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What is a Black Hole
In its simplest terms, black holes can be defined as a region in space created by the collapse of a massive star where the gravitational force is so enormous that even light rays can't escape from its grasp. The final stage in the life of stars varies depending on its initial mass. If it is more than the Chandrasekhar Limit, which is 1.4 solar masses, then those stars will end up as black holes. As the star ages the fusion reaction inside its core produces enormous pressure, which balances the gravitational pull due to its mass. But once it runs out of its fuel there is no counter force, only the gravitational force that will crush the star to form a black hole.
The collapse of such massive stars under their own gravitational pull results in a situation where the entire mass is concentrated in a small volume called a singularity, which is surrounded by the event horizon that we can see. Einstein's General Theory of Relativity explains a singularity as point where infinite space-time curvature exists. The event horizon marks the border of a black hole, as any object which passes through the event horizon ceases to exist from our point of view, as it is pulled into the back hole by the massive gravitational force.
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Three Types of Black Holes
Black holes are classified by their mass, magnetic field and spin. On the basis of mass, black holes are classified into three, namely, miniature, stellar and super-massive black holes.
1. Miniature Black Holes: It is believed to have formed during the Big Bang and there is no evidence to substantiate its possible existence during the evolution of the universe. The mass of such black holes would have been considerably smaller than that of the Sun and they only formed due to the extreme external pressures during the Big Bang that compressed the mass to create the singularity.
2. Stellar Black Holes: This type of black holes originates from the death of a massive star.
3. Supermassive Black Holes: These types are the biggest among the three. Scientists believe that these black holes, which occupy the center of a galaxy, increase in size as they pull in more and more material from the dense core of the galaxy. Unlike the outer parts of the galaxies, the inner region is crowded with stars, and this creates favorable conditions for the supermassive black holes to grow. The size of these black holes are in the order of hundred thousand solar masses.
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A Different Method of Classification
Black holes are also classified by their spin and magnetic field. This classification is based on Einstein's General Theory of Relativity.
1. Schwarzschild Black Hole: This the simplest among the three classes and has only a theoretical existence. It has neither magnetic field nor the spin. This type was proposed by Karl Schwarzschild and is characterized by a point of singularity and an event horizon.
2. Kerr Black Hole: This type of black hole posses both spin and magnetic field and is the most common type of black hole found in space. These uncharged black holes have a characteristic rotation about a central axis. Named after the scientist Roy Kerr, who proposed its existence, Kerr black holes have special features like ring singularity, two event horizons (one inner and one outer), ergo-sphere and a static limit. The Ergo-sphere is an ellipsoidal region outside the outer horizon, in which the space-time continuum rotates with the rotation of the black hole. This rotation of the space-time continuum is called Lense-Thiring Effect. Beyond the static limit, space is normal. Particles which move through the ergo-sphere have an opportunity to escape by gaining energy from the rotational energy created by the black hole's rotation. But once it crosses the event horizon there is no return journey. Another interesting fact about this type of black hole is the closed time like paths.
3.Reissner-Nordstrom Black Hole: Characterized by singularity and two event horizons this group of black holes show no rotation. Since all black holes identified have a rotation this group, too, has only a theoretical basis.
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In the absence of light from the black holes scientists have relied on indirect methods to detect its presence. The stellar objects which move closer to the event horizon start to rotate with it at a very high velocity and thus the gets heated up. This forms an accretion disk around each black hole which is visible. The binary stars, too, provide evidence for the existence of black holes. When one of them becomes a black hole, it will tear off parts of its companion star forming accretion disk around the back hole.
Quite a few super massive black holes have been detected. The black holes found in the galaxy M87 and OJ 287 in the constellation Cancer are examples of supermassive black holes. Cygnus X -1 is a stellar black hole found in the constellation Cygnus.
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