Ageing stars are slow on the outside but fast on the inside

8 December 2011

The core of red giants spin at least 10 times faster than the outer layers.
The core of red giants spin at least 10 times faster than the outer layers.

Scientists have made a new discovery about how old stars called 'red giants' rotate, giving an insight into what our sun will look like in five billion years.

The international team of scientists, including University of Sydney astronomers Professor Tim Bedding and Dr Dennis Stello, has discovered red giants have slowed down on the outside, while their cores spin at least 10 times faster than their outer layers.

The finding, just published in the prestigious journal Nature, tells us what the sun will look like in five billion years when it develops into a red giant.

"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," said Professor Tim Bedding from the University of Sydney's School of Physics.

Using NASA's Kepler space telescope, the team observed deep inside ageing red giants to make their discovery of the difference in rotation rate between the core and outer layers of the stars.

The team, led by Paul Beck from Leuven University in Belgium, 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," explained Dr Dennis Stello, from the University of Sydney's School of Physics.

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

"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.

"A spinning ice skater will slow down if their arms are stretched far out, like the expanded outer layers of the red giants. The ice skater will spin faster if their arms are pulled tightly to the body, like the fast spinning contracted core of red giants."

The Kepler space telescope - one of NASA's most successful space missions - is searching in the constellation Cygnus for potentially habitable planets by focussing on those similar in size to Earth.

"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," explained Dr 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," said Dr Stello.

Different waves probe different parts of the star, and by a detailed comparison of the depth to which these waves travel inside the star the team found the rotation rate dramatically increased towards the stellar core.

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.

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