The sounds of the stars

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Data from NASA’s Kepler space telescope have revolutionized the search for planets outside the Solar System — and are now doing the same for asteroseismology.

Ron Cowen
Most astronomers gaze at the heavens and see stars. William Chaplin hears an orchestra — a celestial symphony in which the smallest stars are flutes, the medium-sized ones are trombones and the giants are reverberating tubas.

The sounds are internal vibrations that reveal themselves as a subtle, rhythmic brightening and dimming of a star, explains Chaplin, an astrophysicist at the University of Birmingham, UK, and a specialist in asteroseismology. These waves provide information that astronomers can’t get in any other way: triggered by the turbulent rise and fall of hot gases on the star’s surface, the vibrations penetrate deep into the stellar interior and become resonating tones that reveal the star’s size, composition and mass (see ‘Celestial music’). So by watching for the characteristic fluctuations in brightness, says Chaplin, “we can literally build up a picture of what the inside of a star looks like”.

Better still, he adds, asteroseismologists are now hauling in the data wholesale. After years of being hampered by Earth’s turbulent atmosphere, which obscures the view of the Universe and has limited asteroseismology to about 20 of the brightest nearby stars, researchers have been astonished by the trove of information coming from a new generation of space observatories. Thanks to the French-led Convection, Rotation and Planetary Transits (COROT) space telescope, launched in 2006, and NASA’s Kepler space telescope, launched in 2009, they can now listen in on hundreds of stars at a time.(….)
Stellar serendipity
Asteroseismology isn’t the main mission of either COROT or Kepler: they are intended to hunt for planets outside the Solar System (exoplanets) that have roughly the size and orbital radius of Earth. But because they both look for the tiny dip in brightness caused when a planet transits, or passes in front of, its parent star, they both have to record a drop in stellar brightness of no more than 1 part in 1,000. And that, in theory, makes them able to detect the effects of the stellar sound waves.
Before launch, no one could say whether the satellites would make good on this. Kepler’s exoplanet search has, in fact, been hindered by stellar oscillations that obscure transits, but are caused by magnetic activity1, so are unrelated to sound waves. Acoustic oscillations and transits don’t interfere with each other: sound waves cause the brightness of Sun-like stars to vary on time scales of 5–15 minutes, whereas planetary transits last for hours. So the planners for both COROT and Kepler were happy to include asteroseismologists in their mission teams. “We are riding on the back of the planet hunters,” says Douglas Gough, an asteroseismologist at the University of Cambridge, UK.

As it turned out, the sound-wave data came down in an avalanche — especially from Kepler, which has a 0.95-metre-aperture telescope — nine times the sensitivity of COROT’s — plus the ability to look at a larger group of stars for a longer period of time than COROT………………
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Written by physicsgg

January 5, 2012 at 10:22 pm


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