UH finds way to derive inner volatility of stars
By Jan TenBruggencate
Advertiser Science Writer
University of Hawai'i astronomers have found a way to determine how turbulent a star's insides are by looking at its surface.
The technique uses calculations based on the amounts of the light elements beryllium and lithium that are detected at the surface.
The information is "awfully interesting" to astronomers, but probably is of little importance to average folks on Earth, said University of Hawai'i astronomer George Herbig.
University of Hawai'i professor Ann Boesgaard led a team that helped solve a mystery about star characteristics, using the Keck I telescope atop Mauna Kea.
Researchers know that some stars have reduced amounts of lithium detectable at their surfaces, and some have reduced amounts of both lithium and beryllium. Both elements can be destroyed in the exceedingly high heat deep inside stars — lithium at 2 million degrees Kelvin and beryllium at 3 million degrees.
In the down-currents and up-currents that occur in a star's interior, these elements are carried down from the cooler surface to the hotter lower depths, and if they're not destroyed, back up to the surface. If the circulation is deep enough, the lithium is destroyed. If it's even deeper, the beryllium is destroyed, too.
One of the mysteries is how to determine what causes one star to have deeper circulation than another. In the past, Herbig said, astronomers focused on a star's age and its mass. But Boesgaard's team concluded there needs to be something else, and they figure it's how fast a star is spinning.
"It looks like not simply a matter of mass and age, but depth of convection also depends on a third parameter — how fast a star is spinning," said Herbig, a former professor of Boesgaard's.
Boesgaard was on the Mainland on Monday presenting a paper on the findings. She worked on the lithium-beryllium project with Eric Armengaud, a French student working in Hawai'i, and Jeremy King, an assistant professor at the University of Nevada at Las Vegas.
Our own sun has a surface temperature of about 6,000 degrees and a core temperature of between 10 million and 15 million degrees. It has little lithium detectable at the surface — only about 1 percent of the amount originally believed to have been present. But most of its beryllium remains.
This suggests that the active circulation of our sun extends down only to the 2 million-degree zone, but not much deeper.
"This is a 'skin' phenomenon," Herbig said.
It also takes a lot of time to destroy these elements. Stars must be 200 million years old or more before they appear to experience what astronomers are calling lithium and beryllium "dip."
