COSMOLOGY
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.