A simple explanation for the Formation of black holee

Understanding the formation of black Hole

To understand how a black hole might be formed, we first need an understanding of the life cycle of a star. A star is formed when a large amount of gas, mostly hydrogen, starts to collapse in on itself due to its gravitational attraction and finally fuse together to form helium. The heat released in this reaction makes the star shine. It is this heat which in-creases the pressure of the gas until it is sufficient to balance the gravitational attraction and the star stops contracting further. Stars will remain stable like this for a long time, with heat from the nuclear reactions balancing the gravitational attraction. Eventually, however, the star will run out of its hydrogen and other nuclear fuels.

The Role of Exclusion principle

After all the fuel for nuclear reaction has been used up, gravity meets a fresh opponent, the exclusion principle. All particles of ordinary matter obey this principle. The exclusion principle says that no more than two electrons can occupy the same region of space at the same time. Electrons are paired together in their orbits and protest their confinements in these orbits by moving erratically, shaking, flying around, and kicking forcefully against adjacent electrons. This motion is called degenerate motion, and the pressure it produces is electron degeneracy pressure. This pressure keeps electrons form being pulled in to the nucleus of the
atom. As a star collapses, the clouds of electrons around the nuclei of the atoms in the star get squashed until the electrons are confined in cells many times smaller than the usual . In thIs situation e electrons ehave like a wave. Due to high energy and fast motion of the waves larger electron degeneracy pressure is being produced and this pressure that will continue to support the star against gravity. As the pull of gravity increases, so would the pressure opposing it. There is, however, a hitch. Our universe has a speed limit that for all practical purposes seems to be unbreakable. That speed limit is the speed of light, approximately 300,000 kmlsec. Degenerate electrons cannot move faster than the speed of light But even short of that, when matter is so dense that degenerate electrons move at near the speed of light, matter has serious difficulty supporting itself against the squeeze of gravity. So the pressure due to the exclusion principle fails and the star starts contracting further.

As a star ends up its life much of its mass is thrown of in supernova explosion or in planetary nebula, what remains after this process is called the stellar remnants. The stellar remnant formation takes place in three possible ways depending on the initial masses of the star. Some stars may end up their life, as white dwarfs while others collapse to neutron stars
and still others are doomed to crunch all the way down to a black hole.

The Role of Chandrasekhar Limit

In the late 1920s Subrahmanyan Chandrasekhar, a young Indian physicist calculated that if a star's mass is less than 1.4 times the mass of our sun, gravity will not be able to overpower this exclusion principle repulsion among the electrons. The star shrinks and becomes a white dwarf.
A white dwarf is supported by the exclusion principle repulsion between the electrons in its matter. Only in a star whose mass is more than the Chandrasekhar limit will overcome the exclusion principle among the electrons and be the victor in this second competition.

There is another possible final state for a star with mass slightly larger than the Chandrasekhar limit. These stars would be supported by the exclusion principle repulsion between neutrons and protons, rather than between electrons. They are therefore called neutron stars. When gravity has overpowered the exclusion principle repulsion among the
electrons, as the star squashes down, electrons are squeezed into the atomic nuclei and combine with protons in the nuclei to form additional neutrons. After a while the core of the star is almost entirely made of neutrons. Neutrons must obey the exclusion principle as surely as electrons must. The resistance to squeezing due to degeneracy pressure and the strong
nuclear force is stronger than it was previously among the electrons which
present gravity with an even more formidable opponent.

Stars with masses much higher than the Chandrasekhar limit, on the other hand, have a big problem when they come to the end of their fuel. In some cases they may explode or manage to throw off enough matter to reduce their mass below the limit and so avoid catastrophic gravitational collapse, but it is difficult to believe that this always happened, no matter how big the star.9 Chandrasekhar had shown that the exclusion principle could not halt the collapse of a star more massive than the Chandrasekhar limit. 10 In 1939 by the theoretical invention of Black holes with the help of general relativity a young American scientist, Robert Oppenheimer first solved the problem of understanding what would happen to such a star. A star may lose a considerable amount of mass in a late-in life explosion. However, if it doesn't lose enough to bring it below the maximum allowed mass for neutron stars gravity will overpower both the
exclusion principle repulsion among the neutrons and the strong nuclear force, and the star will continue to collapse and form a black hole.
General Theory of Relativity as the Theoretical Foundation
of Black Holes

The General theory of Relativity is the prominent theoretical foundation to black holes.
The general theory of relativity describes gravity entirely in terms
of the geometry of space and time. This theory describes how space and
time are distorted by the presence of massive bodies. The massive bodies
in the universe cause the cosmic fabric to become distorted and the objects
travelling through flat regions of space-time continue along straight-line
paths while those traveling through curved regions move in curved
trajectories. The gravitational field of the star changes the paths of light rays in
space-time. The light cones, which indicate the paths followed in space
and time by flashes of light emitted from their tips, are bent slightly inward
near the surface of the star. As the star contracts, the gravitational field at
its surface gets stronger and the light cones get bent inward more. This
makes it more difficult for light from the star to escape, and the light
appears dimmer and redder to an observer at a distance. Eventually, when
the star has shrunk to a certain critical radius, the gravitational field at the
surface becomes so strong that the light cones are bent inward so much
that light can no longer escape. This effect due to which even light cannot
escape the gravity of the star makes it invisible to us, thus appearing
invisible to us.
According to the theory of relativity, nothing can travel faster than
light. Thus if light cannot escape, neither can anything else; everything is
dragged back by the gravitational field. So one has a set of events, a region
of space-time, from which it is not possible to escape to reach a distant
observer. If we could view the vicinity of a collapsing star, as it becomes
a black hole, we would notice that the gravity from the star is increasing
as the collapse proceeds. At first the gravity is weak and the light is bent
very little. As the collapse proceeds the bending becomes greater and
greater until light is bent so much that it orbits the star. Nothing can move
faster than the speed of light, so when the gravity has increased to make
the escape velocity equal to the speed of light, nothing can escape. The
surface of the collapsing object at this moment is called the event horizon
and its radius is the Schwarzschild radius. I The collapse continues inside
the event horizon, but observers on the outside can never see it. The
object collapses into a mathematical point called the singularity
black hole space-time is severely distorted and time itself is affected. As
measured by an outside observer, time stops on the event horizon. Because
the original object is no longer accessible from our universe, we cannot
know very much about a black hole.
There are some special characteristic features those are specific to
black holes. According to the General relativity, there must be a singularity
of infinite density and space-time curvature within a black hole. At this
singularity the laws of science and our ability to predict the future would
break down. Another important feature is the event horizon, or surface,
that surrounds a black hole. Inside a black hole, a powerful gravity distorts
the structure of space time so severely that the directions of space and
time become interchanged. Just as no force in the universe can prevent
the forward march of time from past to future outside a black hole, no
force in the universe can prevent the inward march of space from event
horizon to singularity inside a black hole. These characteristic features
of black holes put forward many challenges to the existing theories of
physics. Because of these challenges black holes where always remained
as a mystery or an object for constant new discoveries. Earlier scientists
believed that what ever goes inside a black hole never come back. But
today new theories have been developed saying that what ever goes inside
a black hole will be radiated back

I have taken this article from Indian General of science and religion. It is written by Shibhu joseph


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