Black Holes
Black holes are objects so dense that not even light can escape their gravity,
and since nothing can travel faster than light, nothing can escape from inside a
black hole . Loosely speaking, a black hole is a region of space that has so
much mass concentrated in it that there is no way for a nearby object to escape
its gravitational pull. Since our best theory of gravity at the moment is

Einstein\'s general theory of relativity, we have to delve into some results of
this theory to understand black holes in detail, by thinking about gravity under
fairly simple circumstances. Suppose that you are standing on the surface of a
planet. You throw a rock straight up into the air. Assuming you don\'t throw it
too hard, it will rise for a while, but eventually the acceleration due to the
planet\'s gravity will make it start to fall down again. If you threw the rock
hard enough, though, you could make it escape the planet\'s gravity entirely. It
would keep on rising forever. The speed with which you need to throw the rock in
order that it just barely escapes the planet\'s gravity is called the
"escape velocity." As you would expect, the escape velocity depends on
the mass of the planet: if the planet is extremely massive, then its gravity is
very strong, and the escape velocity is high. A lighter planet would have a
smaller escape velocity. The escape velocity also depends on how far you are
from the planet\'s center: the closer you are, the higher the escape velocity.

The Earth\'s escape velocity is 11.2 kilometers per second (about 25,000 M.P.H.),
while the Moon\'s is only 2.4 kilometers per second (about 5300 M.P.H.).We cannot
see it, but radiation is emitted by any matter that gets swallowed by black hole
in the form of X-rays. Matter usually orbits a black hole before being
swallowed. The matter spins very fast and with other matter forms an accretion
disk of rapidly spinning matter. This accretion disk heats up through friction
to such high temperatures that it emits X-rays. And also there is some X-ray
sources which have all the properties described above. Unfortunately it is
impossible to distinguish between a black hole and a neutron star unless we can
prove that the mass of the unseen component is too great for a neutron star.

Strong evidence was found by Royal Greenwich Observatory astronomers that one of
these sources called Cyg X-1 (which means the first X-ray source discovered in
the constellation of Cygnus) does indeed contain a black hole. It is possible
there for a star to be swallowed by the black hole. The pull of gravity on such
a star will be so strong as to break it up into its component atoms, and throw
them out at high speed in all directions. Astronomers have found a half-dozen or
so binary star systems (two stars orbiting each other) where one of the stars is
invisible, yet must be there since it pulls with enough gravitational force on
the other visible star to make that star orbit around their common center of
gravity and the mass of the invisible star is considerably greater than 3 to 5
solar masses. Therefore these invisible stars are thought to be good candidate
black holes. There is also evidence that super-massive black holes (about 1
billion solar masses) exist at the centers of many galaxies and quasars. In this
latter case other explanations of the output of energy by quasars are not as
good as the explanation using a super-massive black hole. A black hole is formed
when a star of more than 5 solar masses runs out of energy fuel, and the outer
layers of gas is thrown out in a supernova explosion. The core of the star
collapses to a super dense neutron star or a Black Hole where even the atomic
nuclei are squeezed together. The energy density goes to infinity. For a Black

Hole, the radius becomes smaller than the Schwarzschild radius, which defines
the horizon of the Black Hole: The death explosion of a massive star, resulting
in a sharp increase in brightness followed by a gradual fading. At peak light
output, supernova explosions can outshine a galaxy. The outer layers of the
exploding star are blasted out in a radioactive cloud. This expanding cloud,
visible long after the initial explosion fades from view, forms a supernova
remnant. So, a black hole is an object, which is so compact that the escape
velocity from its surface is greater than the speed of light. The