London: Scientists claim to have made a new discovery about how old stars called "red giants" rotate, thus giving an insight into what the sun would actually look like in five billion years.

An international team has found that red giants have slowed down on the outside, while their cores spin at least 10 times faster than their outer layers, a finding, which says what the sun will look like 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 Prof Tim Bedding from University of Sydney, a team member.

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," said another team member Dr Dennis Stello.

"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.

"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," he said.

The findings have been published in the latest edition of the 'Nature' journal.