<p>There are two phases to this problem.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2020-10-27T15:24:52.567Z
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
CC BY-SA 4.0
2020-10-27T15:40:18.717Z
@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.
ProfRob
CC BY-SA 4.0
2020-10-26T20:32:42.927Z
2020-10-26T23:51:06.730Z
2020-10-26T23:51:06.730Z
ProfRob
<p>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?</p>

CC BY-SA 4.0
2020-10-26T20:22:24.473Z
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.
void_ptr
CC BY-SA 4.0
2020-10-26T22:52:05.790Z
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.
Henlo Jibbab
CC BY-SA 4.0
2020-10-26T23:44:33.070Z
"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?
void_ptr
CC BY-SA 4.0
2020-10-27T18:26:49.953Z
@HenloJibbab I think you mean "higher rotational frequency." Higher period (more time per revolution) means lower frequency/angular velocity.
reirab
<star><star-formation><nebula>
Why do stars rotate slower than they're expected to?
Henlo Jibbab
# Why do stars rotate slower than they're expected to?

(Asked by: username_0 on 2020-10-26)

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?

**Tags:** star, star-formation, nebula

> **Comments:**
> **username_1**: 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.  
> **username_0**: 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.  
> **username_1**: "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?  
> **username_2**: @username_0 I think you mean "higher rotational frequency." Higher period (more time per revolution) means lower frequency/angular velocity.  

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## 1 answers

### Answer by username_3 on 2020-10-26. Score: 9 | Accepted answer

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.

> **Comments:**
> **username_4**: 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.  
> **username_3**: @username_4 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.  

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