Established Member
This report from the Guardian newspaper reports on measurements which show that the sun's diameter is 1,400,000 km and the difference between polar diameter and equatorial diameter is only 10 km. Seeing that it is a spinning object, this comes as something of a surprise.
Established Member
Nice reminder why comments to news articles shouldn't be read.
Anyway, one of the very few intelligent comments pointed out, the Sun rotates relatively slowly, but has tremendous gravity, so I wonder why they would even compare it to Jupiter and Saturn's bulges.
Administrator
One difference between the Sun and Jupiter is that Jupiter's shape depends on PV=nRT, and the Sun has to include some radiation pressure in the equation.
Forming opinions as we speak
Established Member
Wouldn't the radiation be approximately equal all around, or are there dynamics that make different pressure at the poles/equator?
Order of Kilopi
The reported oblateness is a lot less than what I would expect the equipotential spheroid to be. Either something in the way of uneven limb darkening is goofing their measurements, or else some dynamic effect is offsetting some of the centrifugal effect.
Order of Kilopi
Retraction: I messed up my calculations the first time around. Now I get only about twice what they reported. Since they did not say what they expected and did not give any error bars for the 10 miles they measured, I am not sure whether or not there is a significant problem here.
Moderator
This article, at sciencemag.org, says they expected *four* times as much :)Originally Posted by Hornblower
http://news.sciencemag.org/scienceno...ed.html?ref=hp
Also:Besides the unexpected steadiness of the sun’s shape, the team found that the sun's oblateness is about 25% smaller than previously estimated, Kuhn says. The oddly low measurement may be caused by misunderstandings about how turbulence or magnetic forces influence the outermost layers of the sun, he notes, "but neither of those adjustments would agree with our current models." Instead, he suggests, it’s most likely that the layers of the sun just below its visible surface are rotating between 3% and 10% more slowly than expected.
This used to be a contentious issue, because if the sun were just a little bit more oblate (and the error bars were bigger back then, the early fifties), it could have explained the precession of Mercury without recourse to general relativity.An alternate explanation for the result, Gough suggests, might be that the atmosphere over the sun's polar regions is slightly different than that over the sun’s equator. Such a disparity, he notes, could cause distortions in the paths of light headed to Earth from the various regions, thereby skewing the estimate of oblateness lower than expected.
Established Member
This article, at sciencemag.org, says they expected *four* times as much
25% smaller is 75% of the expected value, not 25%.is about 25% smaller
Established Member
The photosphere has a density about the same as Earth's atmosphere at 100km altitude. So it is not something of real substance. Maybe there is really an oblateness of the matter, but that is compensated in some way by greater energy flux towards the poles. Or maybe something to do with the magnetic field.
Moderator
O wow how did I misread that? Thanks for pointing that out.
This new result then is not such a drastic difference. Hornblower, in your calculations, did you take the density profile of the sun into account?
Order of Kilopi
No. I just made the simplistic assumption that for small amounts the oblateness would be equal to the ratio of the equatorial centripetal acceleration to the gravitational acceleration, and that the body was rotating as a unit. That was roughly in agreement with Earth and Jupiter.
Established Member
The sun is not rotating as a unit though. It has differential rotation with latitude, at least for the surface layers.
At about 1/3 of the way under the "surface", the sun does rotate as a unit. See the wikipedia page:
http://en.wikipedia.org/wiki/Solar_rotation
So there's a more complicated calculation involved I'm sorry to say. I can't do it
Order of Kilopi
That, among other things, definitely complicates the calculations. When we throw in electromagnetic effects all bets may be off. The author of the Guardian piece did us a disservice by not explaining how the scientists calculated the oblateness they expected to see, or how big the error bars are for the observed value.The sun is not rotating as a unit though. It has differential rotation with latitude, at least for the surface layers.
At about 1/3 of the way under the "surface", the sun does rotate as a unit. See the wikipedia page:
http://en.wikipedia.org/wiki/Solar_rotation
So there's a more complicated calculation involved I'm sorry to say. I can't do it
Insert awesome title here
The Greek philosophers were right after all.
The surface of the Sun is not a uniformly rotating shell. What is the distribution of rotational speed with latitude? How does that effect the net centrifugal force?
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