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NASA's Kepler Mission Helps Reveal the Inner Secrets of Giant Stars for the First Time
March 31, 2011

Stellar evolution chart Click image for full-resolution.
Studies of oscillation frequencies of many stars with very high precision gives insights into stellar evolution by knowing how the cores of stars change (starting in the bottom left corner in the sequence above) from hydrogen fusion-burning cores to helium fusion-burning cores, with intermediate stages where hydrogen fusion-burning shells expand into red giant sizes. A Hydrogen shell fusion star and a Helium core fusion star are indistinguishable when looking only at their surface properties. On the inside, they are radically different.
Image credit: Thomas Kallinger, University of British Columbia and University of Vienna

Ages of stars Click image for full-resolution.
Kepler, the paparazzi of the celestial stars, takes snapshots of oscillations that can can be used to tell the size and age of the star. As a star "burns" hydrogen in fusion reactions, helium builds up in the star's core. Helium is more dense than hydrogen, and since waves travel more quickly through denser material, waves travel faster through the core as helium builds up there. Waves that go straight through the center (white) line and waves that bounce around outside the core (colored lines) produce oscillations in surface brightness.
Image credit: Travis Metcalfe, National Center for Atmospheric Research
University of Sydney astrophysicists are behind a major breakthrough in the study of the senior citizens of our galaxy: stars known as Red Giants. Using high precision brightness measurements taken by the Kepler spacecraft, scientists have been able to distinguish profound differences inside the cores of stars that otherwise look the same on the surface.

The discovery, published in the latest edition of the journal Nature and made possible by observations using NASA's powerful Kepler space telescope, is shedding new light on the evolution of stars, including our own sun.

The paper's lead author, the University of Sydney's Professor Tim Bedding, explains, "Red giants are evolved stars that have exhausted the supply of hydrogen in their cores that powers nuclear fusion, and instead burn hydrogen in a surrounding shell. Towards the end of their lives, red giants begin burning the helium in their cores."

The Kepler space telescope has allowed Professor Bedding and colleagues to continuously study starlight from hundreds of red giants at an unprecedented level of precision for nearly a year, opening up a window into the stars' cores.

"The changes in brightness at a star's surface is a result of turbulent motions inside that cause continuous star-quakes, creating sound waves that travel down through the interior and back to the surface," Professor Bedding said.

"Under the right conditions, these waves interact with other waves trapped inside the star's helium core. It is these 'mixed' oscillation modes that are the key to understanding a star's particular life stage. By carefully measuring very subtle features of the oscillations in a star's brightness, we can see that some stars have run out of hydrogen in the center and are now burning helium, and are therefore at a later stage of life."

Astronomer Travis Metcalfe of the US National Center for Atmospheric Research, in a companion piece in the same Nature issue which highlights the discovery's significance, compares red giants to Hollywood stars, whose age is not always obvious from the surface. "During certain phases in a star's life, its size and brightness are remarkably constant, even while profound transformations are taking place deep inside."

Professor Bedding and his colleagues work in an expanding field called asteroseismology. "In the same way that geologists use earthquakes to explore Earth's interior, we use star quakes to explore the internal structure of stars," he explained.

Professor Bedding said: "We are very excited about the results. We had some idea from theoretical models that these subtle oscillation patterns would be there, but this confirms our models. It allows us to tell red giants apart, and we will be able to compare the fraction of stars that are at the different stages of evolution in a way that we couldn't before."

Daniel Huber, a PhD student working with Professor Bedding, added: "This shows how wonderful the Kepler satellite really is. The main aim of the telescope was to find Earth-sized planets that could be habitable, but it has also provided us with a great opportunity to improve our understanding of stars."

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