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Study finds Sun rounder than previously thought


New measurements show the shape of the Sun is more constant and spherical than expected (Source: TRACE Team, NASA)

Solar constant Scientists have discovered that our Sun is an almost perfect sphere with a far more constant shape than previously thought.

The findings appear today in the journal Science.

The Sun's solar cycle has been linked to climatic changes on Earth, with long term cooler periods coinciding with weaker solar maximums having less sunspot activity.

Scientists including Dr Jeffery Kuhn from the University of Hawaii, looked for changes in the Sun's shape which could provide clues about the strength of the solar cycle.

"Most people think of the Sun as steady, heating the Earth and affecting the weather," says Kuhn. "But the Sun's not steady, it goes through an 11 year solar cycle which also effects its brightness."

"We want to know what's causing this, and looking at changes in the Sun's shape is one way."

According to Kuhn, the researchers expected to see changes in the Sun's oblateness, or bulge, matching the solar cycle.

Oblateness is caused by the rotation of the Sun causing its equatorial diameter slightly larger than its pole-to-pole diameter.

However the Sun's shape is also affected by the way mass is distributed below its surface.

"So changes in the distribution of mass also mean changes in shape," says Kuhn. "But because the Sun's an almost perfect sphere we're talking about very small numbers."

"If you compared the 1.4 million kilometre wide Sun to a beach ball, the oblateness would be less than the width of a human hair."

Surprise finding

To gain an exact figure, Kuhn and colleagues measured the Sun for two years using NASA's Solar Dynamics Observatory spacecraft.

"Despite expectations, we found no change in the Sun's oblateness, regardless of the cycle of sunspots on its surface," says Kuhn.

The researchers also found the Sun's oblate shape is less pronounced than expected.

"The shape is even more spherical than our best models predicted," says Kuhn.

The discovery means scientists will need to rethink existing theories about the forces affecting the Sun's internal structure.

"It's very exciting because when our theories don't match our observations that's when we learn new things."

Tags: physics, stars