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Stellar Stethoscope Pushes the Envelope

By: Adrian Cho
By: Adrian Cho

Like a gigantic bell, the sun reverberates at myriad frequencies, and by studying such oscillations, scientists have deciphered the structure of the star closest to our planet and our hearts. Now, new satellite measurements of distant stars may pave the way to similar studies of their interiors, too.
The results come from the COnvection ROtation and planetary Transits (CoRoT) satellite. Researchers say they open an era of precision studies in a field called astroseismology. "This paper is a milestone," says Jørgen Christensen-Dalsgaard, an astrophysicist at the University of Aarhus in Denmark.

Just like Earth, the sun vibrates constantly as sound waves ricochet through its interior. And just as a bell rings only at specific pitches that depend on its shape and the material from which it's made, the sun oscillates only in particular patterns, or "modes," with specific frequencies. For decades, solar scientists have studied those modes, and by combining the data with computer models, they have been able to determine, for example, that the relatively solid core of the sun extends from its center to 71% of its radius. Above that, the sun roils as hotter material rises and cooler material falls. This convection also gives the surface of the sun its mottled, grainy appearance.

Observatoire de Paris-LESIA/Eric Michel/ Institut d'Astrophysique de Paris/Jean Mouette

Stellar songs. Each of the three stars has its own vibrations; they have been transposed by 18 octaves in this video so human ears can hear them.
France's CoRoT satellite, launched in December 2006, has taken the best look yet at such vibrations in other stars. The orbiting telescope peered at three stars a couple hundred light-years away that weigh 20% to 40% more than the sun. It tracked one of these stars for more than 60 days and the other two for 156 days. Researchers constantly monitored each star and noted changes in its brightness as small as a part in a million. Then they teased out the precise frequencies at which the stars were pulsing, a process similar to breaking a musical chord into its component notes.
The data indicate that in these stars, the amplitude of oscillations is about 50% bigger than in the sun, as they report this week in Science. Still, models had predicted even larger oscillations for the distant suns, which suggest the models may need some revising. From the variations in brightness, researchers also showed that the surfaces of the three stars are up to three times grainier than the sun.

To be sure, others have seen such oscillations before. In recent years, both ground- and space-based telescopes have seen clear evidence of oscillations in stars roughly similar to the sun. But the new measurements open up the possibility of testing models of stellar interiors in detail, says Eric Michel, an astrophysicist with the Observatory of Paris and lead author on the paper.

"The level of precision is indeed unprecedented and necessary in this field," says Michel Breger, an astrophysicist at the University of Vienna in Austria. But CoRoT may not enjoy its advantage for long. NASA's Kepler satellite, scheduled for launch next year, will also study such vibrations, and researchers in Europe are hoping the European Space Agency will fund a satellite called PLATO that will make even more precise measurements.

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