Sagittarius A*, the supermassive black hole at the center of our galaxy, made the news over the past week, picking up that scientists recently reported they'd calculated the speed of its spin: near the theoretical maximum for a black hole of its size. I couldn't find that broken down in layman's terms, other than "a significant fraction of the speed of light."
Black holes are simple things when you get right down to it, so to calculate A*'s rotational speed via "the outflow method," all they needed to know was its mass—just over 4 million Suns—as well as its energy output in radio waves—much of it in the form of a jet, the readings suggest—and X-rays (from its accretion disc), the latter two values being obtainable from existing NASA Chandra X-ray space telescope readings.
A*'s rotational speed had been calculated before using different methods, but the "outflow method" is apparently thought to be pretty darned solid; it had already "been applied to over 700 supermassive black holes."
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^ From "The Master Plan of Doctor Doom!" in Fantastic Four #23, February 1964. Stan Lee plot, Jack Kirby pencils, George Roussos inks, Sam Rosen letters. Original colorist unknown. Photoshop color reduction.
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