When a star dies it explodes in a supernova. This happens when heavier elements like iron is fused in the core of the star. Then the star collapses under its own gravity. This leaves a black hole behind. Now in order to conserve momentum the star must spin. So does a black hole spin? If so at what speed does a black hole spin?
I think I have the answer. Suppose Mass of the star is 10 kg and its velocity is 100 m/s initially. So initial momentum is 1000 kg m/s. Now some mass has exploded into a supernova and what remains is a dense ball of matter which is caused due to the collapse of the gravity of the star.
Now mass is say 5 kg after the supernova (Which is mass of the black hole) so velocity must be 200 m/s. So final momentum is again 1000 kg m/s. This way we can apply conservation of momentum and derive the velocity. Is this correct?