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A
black hole is a region of space-time from which
nothing can escape, even light.
{Please
scroll down the page for more information and
related links.}
To
see why this happens, imagine throwing a tennis
ball into the air. The harder you throw the
tennis ball, the faster it is traveling when it
leaves your hand and the higher the ball will go
before turning back. If you throw it hard enough
it will never return, the gravitational
attraction will not be able to pull it back
down. The velocity the ball must have to escape
is known as the escape velocity
and for the earth is about 7 miles a second.
As
a body is crushed into a smaller and smaller
volume, the gravitational attraction increases,
and hence the escape velocity gets bigger.
Things have to be thrown harder and harder to
escape. Eventually a point is reached when even
light, which travels at 186  thousand
miles a second, is not traveling fast enough to
escape. At this point, nothing can get out as
nothing can travel faster than light. This is a
black hole.
Do
they really exist?
It
is impossible to see a black hole directly
because no light can escape from them; they are
black. But there are good reasons to think they
exist.
When
a large star has burnt all its fuel it explodes
into a supernova. The stuff that is left
collapses down to an extremely dense object
known as a neutron star. We know that these
objects exist because several have been found
using radio telescopes.
If
the neutron star is too large, the gravitational
forces overwhelm the pressure gradients and
collapse cannot be halted. The neutron star
continues to shrink until it finally becomes a
black hole. This mass limit is only a couple of
solar masses, that is about twice the mass of
our sun, and so we should expect at least a few
neutron stars to have this mass. (Our sun is not
particularly large; in fact it is quite small.)
A
supernova occurs in our galaxy once every 300
years, and in neighboring galaxies about 500
neutron stars have been identified. Therefore we
are quite confident that there should also be
some black holes.
Black
Hole in NGC4261
This
Hubble Space Telescope image contains three main
features. 
The
outer white area is the core or center of the
galaxy NGC4261.
Inside
the core there is a brown spiral-shaped disk. It
weighs on hundred thousand times as much as our
sun.
Because
it is rotating we can measure the radii and
speed of its constituents, and hence weigh the
object at its center. This object is about as
large as our solar system, but weighs
1,200,000,000 times as much as our sun.
This
means that gravity is about one million times as
strong as on the sun. Almost certainly this
object is a black hole.
Black
Hole in M87
M87
is an active galaxy, one in which we see
interesting obje cts. Near its core (or center)
there is a spiral-shaped disc of hot gas. The
first picture places it in context.
The second
superposes spectra from opposite sides. This allows us to determine the speed of rotation of
the disk and its size. From this we can weigh
the size of the invisible object at the center.
Although
the object is no bigger than our solar system it
weighs three billion times as much as the sun.
This means that gravity is so strong that light
cannot escape. We have a black hole.
Also
see Cosmology
(There is more below.
Please scroll down.)
History
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1687
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Sir
Isaac Newton
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Described
gravity in his publication,
"Principia."
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1783
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John
Michell
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Conjectured
that there might be an object massive
enough to have an escape velocity greater
than the speed
of light.
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1796
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Simon
Pierre LaPlace
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Predicted
the existence of black holes. "...[It]
is therefore possible that the largest
luminous bodies in the universe may,
through this cause, be invisible." --
Le Système du
Monde
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1915
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Albert
Einstein
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Published
the Theory of General Relativity, which
predicted spacetime
curvature.
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1916
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Karl
Schwarzchild
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Used
Einstein's Theory of General Relativity to
define a black hole. Defined gravitational
radius of black holes, later called the Schwarzchild
radius.
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1926
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Sir
Arthur Eddington
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Relativity
expert who, along with Einstein, opposed
black hole theory.
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1935
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Subrahmanyan
Chandrasekhar
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Pioneer
in theory of white
dwarfs that led to an understanding
of mass limits that decide whether a star
will die as a white dwarf, neutron star or
black hole.
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1964
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John
Wheeler
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Coined
the term, "black hole."
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1964
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Jocelyn
Bell-Burnell
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Discovered
neutron stars
that, at the time, were the densest matter
found through observations.
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1970
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Stephen
Hawking
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Defined
modern theory of black holes, which
describes the final fate of black holes.
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1970
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Cygnus
X - 1
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The
first good black hole candidate that
astronomers found. It emits x-rays and has
a companion smaller than Earth but with a
mass greater than that of a neutron star.
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1994
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Hubble
Space Telescope
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Provides
best evidence to date of supermassive
black holes that lurk in the center of
some galaxies. The Space Telescope Imaging
Spectrograph (STIS) revealed large
orbiting velocities around the nucleus of
these galaxies, suggesting a huge mass
inside a very small region.
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SOURCE-{damtp.cam.ac.uk
and NASA}
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