In the manner described in the previous subsection, material falling onto the accretion disk around a forming star causes the disk to heat up. The amount of heating depends on where the material hits the disk. Material hitting the inner part of the disk (the “inner disk”) has fallen farther and picked up greater speed within the gravitational field of the forming star than has material hitting the disk farther out. Like a rock dropped from a tall building, material striking the inner disk is moving quite rapidly when it hits, so it heats the inner disk to high temperatures. In contrast, material falling onto the outer part of the disk (the “outer disk”) is moving much more slowly (like a rock dropped from just a foot or so). So the temperature at the outermost parts of the disk is not much higher than that of the original interstellar cloud. Stated another way, material falling onto the inner disk converts more gravitational potential energy into thermal energy than does material falling onto the outer disk.