If a supermassive black hole exists at the nucleus of a galaxy, it may accumulate matter from the stars and gas that surround it. If this matter is accumulated rapidly—at a rate of a few Earth-masses per minute or greater—tremendous amounts of energy can be generated as the matter falls toward the black hole. The energy that is released in this way can be much greater than that of the nuclear fusion of a star. In fact, such a supermassive black hole system can radiate more energy in a few seconds than our Sun can produce in thousands or even millions of years. These systems are called active galactic nuclei, or AGN.