Swiss team hears stellar heartbeat

Sound waves resonate in the interior of a solar-like star (Picture: European Southern Observatory) swissinfo.ch

For the first time, astronomers have detected sound waves running through a star other than the Sun.

The Swiss team, which made the discovery, says it will provide them with plenty of information about the interior of the star and not just the outer layers normally visible to telescopes.

Francois Bouchy and Fabien Carrier from the Geneva Observatory picked up tiny oscillations in the star, Alpha Centauri A, at the La Silla Observatory in Chile.

Their measurements showed that the star, which is 875,000 kilometres in radius, “breathed” in and out by some 40 metres.

“When you can detect the frequency of pulsation of a star, it gives you precious information on the structure of the star itself,” said Bouchy.

Just as geologists monitor how seismic waves generated by earthquakes spread through the Earth and learn about the inner structure of our planet, the same technique works for the stars.

In the Sun, heat is bubbling up from the central regions where enormous amounts of energy are created by nuclear reactions. This turbulent convection creates noise and these sound waves then spread through the solar interior and are reflected on the surface making it oscillate.

In the case of the Sun, such waves have been investigated since the 1960s and have greatly improved our knowledge about its inner properties. However, it has proved very difficult to detect similar waves in other stars because they are much fainter.

Just over four light years away, Alpha Centauri A is the closest star visible to the naked eye. Being a near twin to the Sun and very nearby, in astronomical terms, made it the ideal candidate in the search for tiny oscillations.

The acoustic waves make the surface of the star periodically pulsate in and out. The new measurements from the European Southern Observatory show that it pulsates with a seven-minute cycle, similar to the Sun’s five-minute oscillation.

The observations of Alpha Centauri A were conducted in May. The star’s light was analysed with the Coralie spectograph, a device better known for its successful detection of planets circling nearby stars.

This branch of modern astrophysics, known as asteroseismology, looks set to become increasingly important.

A state-of-the art Harps spectrograph, which will be installed on a telescope at La Silla at the end of 2002, will be able to observe stars which are 100 times fainter than those now detected with Coralie.

“With Harps, we hope to make such observations on a large sample of stars,” said Bouchy. With the Coralie spectograph that we use during this campaign, we are limited to bright stars. With Harps it is a new field of astrophysics, we hope to begin.”

by Vincent Landon