Not so smooth: "Cantaloupe ridges" on the Sun. The glowing white magnetic network is what decreases the roundness of the Sun during times of high solar activity. (Los Angeles astronomer Gary Palmer took this picture in 2005, using a violet calcium-K solar filter.)

Credit: NASA/Gary Palmer

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SYDNEY: Scientists using a space observatory orbiting the Sun, have calculated the star's roundness with unprecedented precision, and found that it is not a perfect sphere. The research could lead to techniques for measuring the Sun's elusive internal core.

Though the Sun is more perfectly round than any of the planets, their new study – reported today in the U.S. journal Science – shows that in years of high solar activity it develops a thin "cantaloupe skin" that significantly changes how wide it is around the equator.

Strong gravity

"The Sun is the biggest and smoothest natural object in the solar system, perfect at the 0.001 per cent level because of its extremely strong gravity," said study lead author Hugh Hudson of the University of California, Berkeley, in the USA. "Measuring its exact shape is no easy task."

The team achieved the calculation by analysing data from NASA's Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), an X-ray/gamma-ray space telescope launched in 2002 to study solar flares.

Although RHESSI was never intended to measure the roundness of the Sun, it has turned out ideal for the purpose. The telescope observes the solar disk through a narrow slit and spins at 15 rpm.

The telescope's rapid rotation and high rate of sampling have made it possible for investigators to trace the shape of the Sun with a remarkably low error rate. The technique is particularly sensitive to small differences in the Sun's polar and equatorial diameters.

Ridges like melon skin

"We have found that the surface of the Sun has rough structure: bright ridges arranged in a network pattern, as on the surface of a cantaloupe [melon] but much more subtle," said Hudson. During periods of high solar activity, these ridges emerge around the Sun's equator, very slightly brightening and fattening the "stellar waist," he added.

At the time of RHESSI's measurements in 2004, ridges increased the Sun's apparent equatorial radius by the equivalent of the width of a human hair viewed one mile away.

"That may sound like a very small angle, but it is in fact significant," says Alexei Pevtsov, part of the RHESSI science team at NASA headquarters in Washington DC. Tiny departures from perfect roundness can, for example, alter the Sun's gravitational pull on Mercury and might skew tests of Einstein's Theory of Relativity, which depends on careful measurements of the orbit of the inner planets.