The Shakura–Sunyaev α-disk model is both thermally and viscously unstable. An alternative model, known as the β β -disk, which is stable in both senses assumes that the viscosity is proportional to the gas pressure ν ν ∝ ∝ α α p g a s . In the standard Shakura–Sunyaev model, viscosity is assumed to be proportional to the total pressure p t o t = p r a d + p g a s = ρ ρ c s 2 since ν ν = α α c s H = α α c s 2 / Ω Ω = α α p t o t / ( ρ ρ Ω Ω ) .