing how stars, gas and light
behave around it. With a black hole nearby, even objects
as big as a star move in a different way, usually faster
than they would if the black hole was not there.
Since we cannot see black holes, they must be detected
by other means. When a black hole passes between us and
a source of light, the light bends around the black hole
creating a mirror image. That effect is called
gravitational lensing. |
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Hawking radiation
Hawking radiation is black body radiation which is
emitted by black hole, due to quantum effects near the
event horizon. It is named after the physicist Stephen
Hawking, who provided a theoretical argument for its
existence in 1974.
Hawking radiation reduces the mass and the energy of the
black hole and is therefore also known as black hole
evaporation. This happens because of the virtual
particle-antiparticle pairs. Due to quantum
fluctuations, this is when one of the particles falls in
and the other gets away with the energy/mass. Because of
this, black holes that lose more mass than they gain
through other means are expected to shrink and
ultimately vanish. Micro black holes (MBHs) are
predicted to be larger net emitters of radiation than
larger black holes and should shrink and dissipate
faster. |
|
 Kiddle:
Black Holes
Wikipedia: Black Holes |
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