I have recently come across the notion that Kerr metric describes the spacetime outside a rotating black hole but not outside a rotating (electrically neutral) star. Unlike Schwarzschild metric, which works both for non-rotating spherically symetric black hole without charge as well as any other object with the same properties. Where does the difference come from? I feel it has something to do with moment of inertia of the spinning star's material but I'm not sure. I am looking forward to your replies.