The Sun is very spherical - allegedly

sophiecentaur

I read this Scientific American link and I found it very interesting.
I have a problem understanding it, though, because I had read that one of the explanations for the existence of Solar Systems is based on Angular Momentum. The argument goes that if all the mass were concentrated in the host star, the rotation rate would be such that material would be shed from it and so the planets have a significant share of the total angular momentum of the system. That would suggest to me that you'd expect the Sun to be Oblate - like many / most planets - rather than the near perfect sphere they seem to have found.

Was that theory commonly held until this recent measurement?

[Edit - or is there still some odd distribution of mass within the Sun?]

H2Bro

Is that theory saying planets formed from the ejecta of proto-stars? Or is it saying that IF all the material went into the star it would likely spin much of it off again?

I am unfamiliar with the former. If the latter case, I can see why it might not hold after this observation but it depends on what is maintaining sphericity. The strength of magnetic, nuclear, and gravitational forces seem to be dominating angular momentum at this rotational speed, at least.

I mean, the mass distribution IS odd, in that it isn't as oblate as one would think... I may have misunderstood your post.

sophiecentaur

Thanks for the reply. You seem to be agreeing that there must be something odd - which is reassuring for me!
I'm being a bit vague about this because it's a vague memory (anno domini, you know) and I'm not sure which of those theories applied to the argument. I think the logic would assume that all the material was part of the same entity, initially.

I suppose that it must be some magnetic effect because the nuclear forces are pretty local. When you think that Jupiter is 'almost big enough' to be a star and it is noticeably oblate, you'd expect stars to be oblate too. Perhaps someone else will contribute and put us wise.

Drakkith

I think I remember reading that the Sun shed angular momentum early in it's life through the interaction of it's magnetic field with the solar wind or something.

sophiecentaur

The plot thickens!! But the Sun does rotate somewhat because I'm sure the sunspots move about. Perhaps that's only the outer layer?? But what about the drag?

Drakkith

Of course the Sun rotates. What drag are you referring to?

sophiecentaur

You wrote that it shed angular momentum - but you didn't imply all of it. So why would it stop doing that, I wonder?

Drakkith

Perhaps the rate of loss slowed as it's rotation slowed?

sophiecentaur

That could make good sense.

H2Bro

I can think of one additional processes that might reduce angular momentum of the sun.

Planets that experience tidal effects from the Sun's gravitation slowly increase their period of orbit, i.e. get further away. At a further distance they take up more of the systems total angular momentum, thus the Sun slows its spin.

Also, large planets like jupiter and saturn may have pulled in sizeable amounts of material that was previously in a closer orbit to the sun. This would also absorb angular momentum from the system.

Dotini

SDO is bringing to light some interesting anomalies well worth questioning.

They also found that the solar flattening is remarkably constant over time and too small to agree with that predicted from its surface rotation. This suggests that other subsurface forces, like solar magnetism or turbulence, may be a more powerful influence than expected.
http://www.sciencedaily.com/releases...0816150801.htm

Respectfully submitted,
Steve

Aimless

Actually, I wonder about this. The surface rotates, but do we have any direct observations of the rate of rotation of the core? Or is it just all inferred?

Drakkith

No idea. I would expect the core to rotate given that the rest of the Sun does, but I really have no clue.

davenn

from Wiki.....
that I didnt know till now ;)

Dave

H2Bro

A priori it would be nigh impossible for a great cloud of material to fall in on itself and have a net zero angular momentum.

sophiecentaur

So where does this all take us in the "why is it so damned spherical?" question? Would all stars be the same, do you suppose?

Drakkith

Nope. Vega is spinning VERY fast. It spins at 87.6% the speed that would tear it apart.

http://en.wikipedia.org/wiki/Vega#Rotation