Does a star have to rotate?

[RichieJarvis]

Hi All,

A Q occurred to me - does a star have to rotate? I mean, I understand why they do, but surely a situation could occur where they dont?

Cheers,

Richie

[beamish]

I have no definitive answer to this but my intuition tells me that it could never happen, except if it was a dead, cold, dark star.

[Chiz]

Even a dead, cold, dark star would continue to rotate. In fact as it collapsed it would rotate faster & faster to conserve the angular momentum.

[beamish]

I assumed over time it would decelerate and eventually stop ?

[RichieJarvis]

But surely angular momentum doesn't need to be generated when the star forms?

[beamish]

This is gonna prove to be interesting !

Richie can you give us your thoughts on when a situation might arise?

I would have thought it is inevitable as the gas starts to collapse under gravity during the initial stages of star formation . Hope there's a boff somewhere close who might elucidate

[RichieJarvis]

Well - my thought is this. In an infinite universe, somewhere the gas must have coalesced from equal pressures from all directions. If so, then where does the energy come from to begin the rotation of the star?

Also - if the star stopped spinning (hypothecially) how would that effect its fusion?

Cheers,

Richie

[FraserClarke]

I think, In theory, you could make a non-rotating star. You'd have to have a perfectly homogenous and spherical gas cloud with no intrinsic rotation to start with; which is completely unphysical. As soon as you have any interactions with anything nearby (e.g. passing another star), you'd have a torque on the star and start it rotating.

I don't think it would affect the nuclear fusion very much; main sequence stars like the Sun spin in many 10's of days, which I suspect is practically non-rotating as far as nuclear fusion is concern. You do see some atmospheric affects in very fast rotating stars (with periods of a few hours), which you need to take into account if you want to understand the atmospheres properly.

Stars do spin down as they evolve, transferring angular momentum away through magnetic fields and stellar winds. I'm not sure they ever stop though.

[RichieJarvis]

Hmmm - interesting - so would an accretion disk form around our hypothetical non-rotating star?

[beamish]

I assumed over time it would decelerate and eventually stop ?

I can't believe I said that ! Obviously in the void of space there wouldn't be any forces acting upon the spinning star hence NO deceleration - god what a numpty !

[brantuk]

Surely the effect of gravity from other objects would start it spinning? The universe is expanding so it would be moving past other stuff (or other stuff past it) even if it did form perfectly symetrically (wich is unlikely).

[RichieJarvis]

So the only way a star could form and not rotate would be in a completely closed system then. Or one where the forces just happen to be in perfect balance. Of course, that balance would have to continue for the star not to rotate I guess.

Ok - that makes sense....

[LukeSkywatcher]

Interesting. In my ignorance i assumed only certain types of stars rotated.

[FraserClarke]

Hmmm - interesting - so would an accretion disk form around our hypothetical non-rotating star?

No, you get the disc because of angular momentum. If you had no angular momentum (no rotation), you'd have a spherical shell of accreting material. Again, you'd need perfect homogeneity in the material and the radiation pressure from the forming star...

[Macavity]

To some level it (usually!) has to rotate? As an aspect of... Spontaneous symmetry breaking - Wikipedia, the free encyclopedia. But I admit to genuine ignorance re. application thereof to "macroscopic" systems. On the other hand, I never could balance a pencil on it's point...

[RichieJarvis]

Ahhh - thanks TD - thats what I was wondering

[steveangel]

i agree with brantuk. from what iv learnt as a physics undergrad, regions of higher density within the dust / gas cloud start to contract due to gravity. This then attracts more gas and dust, including the other regions of higher density. These form "clumps" within the cloud which begin to swirl and spiral around due to the gravitational forces from other clumps. As the "clumps" eventually clump together the star takes shape with the accretion disk to conserve angular momentum as Teadwarf correctly stated.