The Sun is Round

SUBHEAD: Even the Earth bulges a little as it spins, but the Sun...the Sun is nearly perfectly round. By Jan TenBruggencate on 21 August 2012 for Raising Islands - ( Image above: Near sunset off Puolo Point in Hanapepe, Kauai. Photo by Juan Wilson. The Sun is rounder than almost anything else--and that's a surprise. University of Hawai`i scientists were among those who recently conducted state-of-the-art measurements using a device called the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory satellite. They found that it is amazingly round. The sun spins, an activity that tends to widen objects at the equator and flatten them between the poles. But not Ol’ Sol. The solar research team on this project includes Jeff Kuhn and Isabelle Scholl of the Institute for Astronomy, University of Hawaii at Manoa, Rock Bush of Stanford University, and Marcelo Emilio, of the Universidade Estadual de Ponta Grossa, Brazil. They reported their findings in the August 16, 2012, Science Express in an article entitled The Precise Solar Shape and Its Variability. The abstract is here.
First, the measurement. They found that if you shrank the Sun to a ball one meter across, then the distance measured through the poles would be only 17 millionths of a meter less than the distance measured at right angles to the poles, through the equator—the equatorial diameter. A sheet of paper is five or six times thicker than that. Most human hair is significantly thicker. Our Sun spins fully every 28 days and it ought to flatten more than that, according to predictions based on that rotation. With all the sunspots and moving plasma and other stuff, you might also think there would be lots of variance in its shape. There isn’t. "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,” lead author Kuhn said. In fact, the sunspot cycle seems to have no role, the authors say. It is “completely unaffected by the solar cycle variability seen on its surface.” Their best guess: subsurface forces like solar magnetism may be having a much more powerful impact than anyone predicted. The sun's massive gravity, along with other subsurface forces, may counteract the effects of its spin, and keep it in a rounder shape. This work was supported by NASA grants to Stanford University and the University of Hawaii. More about the Solar Dynamics Observatory here and here.
The University of Hawaii press release on the discovery is here.

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