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]

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.

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]

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.

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]

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?

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]

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

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

 

[sophiecentaur]

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

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/2012/08/120816150801.htm

Respectfully submitted,
Steve

 

[Aimless]

Of course the Sun rotates.

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]

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?

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

 

[davenn]

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

from Wiki...

Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone

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]

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?

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

 

[sophiecentaur]

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

How spherical is Vega? Has the spin affected its shape? Does it have much mass around it in the form of a solar system (which could have pinched some of its angular momentum)?

 

[H2Bro]

Vega is also a very young star, still surrounded by its proto-stellar cloud of dust and gas. It could be stars have initially high spin which slows over time through interaction with their own nebular disk.

For our Sun, it implies that there is still quite a bit more to learn about what's really going on inside :)

 

[Chronos]

It is generally believed much of the solar angular momentum is carried off via the solar wind. It is also likely some of it is lost via the accretion process during the protostar phase. Magnetic angular momentum transport is also favored by most models. For discussion see http://farside.ph.utexas.edu/teaching/plasma/lectures/node69.html and http://arxiv.org/abs/0911.1121

 

[Drakkith]

How spherical is Vega? Has the spin affected its shape? Does it have much mass around it in the form of a solar system (which could have pinched some of its angular momentum)?

Vegas equatorial radius is about 19% larger than its polar radius. So it is VERY bulgy!

 

[onomatomanic]

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.

This is true, but I'm near-certain that it's a vanishing contribution.

The characteristic time to tidal lock is, scaled to the Earth-Moon case for convenience, something like

T_lock ~ (10^10 years) / ((T in days) (F_tidal/F_tidal|lunar)^2)​

where T is the period of rotation of the body undergoing tidal lock and F_tidal the tidal force exerted upon it. The tidal forces exerted upon the Sun are bound to be dominated by either Venus (close and much more massive than even-closer Mercury) or Jupiter (massivest). For the former case, they're at least five orders of magnitude lower than the lunar force on Earth. For the latter, I have to look at the numbers... *looks at the numbers* ... they're also at least five orders of magnitude lower than the scaling value. So, that gives us at least 10^20 years on top, and (T in days) at the bottom, which isn't going to be far away from 1, relatively speaking. As I suspected, this is not going to amount to anything during the entire solar lifetime.

 

[H2Bro]

Ah yes, I was not speaking of tidal locking, which indeed would be vanishing, rather that if one has two bodies in orbit around the sun with both the same velocity the furthest body will have more angular momentum (wrt to the sun) than the closest. Therefore a body maintaining constant velocity but increasing it's semi-major axis would draw angular momentum away from the sun's rotation (insert figure skater reference here). Meaning a planetary body that has viscous surface which is attracted by the sun and exhibits tides - in the normal sense - will slowly drift further out as gravitational energy is consumed moving the viscous liquid against the planet's surface friction (This effect is observed with the moon, which is slowly drifting further from the Earth as it pulls the oceans along whoses movements are resisted by the continents).

This is all fine and dandy except, as I realized shortly after posting, that as a body moves further from the Sun its orbital period increases, something Kepler figured out while most of our families were serfs on someone's manor.

Anecdotally, one can note the trivial effect of tidal locking in Sun-planet systems by considering the degree of tidal drag proportionate to the mass of the effecting body. In the case of Sun-Venus, the gravity of the Sun is notably ineffective considering Venus rotates in completely the opposite direction as the Sun's rotation, indeed opposite to the movement of all the other planets. Speculation within reasonable limits would guess Venus has had this rotation since the earlier and more hectic era of our solar system, which is to say a few billion years.

 

[onomatomanic]

^ At least one of us is misunderstanding the mechanics involved in tidal locking.

As I understand it, in the familiar Earth-Moon case (considering the Moon only as a source of tidal forces and Earth only as their subject, since the Moon is of course already locked), the Moon creates tides on Earth, and the effect of these tides is to gradually dissipate Earth's rotational energy. Part of it goes into accelerating the Moon, which puts it into a higher orbit, part of it is spend as friction heat et cetera. This will continue until Earth is tidally locked (the tidal bulges have to move relative to surface and/or have varying amplitudes to be able to dissipate energy). Or, rather, it would continue until then, if the timescale didn't exceed the remaining lifetime of the solar system in its present form.

So, yes, you were speaking of tidally locking the Sun when you were speaking of increasing the planets' angular momentum, since those two effects are two sides of the same coin and cannot be separated. If you have a timescale for the one, you have a timescale for the other.

[...] insert figure skater reference here [...]

A figure skater is a solid body, meaning all parts of one have to spin with the same angular speed. If they enlarge the "orbit" of their hands, thus lowering the hands' angular speed below their torso's, the physical link between the two parts (the arms) will transmit torques in both directions, with the result of speeding up the hands and slowing down the torso until the angular speeds are the same once more.

A solar system is gravitationally bound, meaning different parts of one have to spin/orbit with different angular speeds. If something were to enlarge the orbits of its planets, thus lowernig the planets' angular speed per Kepler's laws, there are no arms to transmit a torque to the Sun, so its angular speed wouldn't be affected.

If said something is part of the solar system, then its total angular momentum has to be conserved, just as it is for the figure skater. Other than that, though, the two cases have little in common.

Meaning a planetary body that has viscous surface which is attracted by the sun and exhibits tides - in the normal sense - will slowly drift further out as gravitational energy is consumed moving the viscous liquid against the planet's surface friction.

Tidal forces exerted by the Sun upon a planet (which isn't in tidal lock or resonance yet) dissipate the planet's rotational energy without changing the planet's orbit, as far as I know. There is simply no mechanism to do this. I suppose that if the planet in question is Jupiter, one could consider the Sun's orbit about their common centre of mass in this regard. If the tidal bulge on Jupiter leads or lags, that orbital distance could be affected, and the planet's orbital distance would have to change in turn to compensate. I'm having a bit of trouble fathoming this particular situation, to be honest. Maybe someone else can explain.

Now, tidal forces exerted by a planet upon the Sun (which definitely isn't in tidal lock or resonance yet) dissipate the Sun's rotational energy, and part of that energy can indeed be transferred to the planet's orbit.

That's what I gave you an order-of-magnitude estimate for in the previous post.

This effect is observed with the moon, which is slowly drifting further from the Earth as it pulls the oceans along whoses movements are resisted by the continents).

It is, but you have to think the analogy through very carefully to draw conclusions about the Sun-planet case: Which body takes the place of which?

 

[Dotini]

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?

After skimming the latest NAS decadal report on solar/space physics discoveries and problems, I found no reference to this specific question. However, surmising from the regularity with which the term appears throughout their text, they might say "magnetism" plays a prominent role. Since they consider our sun a variable magnetic star, it seems likely that stellar sphericity would vary with magnetism.

Respectfully submitted,
Steve