Black holes
BLACK HOLES MAY BE DEFINED AS –
The region of space-time exhibiting tremendous gravitational-acceleration so strong that nothing,no matter ,no particles nor even electro-magnetic radiation such as light cannot escape from it.
¡Black holes seem like mystical objects ,but they are an integral part of current astrophysics and are at the centre of attempts to unify quantum physics and general relativity. Humankind has been very curious to know better and more about black holes ,since the objects whoe gravitational fields are too strong for light to escape were first considered in the 18th century by by John Michell and Pierre Simon Laplace.
History of Black Holes
The idea of a body so massive that even light could not escape was briefly proposed by astronomical pioneer and English clergyman John Michell in a letter published in November 1784. Michell's simplistic calculations assumed such a body might have the same density as the Sun, and concluded that such a body would form when a star's diameter exceeds the Sun's by a factor of 500, and the surface escape velocity exceeds the usual speed of light. Michell correctly noted that such supermassive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies. Scholars of the time were initially excited by the proposal that giant but invisible stars might be hiding in plain view, but enthusiasm dampened when the wavelike nature of light became apparent in the early nineteenth century.
If light were a wave rather than a "corpuscle", it is unclear what, if any, influence gravity would have on escaping light waves. Modern physics discredits Michell's notion of a light ray shooting directly from the surface of a supermassive star, being slowed down by the star's gravity, stopping, and then free-falling back to the star's surface.
How are Black Holes foremed ?
When some giant stars burn out,there is nothing to hold them up and they start to collapse under the force of their own gravity. As they shrink , they get denser and denser , and their gravity becomes stronger and stronger ……and so they get denser and their gravity becomes still stronger . Eventually , they shrink to nothing …..and if one goes on shrinking creates a black hole
Structure of a Black Hole
Types of Black Holes
Based on rotation there are 2 types of black holes they are as follows :
¡Karl Schwarzschild-Non-rotating black hole :
In this black hole the core does not turn.It has only singularity and event horizon.
¡Kerr-Rotating black hole :
This black hole rotates because the star from which it developed was rotating. It has singularity , event horizon and ergosphere.¡Based on mass there are 3 types of black holes these are as follows :
¡Miniature black holes :
These are tiniest black holes as small as just one atom , however have the mass of a large mountain.
¡Stellar black holes :
There mass could be 20 times greater than the mass of the sun
¡Supermassive black holes :
These black holes have mass greater than 1 million suns . Scientists have located evidence that
every huge galaxy has a supermessive black hole at it’s centre.
Reasearchers think supermassive black holes were made as the same time as the galaxy
they are in .
Laws and theories by scientists
 | |
up left: Karl Schwarzschild; up right: Stephen Hawwkings;down right:Albert Einstein;down left:Subrahmanyan Chandrasekhar |
Hawking Radiation:
Hawking radiation is black-body radiation that is predicted to be
released by black holes, due to quantum effects near the event horizon. It
is named after the theoretical physicist Stephen Hawking, who provided
a theoretical argument for its existence in 1974. Hawking radiation reduces
the mass and rotation energy of black holes and is therefore also known
as black hole evaporation. Because of this, black holes that do not gain mass
through other means are expected to shrink and ultimately vanish. Micro
black holes are predicted to be larger emitters of radiation than larger black
holes and should shrink and dissipate faster.
General theory of Reletivity:
GENERAL REALATIVITY first predicted the existance of black
holes mathematically . It tells that the enough dense and massive object form
a black hole. actually the dense objects wrap or bend the space time pabrics so they have the great gravitational pull . It tells that mass or the or the density is responsible for the black holes . Density is directly proportional to the gravitational pull .
SCHWARZCHILD RADIUS :
Karl Schwarzschild gave the formula R=2GM/C^2 known as Sc
hwarzschild radius after solving the Einstein's Field equations whichis
also known as the modern solution of Einstein's field equations..
General relativity explains that any object can turn into a black hole
providing the condition that it should do massive or denser.. It means
that the huge mass should be compressed in very small volume of
space.. Then n then only it will form a black hole.. That's the key which
Schwarzschild Equations and
theories explain That formula[R=2GM/C^2] is used to calculate the
Schwarzschild radius of Any body.. And if we compress that object
into such body which will have the radius calculated with the help of
Schwarzschild radius then it will conformaly form a black hole..In this
formula C is Speed of light, M is mass of that body, R is Schwarzschild
radius and G is universal gravitational constant.
The Schwarzschild radius of earth is about 2 c.m.
approximately.. Actually when any mass is compressed in a very
compact volume or space, it warps or bends the space time fabric so
much.. Which results in increment in Gravitational pull or force of that
object tremendously.. That's the simple explaination behind Black
holes tremendous Gravitational pull it Schwarzschild radius as well!!!
CHANDRASEKHAR LIMIT :
There is a limit to the repulsion that the exclusion
principle can provide. The theory of relativity limits the maximum
difference in the velocities of the matter particles in the star to the
speed of light. This meant that when the star got sufficiently dense,
the repulsion caused by the exclusion principle would be less than the
attraction of gravity. A cold star of more than about one and half
times the mass of the sun would not be able to support itself against
its own gravity. This mass is known as Chandrasekhar limit.
by:- Advay Kedar Kulkarni
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