Why aren accretion disks around giant stars as hot as accretion disks around black holes?
Material in an accretion disk heats up due to friction within the disk. The basic answer to your question is that the closer you get to a massive object, the more heating occurs, and the hotter the disk gets. Since you can get closer to a black hole (or other compact object, such as a neutron star) than you can to a giant star, accretion disks around compact objects are hotter. The way to think about this in detail is to imagine each particle in the accretion disk in orbit around the massive object in the center. Closer to the massive object, the force of gravity will be stronger so the particles there get pulled around in faster orbits than the particles which are further away. Therefore, if you imagine two adjacent “rings” of particles centered on the massive object, the ring that is closer to the massive object will be spinning around faster and so it will rub against the ring next to it. This rubbing between the two “rings” will heat them up due to friction between them. The rate a
