The Really Round Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields. The sun rotates every 28 days, and because it doesn't have a solid surface, it should be slightly flattened. This tiny flattening has been studied with many instruments for almost 50 years to learn about the sun's rotation, especially the rotation below its surface, which we can't see directly. Jeff Kuhn and Isabelle Scholl (Institute for Astronomy, University of Hawaii at Manoa) and others have used the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory satellite to find how astonishingly round the sun really is.
The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye. It is a near-perfect sphere. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely.
All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences.
Because there is no atmosphere in space to distort the solar image, they were able to use HMI's exquisite image sensitivity to measure the solar shape with unprecedented accuracy. The results indicate 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.
They also found that the solar flattening is remarkably constant over time and too small to agree with that predicted from its surface rotation. This suggests that other subsurface forces, like solar magnetism or turbulence, may be a more powerful influence than expected.
Kuhn, the team leader and first author of an article published today in Science Express, said, "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."
For further information see Round Sun.
Sun image via NASA.