Why is the sun so round?

Posted by Kate Taylor

We all know the sun is round. But according to astronomers, it's a lot rounder than it ought to be, and now they're trying to work out why.

The sun rotates every 28 days. And, because it doesn't have a solid surface, it should be slightly flattened by that rotation. But an international team of scientists using the Solar Dynamics Observatory satellite have come up with the most accurate measurement yet, showing just how perfectly round it is.

Data from the SDO's Helioseismic and Magnetic Imager (HMI) indicates that if the sun were shrunk to a ball one meter in diameter, its equatorial diameter would be only 17 millionths of a meter larger than the diameter through its North-South pole, which is its rotation axis.

If scaled to the size of a beach ball, it would be so round that the difference between the widest and narrow diameters would be much less than the width of a human hair.

The team also found that the solar flattening is remarkably constant over time - and too small to agree with that predicted from its surface rotation. The implication is that other subsurface forces, like solar magnetism or turbulence, may be exerting more of an effect than expected.

"For years, we've believed our fluctuating measurements were telling us that the sun varies, but these new results say something different," says Jeff Kuhn of the University of Hawaii at Manoa.

"While just about everything else in the sun changes along with its 11-year sunspot cycle, the shape doesn't."