A protostellar disk is created as the gas and dust of a stellar nursery collapse gravitationally in a process that eventually creates stars and solar systems. As the gas clumps together at the center of the cloud, physical forces pull it into a spinning disk shape.
The collapse of a large cloud imparts energy to the individual particles as they fall toward the center. The conservation of angular momentum forces these particles into a flattened disk that spins faster and faster as it is drawn into the central mass. In time, this central mass ignites as a star and blows away the lightweight remnants of the disk.