Why do discs form around protostars?
Many processes play a role in disc formation, but angular momentum is the principal quantity. The protostar and its disc are formed from the same cloud composed of gas and dust, which initially has some rotation – it has angular momentum, which is a measure of the direction and speed that something is rotating. The cloud collapses due to its own gravity. However, angular momentum hinders this process, and the cloud collapses into a rotating ‘pancake’ rather than a sphere. If the central region contains enough dense gas, then a rotating protostar will form.
The gas and dust surrounding the new star continues to rotate. The amount of angular momentum that this gas and dust contains is one of the primary quantities that will determine its fate. If there is too little angular momentum, then the gas and dust will fall onto the star and increase its mass. However, if there’s enough angular momentum then the gas and dust will remain as a rotationally supported protostellar disc. At a large enough distance from the protostar, the orbiting gas and dust will no longer be rotationally supported, indicating the outer edge of the disc. This edge is reasonably well defined, making the disc distinct from its surroundings.