The alpha disk model (or α disk or Shakura-Sunyaev disk or thin disk) is a model of an accretion disk developed in the 1970s to explain black hole behavior. Among its assumptions is the disk is thin, and the type of accretion has been referred to as thin disk accretion. It posits that viscosity is based upon the distance from the central object, the speed of sound in the material, the scale height (rate at which density falls off "up" or "down" from the disk) and a free parameter labeled α, which is presumed to be related to turbulence in the disk. Since for observed disks, the other values can sometimes be worked out or estimated, given observational data, the model can yield an equation with just α as unknown, allowing α for the disk to be worked out, and it can be taken as a measure of the disk's turbulence.
The alpha disk is a model worked out by analytical means, and is often used as a starting point, though current research efforts incorporate more factors (e.g., MHD), more observation data, and the results of numerical simulations.
Another disk scenario is the ADAF, the case where the disk is unable to radiate much of its energy.