Probing the interiors of the Sun and other stars might sound tricky, but a technique similar to seismology provides a way. Just as seismic waves on Earth reveal information about the planet's interior, sound waves that travel through the Sun provide astronomers with clues about what is going on beneath its surface.

On page 398, Joris De Ridder, a postdoc at the Institute of Astronomy in Leuven, Belgium, and his collaborators reveal that 'asteroseismology' can be applied to the study of red giants — stars approaching the end of their evolution. “If you go to the doctor, he might listen to the sound of your heart to find out about its condition,” says De Ridder. “In a similar way, we are listening to the sound of a star to understand its interior.

Sound waves travelling through a star can be detected as variations in the star's brightness. De Ridder and his co-workers had previously tried to detect such variations in red giants using Earth-based telescopes. But the results turned out to be the subject of some debate. One group of researchers, including De Ridder, suggested that the oscillations seen in these red giants were only radial — consisting of spherically symmetrical expansion and contraction — and would therefore contain little information. Others believed that some of the oscillations were non-radial — a type that is potentially a lot more useful for asteroseismology.

Because of this work, De Ridder was asked in 2004 to join the community of European and Brazilian scientists using France's Convection, Rotation and Planetary Transits (COROT) space telescope, launched in December 2006, and to head a group responsible for observing and analysing red giants. As soon as the data from COROT arrived, De Ridder and his colleagues realized that they contained evidence of non-radial oscillations. “I was very happy to be proved wrong,” laughs De Ridder.

But the real work is just beginning. “We have collected the sounds giant stars make,” says De Ridder. “Now we have to learn how to listen.” To analyse the data, De Ridder and his team are collaborating with theoretical astrophysicists. The idea is to develop mathematical models that predict what stars' oscillations would look like on the basis of their internal properties, and then compare those models with observed data. “There was some theoretical work done before that suggested that non-radial oscillations would not be visible in red giants,” says De Ridder. “So now we have to go back to the drawing board to figure out why we do see non-radial oscillations.

One detail that De Ridder would ultimately like to extract from the data is the density of a red giant's core, because that measurement relates to a star's evolution. “Red giants are elderly stars. Our own Sun will someday become a red giant. So we want to understand how this evolution occurs,” he says.

De Ridder has now been asked to coordinate another project to observe red giants with NASA's Kepler telescope, which was launched into space earlier this year. The instrument will monitor the brightness of more than 100,000 stars for three and a half years. “Among other things, we hope to obtain information about how the stars rotate internally,” says De Ridder. “Red giants rotate very slowly, so we need to observe them for a long time.”