Astronomers think that almost every galaxy in the universe contains a supermassive black hole at its center. As matter falls toward the black hole, drawn by its immense gravity, it forms an accretion disk of gas and other debris outside the event horizon. This flattened disk is very hot and radiates across the electromagnetic spectrum from low-energy radio emissions to high-energy gamma rays. This illustration shows such an active galactic nucleus. Learn more at AAS Nova. [NASA/JPL-Caltech]