Therefore, if a black hole rotates with the spin a = M {\displaystyle a=M} , its total mass-equivalent M {\displaystyle M} is higher by a factor of 2 {\displaystyle {\sqrt {2}}} in comparison with a corresponding Schwarzschild black hole where M {\displaystyle M} is equal to M i r r {\displaystyle M_{\rm {irr}}} . The reason for this is that in order to get a static body to spin, energy needs to be applied to the system. Because of the mass–energy equivalence this energy also has a mass-equivalent, which adds to the total mass-energy of the system, M {\displaystyle M} .