Stars form from the collapse of molecular clouds. During this process, any rotation present in the natal cloud is amplified as the star contracts. If this process was able to continue unaffected, stars would reach rotational velocities great enough to cause them to break up.
The sole fact that we observe stars means a process must exist that enables the rotation of the star to slow down during its formation.
Stellar rotation rates are observed to be slower than expected early on during the star formation process, when the proto-star is about a million years old. At this time, primordial circumstellar discs are present around the proto-stars.
The discs are understood to regulate the stellar rotation rates through interaction with the stellar magnetic field in a process known as 'disc-locking'. This talk will discuss the details of this process and what the implications are for the formation of stars and planetary systems.