News in Science

Red giants harbour fast spinning core

In a spin They may appear big and slow on the outside, but that's just a cover for what is happening on the inside, according to a new study into red giant stars.

An international team of scientists, led by Paul Beck from Leuven University in Belgium, has discovered that the cores of red giant stars spin at least ten times faster than their outer layers.

Their finding, which appears today in the journal Nature, provides a new insight into the fate of our Sun in five billion years time.

"The heart of a star determines how it evolves, and understanding how a star rotates deep inside helps us to understand how stars like our Sun will grow old," says study co-author Professor Tim Bedding of the University of Sydney's School of Physics.

Using NASA's Kepler space telescope, the researchers analysed waves inside the stars, which appear as rhythmic variations in the surface brightness of the stars. The effect of rotation on the frequencies of the waves is so small it took the team nearly two years of almost continuous data gathering from the Kepler satellite to make their discovery.

"Red giants were once stars like our Sun, but as they age their outer layers expand to more than five times their original size and cool down significantly, so they look red," says Dr Dennis Stello, also from the University of Sydney.

"The opposite actually happens to the cores of red giants, as the core contracts and becomes extremely hot and dense."

"We've just discovered that the core spins much faster than the outer layers in these old stars, which makes sense when you consider what happens to other spinning things like, say, an ice skater performing pirouettes."

"The ice skater will spin faster if their arms are pulled tightly to the body, like the fast spinning contracted core of red giants."

Stellar seismology

The Kepler space telescope, which is searching for Earth-sized planets orbiting nearby stars, is also proving to be a useful tool for understanding the processes within stars.

"Kepler is able to detect variations in a star's brightness of only a few parts in a million, so its measurements are ideally suited to detect the tiny brightness fluctuations of stars," says Stello.

"We study these variations in brightness to work out what's going on deep inside stars. It's called asteroseismology - just as geologists use earthquakes to explore Earth's interior, we use star quakes to explore the interiors of stars."

In addition to helping us understand how stars age, asteroseismology will help Kepler's mission of discovering Earth-sized planets outside our solar system by characterising the host stars around which these planets orbit.