Why do stars rotate slower than they're expected to? Due to conservation of angular momentum, I thought most stars would be spinning extremely fast because they have a relatively small diameter. However, it turns out that this is not true and most stars actually spin &quot;slow&quot;? Why is this the case?
There are two phases to this problem.
In order to accrete into stars, a huge amount of angular momentum must be lost to allow so much mass to gather into a small volume.
A second problem is how stars like the Sun end up rotating so slowly, when younger versions of stars similar to the Sun rotate much faster.
The solution to the first problem may be solved by the outward transport of angular momentum in accretion disks. The angular momentum may ultimately be shed by winds originating from the disk surfaces. Another possibility is the loss of angular momentum in jets and bipolar outflows, which are accelerated from the central star-forming core by poorly understood processes. Some of the angular momentum can of course end up in planets.
Stars themselves can then lose angular momentum throughout their lives. Initially this can be through magnetic coupling to the protostellar accretion disk. Later, the principal angular momentum loss process is through stellar winds that become controlled by coronal magnetic fields. The plasma decouples from the co-rotating field at many stellar radii, taking away angular momentum.