The Kepler Space Telescope doesn’t just look for planets. By listening to subtle vibrations of the stars in its field of view, the telescope is recording a stellar symphony that gives an unprecedentedly accurate view of the inner lives of stars.

“We can say Kepler is listening to thousands of musicians in the sky,” said Daniel Huber, a graduate student at the University of Sydney.

Kepler recorded thousands of red-giant stars humming in response to their internal rumblings. In this recording, which was scaled up to the range of human hearing, the deeper, louder tones correspond to larger stars. Audio: Daniel Huber/KASC

To search for planets, Kepler stares unblinkingly at a single patch of sky and waits for a star to slightly dim, a sign that a planet is crossing in front of the star. The telescope’s sharp eyes are sensitive enough to detect stars’ natural brightness variations, the result of vibrations inside the stars.

“Sound waves travel into the star and bring information up to the surface, which Kepler can see as a tiny flickering in brightness of the star,” said astronomer Travis Metcalfe of The National Center for Atmospheric Research.

That flickering “has an underlying order like the notes of a musical instrument,” he added. “We essentially measure the tone of these musical notes from the starlight.”

In the same way that a cello sounds deeper than a violin, larger stars vibrate with lower frequency — or deeper tone — than smaller stars. These vibrations let astronomers measure the radius of a star to within a few percent.

The tones also reveal the stars’ ages. Young stars burn brightly by converting hydrogen into helium and energy deep in their cores, but eventually the hydrogen reserves run out. Sound waves pass through dense helium cores more quickly than through hydrogen, changing the tone heard at the surface and giving a good sense of how long the star has been working as a hydrogen furnace.

Metcalfe presented a star with the uninspiring name KIC 11026764 as the most accurately characterized star in the universe, aside from the sun. The star is 5.94 billion years old, more than one billion years older than the sun, and 2.05 times the sun’s radius. The measurements also show that this star isn’t burning hydrogen fuel anymore — the core is almost entirely helium and is slowly contracting. Over the next several million years, the star will puff off its outer layers of gas until it becomes a bloated red-giant star.

This star does not host any planets, but the same techniques could be used to characterize stars that do. Knowing stars’ sizes and ages could help pin down the sizes and ages of their planets.

“Our knowledge of the planets that Kepler discovers is only as good as our knowledge of the stars that they orbit,” said Kepler co-investigator Natalie Batalha of San Jose State University in California.

Kepler also listened to the vibrations of more than a thousand red-giant stars ranging from a few to several dozen times bigger than the sun. Red giants are the endgame of stellar evolution. In about 6 billion years, the sun will evolve into a red giant as well.

“Kepler allows us to study the future life of our sun in much greater detail than ever before,” Huber said.

Bigger red giants give off a lower, louder tone than smaller red giants, the study confirmed. Huber’s results are published in two papers on arXiv.org.

The new observations may also help solve a stellar riddle that has puzzled astronomers for a hundred years. A class of stars called RR Lyrae variable stars can grow brighter or dimmer by a factor of two over just a few hours. Astronomers can measure how bright these stars actually are, as opposed to how bright they look from Earth, making them excellent “standard candles” to determine distances to other objects in the universe.

But some of these stars also show a weird modulation in their brightening and dimming, called the Blazhko effect. When this effect was discovered a century ago, it was thought to be rare and unusual.

The new Kepler data suggests that the effect “may be a rule rather than an exception,” said astronomer Katrien Kolenberg of the Institute of Astronomy in Vienna, Austria.

Kepler observed the prime example of these shifty stars, known simply as RR Lyrae, plus 40 of its counterparts: “the most accurate and extensive measurement of these stars ever made,” Kolenberg said. The observations showed a second variation in the stars’ brightness that happens at half the speed of the main variation, which Kolenberg believes is related to the mysterious Blazhko effect.

“It is striking that only a few months of uninterrupted Kepler data of the star RR Lyrae uncover phenomena that were never detected before, not even with a century of high-quality ground-based data,” she said. “This is a dramatic overhaul in our understanding of RR Lyrae stars.”

Because the Kepler Astroseismic Science Consortium is an international group, NASA, which operates the telescope, cannot give it funding. Last summer the consortium began raising research funds through an “Adopt a Star” program, in which anyone can claim a star as their own in Google Sky.

Image: Carter Roberts/Eastbay Astronomical Society.

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