Based on what we know a black hole is an object in space that has so much mass that nothing can escape its gravitational pull.  Why can nothing escape?  Nothing can escape because that the velocity necessary to move away from the “surface” of the black hole is more than the speed of light.  This holds true because massive objects distort time and space around it.  So a super-massive object such as a black hole would distort space so much that it would begin to exhibit actions that are seen no where else.  So as the object’s mass increases the faster speed is necessary to escape the surface.  Furthermore, as an object nears a black hole the velocity necessary for escape increases to almost the speed of light and then you cross the event horizon and there is no escape.  That is why they are called black holes, because nothing radiates from them because nothing is fast enough to escape. 
    To better explain imagine a black hole like a planet in the sense that it has a core and a surface.  Starting at the “surface” we have what is called the event horizon.  This is the point of no return.  Once anything passes through this boundary it is no longer able to escape.  You continue on through until you reach the “core”, which is the singularity.  The singularity is interesting in that from what is understood all the laws of physics break down completely.  Below is a brief timeline of how a black hole became what it is today.


A History of the Einstein Theories on Black Holes

    In 1915, Albert Einstein developed the theory of gravity called general
            relativity.

    A few months later Karl Schwarzschild gave the solution for the 
            gravitational field of a point mass and a spherical mass, showing
            that a black hole could, in theory, exist. The Schwarzschild
            radius is now known to be the radius of the event horizon of a
            non-rotating black hole. 

    In 1939, Robert Oppenheimer and his co-authors used Schwarzschild's
            system of coordinates which were the only coordinates available 
            at the time to prove that at the Schwarzschild radius the
            equations would literally become insolvable at the Schwarzschild
            radius because some of the terms were infinite. This indicated
            that the Schwarzschild radius was really the boundary at which   
            time “stopped”.

    In 1958 David Finkelstein discovered how “stopped time” worked and
            introduced the concept of the event horizon by presenting the
            Eddington-Finkelstein coordinates, which enabled him to show
            that The Schwarzschild surface r = 2 m is not just a single point
            but actually a membrane of sorts that that can be cross but only
            in a single direction.

    In 1963 Roy Kerr extended Finkelstein's analysis by presenting the Kerr
            metric, which basically means that all matter at some point will
            rotate within a massive object’s ergosphere. This Kerr Metric
            showed how it was possible to predict the properties of rotating
            black holes.

    In 1970, Stephen Hawking and Roger Penrose proved that black holes
            are actually a part of all of Einstein’s equations on relativity and
            not just of Schwarzschild's.  This made black holes a space
            property that cannot be avoided in some collapsing objects.

    In 1971, Louise Webster and Paul Murdin, at the Royal Greenwich
            Observatory, and Charles Thomas Bolton, working, on his own, at
            the University of Toronto, observed HDE 226868 wobble, as if
            orbiting around an invisible but massive companion. Further
            research made astronomers realize that the space object, Cygnus
            X-1, was in fact a black hole.


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