The Sun's Unchanging Size Baffles Scientists
The sun's disk showing active region 10486, which became the largest sunspot seen by SOHO, the satellite Dr. Jeff Kuhn and collaborators used to monitor the sun's diameter.
Credit: SOHO/MDI consortium

The surface of the sun undergoes violent changes on a daily basis, but a group of astronomers has found that the size of our nearest star has been perplexingly constant in recent years.

The new study shows that the sun's diameter has changed by less than one part in a million over the last 12 years. The sun's width today is a steady 932,057 miles (1,500,000 km) across, the researchers found.

"The sun is remarkably constant," lead researcher Jeff Kuhn, the associate director of the University of Hawaii Institute for Astronomy, told "We're measuring that the diameter changes by less than a kilometer (0.62 miles).

"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," Kuhn said.

The puzzling results also contradicted other measurements of the sun taken from the ground, raising further questions on what could be causing the discrepancies.

"What this really means is that, if we believe the ground measurements, then what we're seeing is long-term fluctuations in the Earth's atmosphere," Kuhn said. "The sun is influencing the atmosphere of the Earth in very significant ways."

Kuhn's work is one of several worldwide efforts to understand the influence of the sun on Earth's climate.

"We can't predict the climate on Earth until we understand these changes on the sun," Kuhn said.

Kuhn and his colleagues used NASA's Solar and Heliospheric Observatory (SOHO) satellite to monitor the sun's diameter. They will soon repeat the experiment with much greater accuracy using NASA's new Solar Dynamics Observatory (SDO), which launched Feb. 11.

The SDO carries three instruments that constantly study the sun in unprecedented high-definition detail and could help with future examinations of solar size.

According to Kuhn, the ultimate solution to this puzzle will depend on probing the smallest observable scales of the solar surface using the Advanced Technology Solar Telescope (ATST), which is scheduled to be completed on Haleakala (on the Hawaiian island of Maui) in 2017.

"To be able to predict what the sun will do, we need both the big picture and the details," Kuhn said. "Just as powerful hurricanes on Earth start as a gentle breeze, the analogs of terrestrial storms on the sun start as small kinks on the sun's magnetic field."