Accretion disk

An accretion disk is like little bits of stuff (usually gas or dust) that move around in a circle around something else (like a star). The bits of stuff move around faster and faster as they get closer and closer to the middle. As the bits of stuff continue to move faster and faster until it moves so fast that it can't move any faster, a lot of energy is created from the heat and light from the stuff as it squeezes together inside the disk.