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I'm just curious about that the existence of non-rotating stars. Is it physically possible for a star not to rotate at all? Does magnetic braking eventually stop the stellar rotation?

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  • $\begingroup$ "Is it possible for stars not to rotate?" is a really interesting question! Lack of net rotation during the formation process would have a big impact, I'm looking forward to finding out how. However the question on magnetic braking suggests there are possibly two questions here, 1) can stars form with little or no rotation, and 2) are there any mechanisms by which a rotating star can slow itself down over time. $\endgroup$
    – uhoh
    Jan 28 '19 at 3:46
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    $\begingroup$ The probability for a continuous quantity like rotation to be an exact value is zero. There are some very slow rotators though, like Przybylski's Star with a rotation period of 188 years. $\endgroup$
    – user24157
    Jan 28 '19 at 8:37
  • $\begingroup$ How could magnetic braking have an effect? To stop rotation, you need to generate a counter-rotating force, and for a standalone object (the star) this implies that the initial net angular momentum was zero. $\endgroup$ Jan 28 '19 at 19:04
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    $\begingroup$ Magnetic braking occurs through the application of torques to the star either through connection of the magnetic field to the protostellar disc (early in life) or through angular momentum loss through the stellar wind (later stellar life stages). Note that the initial protostellar cloud will have a small amount of rotation from the galactic rotation at a minimum and this would be increased as the protostar contracts. Also proving non-rotation observationally is basically impossible $\endgroup$ Jan 28 '19 at 22:47
  • $\begingroup$ Rotate relative to what? Relative to an observer on Earth, an orbiting satellite, or something else? IF you mean relative to some static background then yes it is possible, but not very plausible. $\endgroup$ Jan 30 '19 at 3:05
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No, this is not possible.

During the stellar formation, some angular momentum will always be present. And any "braking" effects (magnetic, relativistic, tidal etc.) will become weaker as the rotation slows down. So the rotation will never completely stop, because any forces slowing down the rotation will weaken as well.

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    $\begingroup$ well not exactly true, but pretty much true ;-) There could be braking due to tidal effects (or infall) from an object in orbit in the other direction. In that case net angular momentum could change sign, and therefore pass through zero at some instant in time. Then again what does or does not count as "braking". $\endgroup$
    – uhoh
    Jan 30 '19 at 7:36
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This is very well possible. If another star passes the two can exchange angular momentum when they meet tangentially. They can even both come out from this encounter with zero rotatiòn. The chances are small but it is possìble.

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  • $\begingroup$ The first and last sentences seem to contradict, in spirit if not in fact. $\endgroup$
    – RonJohn
    Jun 30 at 9:31
  • $\begingroup$ @RonJohn I see what you mean! Very well possible seems contrary to a very small chance. Maybe I should have written that it is (very well) possible, but... It is very well possible though (in fact). $\endgroup$ Jun 30 at 9:36
  • $\begingroup$ The probability is small, but non-zero? $\endgroup$
    – Ng Ph
    Jul 1 at 13:36
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It is impossible. After the star is "born", it will be rotating because of angular momentum from the place it was formed (most likely a nebula, which is the aftermath of a supernova). Even if there are extremely large amounts of magnetic, tidal, etc. forms of weakening the star's rotation, they will weaken as the star's rotation weakens. The stars rotation could become painfully slow (possibly the slowest speed allowed by the universe), but it is still rotating, just extremely slow.

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  • $\begingroup$ Hi Linus, welcome to StackExchange. While your answer may be correct, it doesn't really say anything the accepted answer doesn't. However, if you could back up your claim with references and/or some math/physics, it would actually be an improvement over the accepted. $\endgroup$
    – pela
    Jun 30 at 14:56
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It's not physically impossible, but it's so improbable that it may as well be so. You would need every tumbling dust grain and swirl of gas to exactly cancel out the rotation of all the other bits of matter that came together to form the star. If that did happen, the most minuscule disturbance would turn it from a non-rotating star into a slowly-rotating star. It's a reasonable assumption that there are no non-rotating stars within the observable universe.

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