A black hole is a point in space where there is extreme gravitational pull, so extreme that light itself cannot escape. The strength of gravity is so strong due to the fact that an immense amount of matter has been contained in a small space. The existence of black holes was first theorized by John Mitchell in 1783. Mitchell accepted Newton’s laws of gravity and suggested that light escaping from the surface of a star would have its speed reduced due to the gravitation pull of the star, and therefore if a star’s gravitation pull was strong enough even light would not be able to escape. Using the approximate speed of light he reasoned that if an object was approximately 500 times the mass of the sun light would not be able to escape and therefore be invisible.  Karl Schwarzchild was a German astrophysicist who used Einstein’s theory of relativity to compute and calculate the gravitation fields of stars. His solution to Einstein’s equation led him to calculate the radius of an object of particular mass for which light could not escape or in other words “that the fabric of time-space would fold in on itself, creating a ‘singularity’ – a region with zero volume and infinite density. 
There are various methods which have suggested and convinced astronomers that black holes exist. In particular, a technique used by astronomers to locate both stellar and super-massive black holes is to observe the motion of stars in the area around the supposed black hole. Most stars in our galaxy are formed in groups of two or more stars, by observing the visible partner of a binary system, astronomers can effectively trace a black hole. Astronomers can calculate the mass of a black hole quite simply, using Keplar’s laws of motion, this is done by determining the velocity at which the partner star is orbiting the black hole. Generally if an object has a mass three times the sun or less it is a neutron star, if it is greater than that it is most likely a...
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