Imposing Giants

In their section “exoplanets”, the CoRoT and Kepler space missions study thousands of stars. Few of them have accompanying planets. What can be made of the data gathered on all these solitary stars? It was collected and used by asteroseismology, the branch of astrophysics that studies the seismic oscillations of stars. Taken at a frequency of ten minute intervals, this data is too spaced out to detect oscillations in stars that are similar to our sun. On the other hand it is adapted to showing the oscillations of red giants, and many surprises were in store!

Launched in 2009, the American space mission Kepler was designed to detect planets around stars belonging to our galaxy. The technology on board the telescope renders it capable of detecting planets as small as our Earth orbiting stars that are similar to our sun. What is its objective? To determine whether earth-like planets are abundant in the  Milky Way. But in fact the results that have just been the object of two publications, one in Nature (1) and the other in Science (2), do not concern these exoplanets. “To understand the context and importance of these Kepler results, we must come back to the CoRoT space mission (Convection Rotation and planetary Transits) of the  CNES and its great discovery relating to red giants”, underlines the astrophysicist  Josefina Montalban, a researcher at the Stellar Astrophysics and Asteroseismology Group at the ULg’s Department of Astrophysics, Geophysics and Oceanography and co-author of the two articles.

The Satellite CoRoT

It was Corot, launched in December 2006, which troubled astrophysicists with regard to the subject of red giants. Having exhausted the hydrogen present in its core, a star begins to burn the helium in its inner layers while its outer layer dilates considerably. This phased transition is marked by a change of colour to red. This is why the star is called a red giant. Even though this will be the status of our sun in five billion years’ time, it has barely been studied by asteroseismology which has tended to focus more on sun-like stars, but also on hot stars and white dwarves, Our sun has been an excellent source for observations thanks to its proximity, which has fuelled a passion for studying its fellow-suns. Moreover, it is around stars such as our sun that astrophysicists search for exoplanets and hope to find other Earth-like planets.

(1) Timothy R. Bedding et al., Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars, Nature 471, 608–611 (31 March 2011).
(2) P. G. Beck et al., Kepler Detected Gravity-Mode Period Spacings in a Red Giant Star, Science (2011).

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