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An artist's conception of the hot gas falling into a supermassive black hole (left) shows how the whirling matter tends to form a flat disk, like the one that created our solar system. The thick gas prevents radiation from escaping out through the sides of the disk. Rather, the radiation blasts into space above and below the disk along narrow jets, propelling matter at close to the speed of light. When one of these jets from a distant quasar happens to point toward us, we call the intensely radiating galaxy a "blazar."
Astronomers can see the characteristic rotational signatures of such disks at the cores of many nearby galaxies. Shown above is a spectrum taken by the Hubble Space Telescope of the inner 26 light-years of the galaxy M84. The Doppler effect (page 139) reveals that stars and gas on one side of the galaxy's core are whirling quickly toward us at a rate of 250 miles per second (purple pixels). On the other side of the core, the stars and gas move away from us at the same rate (red pixels). The physics of gravity suggests that these rapid motions arise from the relentless pull of a central black hole at least 300 million times more massive than our Sun. | 
