I recently attended a lecture by a Caltech professor on the subject of Black Holes. With a large % of non-technical people in the audience, she was trying to avoid getting extremely technical, but may have confused people even more. One question was "what is the size of a black hole of 2 million solar masses?" I found a web site that performs this exact calculation for _non-spinning_ black holes, and the answer seems to be that the radius of the event horizon is ~0.08 AU -- i.e., 8% of the distance from the Sun to the Earth. The next question is what is the size of the fastest _spinning_ black hole with a mass of 2 million solar masses? I wasn't able to find a calculator for this, but I did find a ppt slide that seemed to indicate that an extreme Kerr black hole had 1/2 the Schwarzchild radius of a non-spinning black hole; i.e., 0.04AU for my 2 million solar mass example. Apparently, this counterintuitive result comes from the fact that a large proportion of the spinning black hole mass is in the form of rotational energy. Now a spinning black hole has an ellipsoidal "ergosphere" that touches the Schwarzchild radius at its poles, but is larger (?) at the equator (?) What exactly is the shape of this ellipsoid for the fastest spinning black hole? To be precise, what is the numerical value of the eccentricity of the ellipse? The "singularity" for a spinning black hole is no longer a point, but a _ring_. What formula gives me the _radius_ of this ring for a maximally spinning Kerr black hole ?