Peering deep to see red (stars)

IOWA STATE (US) — Astronomers are getting a clear enough view of changes in star brightness from NASA’s Kepler Mission that they are now able to see what’s happening inside red giant stars.

“No one anticipated seeing this before the mission launched,” says Steve Kawaler, professor of physics and astronomy at Iowa State University. “That we could see so clearly down below a red giant star’s surface was unexpected.”

Kepler tracks tiny, regular changes in star brightness that resemble steady drumbeats at different precise rhythms. Each rhythm can be thought of as an individual tooth of a comb, Kawaler says.

Astronomers have studied these oscillations from ground-based telescopes to determine star basics such as mass and radius, but recently noticed departures from the steady patterns in the data—similar to dandruff on the comb.

These other patterns are caused by gravity mode oscillations and allow researchers to probe a star’s core. The result, according to a paper published in Science, is offering information about the density and chemistry deep inside a star.

Researchers can also tell whether a red giant star burns hydrogen in a shell surrounding the star or whether it has evolved to an age that it burns helium in the core. That’s something astronomers hadn’t been able to determine before Kepler, according to another paper published in Nature.

“The stars burning helium in the core survived a helium flash. That transformation from stars burning a hydrogen shell is mysterious. We think it happens quickly and perhaps explosively. Now we can tell which stars have done that and which stars will do that,” Kawaler says.

That information will help astronomers better understand the life cycle of red giant stars. In about 5 billion years, the sun will evolve into a red giant.

Kepler is like having an instrument that simultaneously studies waves for clues about the ocean’s surface and listens beneath the surface for clues about the ocean depths, says Kawaler.

“But you have to listen very carefully. And you have to have an instrument sensitive enough to see and hear both.”

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