# 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 "slow"? Why is this the case?

• Why do you think that the stars should be spinning faster? Without knowing that, it is impossible to answer why stars are spinning slower than you think they should be spinning. Commented Oct 26, 2020 at 20:22
• Sorry, should have worded it better. According to the law of conservation of angular momentum, a rotating body that has contracted to a lower diameter should have a higher rotational period. According to this logic, I thought that since much of the cloud material in a nebula is contracted into stars, the rotational period of stars should be very high. Commented Oct 26, 2020 at 22:52
• "Very high" compared to what? How high is high? You've not explained anything, instead you just repeated the same thing. You are basically asking "Why do I think that stars should be spinning faster than what is actually observed?" How would anyone know that? Commented Oct 26, 2020 at 23:44
• @HenloJibbab I think you mean "higher rotational frequency." Higher period (more time per revolution) means lower frequency/angular velocity. Commented Oct 27, 2020 at 18:26

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.

• What a compact yet dense answer! Can I paraphrase the last part without straying too much from correct? As the material moves out to several stellar radii still coupled to the co-rotating field, it accumulates much more angular momentum per unit mass than average. So when the mass is lost (decouples) at that point, it takes away much more than it's "fair share" of angular momentum.
– uhoh
Commented Oct 27, 2020 at 15:24
• @uhoh Yes, it is forced to co-rotate (almost) out to the Alfven radius and therfore has a high specific angular momentum. When it decouples, this is lost from the star. Commented Oct 27, 2020 at 15:40