Research also said its shape is more stable than thought, hinting that its outermost part may be rotating more slowly than expected.
Call it a boost to the sun's self-image. Researchers have found that our parent star's midsection is considerably slimmer than believed. Not only that, the sun's overall shape doesn't change as much during the 11-year sunspot cycle as scientists had thought -- a hint that the outermost part, a few percents of the sun, may be rotating more slowly than expected.
Rotational forces cause any whirling blob of gas to be flattened at the poles and bulge at the equator, forming a shape called an oblate spheroid. In our solar system, fast-spinning Saturn is the most squished of the planets, with a polar diameter about 7,000 miles less than its average equatorial diameter. Even rocky planets can be oblate; Earth, which rotates once every 24 hours, has an equatorial bulge of more than 26 miles.
Even though the sun rotates relatively slowly -- once every 27 days or so -- it, too, is oblate, said Jeffrey Kuhn, a solar physicist at the University of Hawaii's Institute for Astronomy in Pukalani. But the distance of the sun's surface from its center depends on several factors other than its gravitational field, he said, including the sun's inner rotation; the convection and turbulence in its outer layers; and magnetic forces, particularly those around sunspots.
Previous data have suggested that the sun's oblateness varies with the solar cycle, with the overall shape subtly changing as the number of sunspots waxes and wanes during the 11-year period. But data recently gathered by sensors aboard NASA's Solar Dynamics Observatory, a satellite that watches the sun from geosynchronous orbit, revealed the sun's shape is steadier and more nearly spherical than previously thought, Kuhn and his colleagues reported online in Science. The satellite takes more than 15,000 images of the sun each day, but for the new study the team analyzed images taken only twice each year, when the satellite's instruments are calibrated.
Analyses of data gathered during five calibration cycles over the past two years -- an interval when solar activity has risen from near nil to near maximum, with 90 or more sunspots visible on the surface each day -- indicate that the sun's oblateness is remarkably steady despite the large changes in magnetic activity at its surface. Results suggest the sun's equatorial bulge is only 7 miles or so, compared with its average diameter of nearly 870,000 miles. Previous data suggesting that the sun's shape varies during a solar cycle, which were gathered by ground-based instruments, may have been affected by atmospheric turbulence -- the same phenomenon that makes distant stars twinkle, Kuhn said. Douglas Gough, an astrophysicist at University of Cambridge in England, called the work "the best attempt so far" at measuring the sun's oblateness.