For most folks, the sun is a constant. The day follows the night, predictably, relentless. We have learned, though, of the solar temper. Its solar flares and coronal mass ejections play havoc with electrical gear on Earth. Its cyclical sunspot cycles can invigorate the planet's magnetic field.
(Image: The sun's disk showing active region 10486, which became the largest sunspot seen by SOHO, the satellite Dr. Kuhn and collaborators used to monitor the sun's diameter. Credit: SOHO/MDI consortium. SOHO is a project of international cooperation between ESA and NASA.)
But in some ways, the sun remains stunningly constant.
That's shown in research conducted by University of Hawai'i Institute for Astronomy astronomers Jeff Kuhn and Isabelle Scholl, and colleagues Marcelo Emilio of Brazil's Universidade Estadual de Ponta Grossa and Rock I. Bush, of Stanford University's Solar Physics Group.
They set out to determine whether, with all its activity, the sun also undergoes any kind of expansion and contraction, like a human chest, breathing in and breathing out.
To do so, they measured the sun's visible radius, using a device called the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) satellite.
They followed the sun over a 12-year period, to track the orb's visible size over an entire sunspot cycle.
This had been tried earlier from Earth, but observations were complicated by the planet's atmosphere. And sure enough, they write, “A significant discrepancy between ground-based observations and these results is clear.”
But the essence of their finding was that the sun is, sure enough, amazingly constant. It doesn't expand and contract significantly—less than a part in a million over the dozen years.
And was a surprise, according to Kuhn.
"This constancy is baffling, given the violence of the changes we see every day on the sun's surface and the fluctuations that take place over an 11-year solar cycle," he said.
© Jan TenBruggencate 2010
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