Song from neighbor star has familiar ring

A graphical representation of acoustic waves in the interior of a solar-like star

By Richard Stenger
CNN

(CNN) -- Sound waves running through the sun have revealed some of its innermost properties. Now for the first time, astronomers have heard the faint song inside another point of light. And the tune sounds quite familiar.

Listening to the inner sun has greatly expanded our knowledge of the energy cycles that churn through its depths. But placing an astronomical stethoscope on other stars has proven difficult because of the faintness of the waves.

Yet Swiss astronomers announced last week that they had detected tiny oscillations in a sun-like star, Alpha Centauri, which is slightly more than 4 light-years away.

RESOURCES MESSAGE BOARD

The new observations, conducted in May using the European Southern Observatory in South America, reveal that the star pulsates with a seven-minute cycle, much like our own sun.

For decades, solar scientists have listened to the depths of the sun, much the same way that geologists monitor seismic waves triggered by earthquakes to learn about the interior workings of Earth.

In the core of the sun, nuclear reactions unleash huge amounts of energy. The almost boiling gas bubbles up nearly at the speed of sound.

The furious convection in the sun generates noise, much like water on a stove that begins to boil. The sound waves resound through the interior of the sun, reflecting on the surface and making it oscillate.

The "ringing" of the sun has been well-observed, providing information about the physical conditions underneath the solar surface. The faint vibrations of other stars have proven elusive.

But Francois Bouchy and Fabien Carrier announced June 28 that they had detected "unambiguous" oscillations in Alpha Centauri A, or Alpha Cen A, the brightest member of a multiple star system in the constellation Centaurus.

In appearance, the stars are virtually identical, sharing nearly the same the mass, temperature and luminosity. But Alpha Centauri A is about 6.5 billion years old, compared to 5 billion years for our sun, and contains twice as many heavy elements.

"We are very interested in solar-like stars because we can learn a lot about the internal structure and evolution of our sun," said Carrier. "In a sense, by looking at Alpha Cen A, we can see what the sun will look like in 1.5 billion years."

The acoustic waves make the surface of Alpha Centauri A pulsate in and out, and the spectra of the stellar surface show corresponding but slight changes in velocity. The pulse periods lasted about seven minutes, compared to five minutes for the sun.

Sound, of course, cannot travel through the vacuum of space. But the oscillations of Alpha Centauri A can be translated into sounds waves. The most prominent note in the stellar symphony is an E flat, 15 or 16 octaves below audible levels, said Carrier. Shifted into the range of human hearing, the waves become what sounds like a simple, eerie organ fugue.

For the breakthrough, the two astronomers used a telescope at the European Southern Observatory's La Silla Observatory in Chile, lwhich has proven instrumental in the discovery of planets outside our solar system. Further study of the results should shed light on the mass, age and chemical composition of the star, they said.

Scientists hope to listen to more star songs in the future, including those with masses, ages, and physical properties much different than the sun or Alpha Centauri A.

After an upgrade in 2002, the La Silla telescope will listen to stars with sound waves one hundred times fainter than current instrumentation allows.

Bouchy and Carrier, who work at the Geneva Observatory in Sauverny, Switzerland, plan to publish their findings in the European journal "Astronomy & Astrophysics."

To tune in to Alpha Centauri A, visit the French language site: http://www.telejournal.ch/TSRtj. Once there, on the right, click on "Archives Video." On the next page, search (resercher) for the term 'astrophysiciens' under the date (date exacte) of 05 07 2001.