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Black holes are objects so massive that nothing can escape from their gravitational pull. Suppose we have an sphere of mass $ M $, which we compress indefinitely. At what point will it become a black hole? From the special theory of relativity we know that the highest speed attainable is the speed of light $ c $. Hence, the criterion is that the escape velocity be greater then the speed of light.

$ \frac{G M}{R} > \frac{1}{2} c^2 $

$ R < R_s = \frac{2 G M}{c^2} $

where $ G $ is the universal constant of gravity. The critical radius is called Schwartzschild radius.

Once something has collapsed into a black hole, all of its previous attributes and properties disappear, except for three conserved quantities: mass, angular momentum and electric charge. For simplicity, we will consider a non rotating, neutral black hole, whose only property is mass. Since this is the only dimensional parameter, many attributes of such black holes can be obtained just with dimensional analysis.