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?

  • $\begingroup$ "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. $\endgroup$ – antispinwards 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. $\endgroup$ – Carl Witthoft 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 $\endgroup$ – astrosnapper 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. $\endgroup$ – N. Steinle Jan 30 '19 at 3:05

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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