Why does an accretion disk become extremely hot?

An accretion disk becomes extremely hot primarily because the material spiraling inward must lose gravitational potential energy and angular momentum to successfully accrete onto the central object. As matter falls into the deep gravitational well of a supermassive black hole, it accelerates to high speeds and undergoes high-speed collisions.1234

The process of accretion converts this gravitational potential energy into heat and radiation. Because the material must shed energy to move closer to the center, the disk material heats up, with temperatures reaching up to several million kelvins. This heat is generated through friction caused by turbulence and viscosity within the disk, which effectively transports angular momentum outward and allows the matter to drift inward. The resulting thermal energy is then emitted as radiation across a broad spectrum, ranging from infrared to X-ray wavelengths.12