Wednesday, 29 July 2020

astrophysics - Is it expected that all stellar black holes will be spinning near the maximum allowed $omega$-velocity?


Using a bit of classical reasoning I'm imagining black hole formation to be much like an ice skater pulling in her arms:


skater pulls in her arms to increase angular velocity


Now, the size difference between a star and its black hole can't even be effectively captured in an image. The black hole for our sun would be much less than a pixel in this image:


size comparison of our sun and planets


That suggests to me that even a very slowly rotating stars would have much more angular momentum than could be supported by their resulting black holes. I haven't done the calculation because I don't really understand the Kerr metric but even with a bunch of classical hand-waving I'd think that just about every black hole formed in a stellar-collapse would be spinning maximally.



So my question is, do we expect nearly all black holes to be spinning maximally? If so, (roughly) how much angular momentum is lost because the star had much more than the black hole could support? And, how is all of this extra angular momentum shed during collapse? Is it just in the form of tons of matter being ejected until the the angular momentum is low enough to allow for the formation of a Kerr black hole?




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