A mystery surrounds the shape of the Sun - it is just too perfectly round, say scientists.
In fact the Sun turns out to be one of the roundest objects ever measured.
Scaled down to the size of a beach ball, the difference between the Sun's widest and narrowest diameters would be far less than the width of a human hair.
Having no solid surface, the Sun's rotation should make it slightly flattened.
But the new measurements show that the flattening is much smaller than expected.
If the Sun was shrunk to a ball one metre across, its equatorial diameter would be only 17 millionths of a metre larger than the diameter between its poles.
Scientists analysed data from the Solar Dynamics Observatory satellite which is studying the Sun.
Previously it was thought that the shape of the Sun varied in cycles, but the new findings show it is remarkably constant.
Lead researcher Dr Jeff Kuhn, from the University of Hawaii, 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."
The research is published in Science Express, the online version of the journal Science.
Sub-surface solar forces, such as magnetism and turbulence, may be having a more powerful influence on the Sun's shape than was previously thought, said the scientists.
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