Why Is The Sun So Perfectly Round?
August 17, 2012

Our Sun Is Nearly The Roundest Object Ever Measured

April Flowers for redOrbit.com - Your Universe Online

On a summer's day, the sun dominates the sky looking for all the world like a big, round ball of light.

Round is exactly what it is, it turns out. It is nearly the roundest object ever measured, according to a study from the University of Hawaii's Institute for Astronomy. If you scaled the sun down to the size of a beach ball, it would be so round that the difference between the widest and narrowest diameters would be much less than the width of a single human hair.

The sun completes its rotation every 28 days, and because it doesn't have a solid surface like a planet, it should be slightly flattened. This tiny flattening has been studied over the past 50 years with many different instruments to learn about the sun's rotation, especially the rotation below its surface, which we can't see directly. Jupiter is a good example of this flattening effect. It's high rate of spin — once every 10 hours — means that it is nearly 7% wider across its equator than the distance from pole to pole.

Now, an international team of researchers has used the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) satellite to obtain what they believe is the definitive and baffling answer.

The SDO is the first mission to be launched in NASA's Living with a Star program, which is designed to understand the causes of solar variability and its impacts on Earth.

There is no atmosphere between the sun and the SDO to distort the image, so the team was able to use HMI's exquisite image sensitivity to measure the solar shape with unprecedented accuracy. Turns out, if you could shrink the sun down to a ball one meter in diameter its equatorial diameter would be only 17 millionths of a meter larger than its polar diameter. The North-South polar diameter is the rotation axis of the sun.

The team also found that the solar flattening is remarkably constant over time, and not nearly as pronounced as previously predicted. This suggests that other subsurface forces, like solar magnetism or turbulence, may be a more powerful influence than expected. The observations are key to learning about the sun's interior, which rotates at different speeds like traffic moving at different speeds on a motorway. This speed distribution can be inferred from measurements of the star's shape and the way that it wobbles.

"For years we've believed our fluctuating measurements were telling us that the sun varies, but these new results say something different. While just about everything else in the sun changes along with its 11-year sunspot cycle, the shape doesn't," said Jeff Kuhn, lead researcher for the study published in Science Express.