How do accretion disks form?

Hint: An accretion disk is a structure formed by diffuse material in orbital motion around a massive central body.

Complete answer:
An accretion disk is a structure formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inwards towards the central body. Accretion disks of young stars and Protostars radiate in the infrared.

An accretion disk is formed whenever the matter being accreted possesses enough rotational or angular momentum that it cannot simply fall inward toward the accretor along a straight line. In directions perpendicular to the accretor's rotation axis, the flow tends to flatten onto a disk because the rotation resists the inflow of the material. In direction parallel to the rotation axis, the matter contracts towards a plane until the thermal pressure inside the disk roughly equals the gravitational force.

Note: Accretion disks are very useful in many astrophysics phenomena and objects such as : Quasars, active galactic nuclei, X- ray binary systems, young stellar objects and gamma ray bursts, supernovae etc. The temperature of the accretion disks range from a few thousand to several million kelvins.