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 almost 12,000 kilometers, or 10 percent, less than its average equatorial diameter.
Even rocky planets can be oblate; Earth, which rotates once every 24 hours, has an equatorial bulge of almost 43 kilometers.
Even though the sun rotates relatively slowly-only once every 27 days or so-it, too, is oblate, says 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 notes, 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 onboard NASA’s Solar Dynamics Observatory, a satellite that continuously watches the sun from geosynchronous orbit, reveals the sun’s shape is steadier and more nearly spherical than previously thought, Kuhn and his colleagues report online Thursday in Science.
SDO takes more than 15,000 images of the sun each day, but for this new study the team analyzed images taken only twice each year, when the satellite’s instruments are calibrated.