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An international team of astronomers, including Dr Saskia Hekker of the University of Amsterdam (UvA), has succeeded in looking deep inside ‘red giants’ and deciding which of these giant stars are still in their infancy, which are reaching puberty and which ones are dying.

Copyright: NOVA

An international team of astronomers, including Dr Saskia Hekker of the University of Amsterdam (UvA), has succeeded in looking deep inside ‘red giants’ and deciding which of these giant stars are still in their infancy, which are reaching puberty and which ones are dying. This discovery, published in the scientific journal 'Nature' on Thursday 31 March 2011, was made using the Kepler space telescope and provides new clues for research into the evolution of stars like our own Sun

Red giants are stars that have reached the end of their life. Our own sun will reach that stage in approximately five billion years time. By that time, the Sun will be more than 10 times larger than it is now, and about 50 times as bright. The color will then have changed from a yellow hue to a red hue, hence the name red giant. The stock of hydrogen in the core is runs out, and the star burns up the hydrogen in a shell around the core. By the end of its life the red giant begins to burn up the helium in its core.

Turbulent movements

The astronomers have studied the light of hundreds of red giants with unprecedented accuracy for almost one year and much has been learned about their cores. ‘The changes in brightness on the surface of the star are the result of turbulent movements in the interior of the star, which continuously cause starquakes,’ says lead author Professor Tim Bedding of the University of Sydney. The sound waves generated as a result of this activity travel to the interior of the star and back again. If the conditions are right, the sound waves interact with other waves in the helium core of the star. It is these mixed fluctuations (oscillations) which say something about the age of the red giant. The very careful measurements of small variations in this process show that some stars burn up helium, while other stars are still only burning up hydrogen.

Co-author Saskia Hekker from the UvA is pleased with the results: ‘On the basis of theoretical models, we knew of the existence of these oscillation patterns, but also that it would be very difficult to observe them. The fact that we have now managed to do this means that we can see what age a red giant star has reached. We can also examine the structure of these stars all the way to the core, which was hitherto not possible.’

Publication details

Timothy R. Bedding, Benoit Mosser, Daniel Huber, Josefina Montalbán, Paul Beck, Jørgen Christensen-Dalsgaard, Yvonne P. Elsworth, Rafael A. García, Andrea Miglio, Dennis Stello, Timothy R. White, Joris De Ridder, Saskia Hekker, Conny Aerts, Caroline Barban. Kevin Belkacem, Anne-Marie Broomhall, Timothy M. Brown, Derek L. Buzasi, Fabien Carrier, William J. Chaplin, Maria Pia Di Mauro, Marc-Antoine Dupret, Søren Frandsen, Ronald L. Gilliland, Marie-Jo Goupil, Jon M. Jenkins, Thomas Kallinger, Steven Kawaler, Hans Kjeldsen, Savita Mathur, Arlette Noels, Victor Silva Aguirre & Paolo Ventura: ‘Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars’, in: Nature, 31 March 2011.