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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?

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marked as duplicate by AccidentalFourierTransform, Jon Custer, anna v, Kyle Kanos, heather Feb 12 '17 at 2:03

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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?

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  • $\begingroup$ One black hole, at the heart of galaxy NGC 1365 is turning at 84% the speed of light. $\endgroup$ – avito009 Feb 10 '17 at 14:21
  • $\begingroup$ That is an unrealistic mass and rotational velocity for a star. $\endgroup$ – Kyle Kanos Feb 10 '17 at 14:42
  • $\begingroup$ @avito009 You are missing liniar momentum with angular momentum. To conserve angular momentum rotational speed needs to increase because as radius shrinks. $\endgroup$ – Pere Sep 13 '17 at 17:39

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