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 that "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."

	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.

The Kepler space telescope has allowed Professor Bedding and colleagues to continuously study star light from hundreds of red giants at an unprecedented level of precision for nearly a year, giving 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 centre and are now burning helium, and therefore at a later stage of life."

	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.

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 emerging 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."

	Previous studies of red giants using ground-based telescopes were limited by disturbances in the atmosphere and interruptions due to daytime/nighttime cycles. Using the Kepler telescope we have detected oscillations in more than 1000 giant stars at a precision never obtained before for such a large set of data. The periods of those oscillations are used to study the interiors of these giant stars, which represent the future life of our Sun. Thanks to Kepler we've found that all red giants show stellar oscillations that help us accurately determine the range of sizes of red giants.

	Variations in brightness of a star are caused by oscillations within the star. The oscillations we observe reveal information about the internal structure of the star in a similar way as seismologists use earthquakes to study the core of the Earth. The oscillations can be thought of as a a sort of pulse of the star or as "tones" like musical instruments produce tones. The many red giant stars that we've studied in the Kepler starfield have many different frequencies and overtones. Larger stars have deeper and louder tones. Image from the 2010 Oct 26 KASC Press Conference

Play sound file: a Kepler Red Giant Concert.
[You can hear a progression from smaller red giants (with higher frequency quieter oscillations) to larger red giants (with lower frequency louder oscillations). With all the star oscillations studied by the Kepler Asteroseismic Science Consortium, it's as if Kepler is listening to thousands of musicians in the sky---a symphony of red giants.]

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."

	The results from the Kepler Asteroseismic Science Consortium (KASC) are of great interest to Kepler for a reason vital to the mission: our knowledge of the planets that Kepler discovers is only as good as our knowledge of the stars that they orbit. The Consortium's ability to measure the sizes of stars allows for accurate determination of the sizes of the planets orbiting those stars. For this reason KASC is an extremely valuable international partner with NASA's Kepler mission. Image from the 2010 Oct 26 KASC Press Conference

Interview contacts: Professor Tim Bedding, 9351 2680, 0425 310 476

Media enquiries: Kath Kenny, 0478 303 173, 9351 1584, kath.kenny@sydney.edu.au
Victoria Hollick, 0401 711 361, 9351 2579, victoria.hollick@sydney.edu.au

See University of Sidney press release.

See also

• Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars, by Timothy R. Bedding et al, Nature 471, Pages: 608–611, 2011 March 31, DOI: 10.1038/nature09935
• Astrophysics: The inner lives of red giants, by Travis S. Metcalfe (Nature 471, Pages: 580–581 2011 March 31 DOI: 10.1038/471580a)
• Kepler-Detected Gravity-Mode Period Spacings in a Red Giant Star, by Paul G. Beck et al, Science Express 2011 March 17 DOI: 10.1126/science.1201939

Kepler Asteroseismic Science Consortium is hosted by the
Danish Asteroseismology Center
(http://astro.phys.au.dk/KASC/seismology/seism.html)

NASA’s Kepler Mission
Michele Johnson, Public Affairs Officer
650-604-6982
michele.johnson@nasa.gov

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