I was at the University Club this evening, listening to Ramesh Narayan speak about relativistic jets shooting away from spinning black holes at .92 the speed of light.
A spinning black hole is a singularity in 2 dimensions; rather than a point, it is a disc with frame-dragging angular momentum that warps magnetic field lines into a spiral around both poles and concentrates local gasses beyond the event horizon, ejecting them outward at terrific speeds. The faster the spin, the greater the ejecta.
Black holes are the simplest things in the universe. Everything about them can be summarized with just two numbers: mass and spin.
Black holes can come in many sizes; any amount of matter greater than 10^-5 grams can be compressed into a black hole. But for small amounts of matter, it takes a lot of energy to induce a flip into this alternative state of being.
On the other end of the spectrum, for any star more than 3x the size of our sun, it will naturally collapse into a black hole once its internal source of fusion heat dies down.
Larger still, every galaxy has a super massive black hole at its center; some are billions of times more massive than our sun. The size and luminosity of the galactic center varies with the size of the central black hole. And the nearby solar systems whip around these black holes like comets around our Sun.
I happened to get a photo of this region at the center of the Milky Way.
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