An accretion disk transports mass inward and angular momentum outward through stresses, while dissipating orbital energy as heat.
Viscous disk evolution combines mass conservation with radial transport of angular momentum by stress. In a steady Keplerian disk, the sum of advective and stress-carried angular-momentum fluxes is independent of radius.
A decretion disk is fed near its inner edge and transports mass outward. Rapidly rotating Be stars can sustain gaseous decretion disks whose outer material is removed by a companion, radiation, or a wind.
A Be star is a rapidly rotating B-type star showing Balmer emission from a circumstellar gaseous decretion disk.
The alpha-disk prescription writes turbulent kinematic viscosity as , with dimensionless . It reflects eddies no larger than the disk thickness and no faster than the sound speed.
The Shakura--Sunyaev thin disk is a geometrically thin, optically thick, radiatively efficient accretion-disk model whose turbulent stress is parametrized by the alpha prescription.
The magnetorotational instability destabilizes a magnetized differentially rotating flow when angular velocity decreases outward. Magnetic tension couples neighboring fluid elements, allowing the inner one to lose angular momentum and fall inward while the outer one gains angular momentum and moves outward.
For a vertical magnetic field in a Keplerian disk, axisymmetric vertical MRI modes are unstable when . The shortest unstable wavelength is .
A local thermal equilibrium is stable when a small temperature increase raises cooling faster than heating at fixed surface density. The standard total-pressure alpha prescription makes a radiation-pressure-dominated, Thomson-opacity disk thermally unstable.
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An accretion disk is a structure formed by diffused material in orbital motion around a central object, such as a star, black hole, or neutron star. The material—composed of gas, dust, and sometimes other celestial debris—spirals inward toward the central object due to gravitational attraction.