B Black Holes: Rotating vs Non-Rotating

[Souhardya Nandi]

I am reading about Kerr black holes and non rotating black holes. But I am unable to understand what decides whether the black hole will not rotate or rotate. And if No Hair theorem suggests, we can know about a black hole through its angular momentum, what implications does zero angular momentum have for a black hole ?

 

[bapowell]

A Kerr black hole forms from a star with nonzero angular momentum, or from a system of stars with nonzero angular momentum.

 

[phyzguy]

In the limit of zero angular momentum, the Kerr black hole solution becomes the Schwarzschild black hole solution. In other words a non-rotating Kerr black hole is identical to a Schwarzschild black hole.

 

[kimbyd]

I am reading about Kerr black holes and non rotating black holes. But I am unable to understand what decides whether the black hole will not rotate or rotate. And if No Hair theorem suggests, we can know about a black hole through its angular momentum, what implications does zero angular momentum have for a black hole ?

Real black holes effectively always rotate. The chance of a black hole not having any angular momentum is pretty much nil.

That said, as a theoretical construct it's easy to consider a black hole that has no angular momentum. Such black holes are much simpler than spinning black holes, lacking features such as the ergosphere or jets. As phyzguy mentions, these are just Schwarzschild black holes which are defined by a single parameter (their mass parameter).

 

[Khashishi]

Angular momentum is conserved, so if the star or matter had some angular momentum before collapsing into a black hole, it will become a rotating black hole.