From the speed of the girl and the size of the circle in which she swirls, we know something of the mass of the invisible companion. That stars close to the center of our galaxy go around as fast as t...From the speed of the girl and the size of the circle in which she swirls, we know something of the mass of the invisible companion. That stars close to the center of our galaxy go around as fast as they do is one of the best indicators we have for the presence, and one of the best measures we have for the mass, of the central black hole, which is itself invisible.
