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Monday, 2 July, 2001, 12:07 GMT 13:07 UK
First stellar 'heartbeat' heard
Simulation of the distribution of sound waves in Alpha Centauri A
Distribution of sound waves in Alpha Centauri A
By BBC News Online science editor Dr David Whitehouse

For the first time, sound waves have been detected racing through the outer layers of a star other than our own.

The effect, easily seen in the Sun, was picked-up in Alpha Centauri A, the nearest bright star to our Solar System.

In a series of remarkably precise measurements, astronomers from the European Southern Observatory (Eso) dissected the star's light to see the effect.

It amounts to the star, 875,000 kilometres (544,000 miles) in radius, "breathing" in and out by only 40 metres (131 feet).

Stellar heartbeat

Oscillations induced by sound waves travelling through our Sun's atmosphere have been detected since the 1960s.

Just as their Earthly counterparts, seismic waves, reveal what it is like inside the Earth, solar sound waves have become an important probe of the Sun's interior.

Alpha Centauri A
Alpha Centauri A
The sound waves, caused by turbulent motions in the Sun's outer layers, cause the Sun to oscillate at certain frequencies. The most-studied solar "heartbeat" is five minutes in duration.

In many ways, Alpha Centauri A is our Sun's twin. At just over four light-years away (39 million million km or 24 million million miles), it is the closest star visible to the unaided eye.

It is almost the same size, colour, temperature and age as our own star. It is therefore a natural target to look for oscillations.

During May, a series of observations were made with Eso's Swiss 1.2-m Leonard Euler telescope at the La Silla Observatory in South America.

Theoretical models

Alpha Cen A's light was analysed with the Coralie spectrograph, a device that has achieved considerable success in detecting planets circling nearby stars.

The new measurements, made by astronomers Francois Bouchy and Fabien Carrier, show that Alpha Cen A oscillates with a seven-minute period, remarkably close to our Sun's five-minute oscillation.

It is in full agreement with expectations from theoretical models of the two stars.

In the future, astroseismology, as the science is called, is likely to become an important probe of Sun-like stars. Soon, the state-of-the-art Harps spectrograph will be mounted on the Eso 3.6-m telescope at La Silla.

When it is in operation it will be able to search for oscillations in stars that are 100 times fainter than those detected with the Coralie spectrograph.

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