FREE ESSAY ON BLACK HOLES

BLACK HOLES

Black Holes
There are many strange and wonderful phenomenons being discovered throughout our
Universe. One of the most intriguing is the concept of a black hole in space. Astronomers
have discovered a black hole just 1,600 light years away from Earth. The National Radio
Astronomy Observatory cataloged this black hole in the constellation Sagittari on a star
called V4641. 
A black hole is one form of a dead star. A star has three choices when it dies, it can:
shrink until it is a white dwarf, shrink until it is a neutron star, or keep on shrinking
until it is a point in space with an infinite density known as a black hole. A black hole
is an extremely dense outer space body that has been theorized to exist in the universe.
The gravitational field of a black hole is so strong that, if the body is large enough,
nothing, including electromagnetic radiation, can escape from its area. The body is
surrounded by a spherical boundary, called a horizon, through which light can enter but
not escape; it therefore appears totally black. 
The idea of a mass concentration so dense that even light would be trapped goes all the
way back to Laplace in the 18th century. Almost immediately after Einstein developed
general relativity, Karl Schwarzschild discovered a mathematical solution to the
equations of the theory that described such an object. The radius of the horizon of a
Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi)
times the mass of the body in solar units (the mass of the body divided by the mass of
the sun). If a body is electrically charged or rotating, Schwarzschild's results are
modified.
According to general relativity, gravitation severely modifies space and time near a
black hole. As the horizon is approached from outside, time slows down relative to that
of distant observers, stopping completely on the horizon. Once a body has contracted
within its Schwarzschild radius (radius of the horizon), it would theoretically collapse
to a dimensionless object of infinite density.
Although Schwarzschild discovered black holes, it wasn't until much later with the work
of people such as Oppenheimer, Volkoff, and Snyder in the 1930's, that people thought
seriously about the possibility that such objects might actually exist in the Universe.
These researchers showed that when a sufficiently massive star runs out of fuel, it is
unable to support itself against its own gravitational pull, and it should collapse into
a black hole. 
In general relativity, gravity is a manifestation of the curvature of spacetime. Massive
objects distort space and time, so that the usual rules of geometry don't apply anymore.
Near a black hole, this distortion of space is extremely severe and causes black holes to
have some very strange properties. In particular, a black hole has something called an
'event horizon.' This is a spherical surface that marks the boundary of the black hole.
You can pass in through the horizon, but you can't get back out. In fact, once you've
crossed the horizon, you're doomed to move inexorably closer and closer to the
'singularity' at the center of the black hole. 
You can think of the horizon as the place where the escape velocity equals the velocity
of light. Outside of the horizon, the escape velocity is less than the speed of light, so
if you fire your rockets hard enough, you can give yourself enough energy to get away.
But if you find yourself inside the horizon, then no matter how powerful your rockets
are, you can't escape. 
The horizon has some very strange geometrical properties. To an observer who is sitting
still somewhere far away from the black hole, the horizon seems to be a nice, static,
unmoving spherical surface. But once you get close to the horizon, you realize that it
has a very large velocity. In fact, it is moving outward at the speed of light! That
explains why it is easy to cross the horizon in the inward direction, but impossible to
get back out. Since the horizon is moving out at the speed of light, in order to escape
back across it, you would have to travel faster than light. You can't go faster than
light, and so you can't escape from the black hole. 
Scientists had no proof that black holes existed until 1994, when the Hubble Space
Telescope uncovered the first convincing evidence that black holes exist in Galaxy M87.
Second and third black holes were discovered in 1995 in Galaxy NGC 4258 and NGC 4261.
Incidentally, the name 'black hole' was invented by John Archibald Wheeler, and seems to
have stuck because it was much catchier than previous names. Before Wheeler came along,
these objects were often referred to as 'frozen stars.'