Listening to stars' 'songs'

The National Aeronautics and Space Administration (NASA) launched Kepler in 2009 with the primary mission of seeking out Earth-like alien worlds. However, the spacecraft's photometer provided astronomers with data of such a quality that they changed their view of how stars work.

At the Harvard–Smithsonian Center for Astrophysics (CfA), scientists from around the world are working on the analysis and interpretation of Kepler's data for more than 150 000 stars. The 'Space asteroseismology &RR Lyrae stars' (SAS-RRL) project was initiated to study a class of pulsating stars used to measure cosmological distances.

The first member of the class, the RR Lyrae star has been studied for more than 100 years. Its brightness oscillates with a time period of about 13.5 hours. During this period, smaller, cyclic changes occur. SAS-RRL researchers found that this behaviour, known as the Blazhko effect, is a rule rather than an exception in RR Lyrae stars.

Project researchers also found signs of the RR Lyrae star's period doubling in data from the French-led Convection, Rotation and planetary Transits (COROT) mission. The stars' light curves were then processed and analysed with standard Fourier technique to follow the time evolution of oscillations' periods.

The varying periods suggest that brightness variations result from a complicated interplay of radial and non-radial pulsations of the star's surface. The omnipresence of smaller and more frequent oscillations in all types of RR Lyrae stars open the way to using their brightness for study of their internal structure.

Stars' internal structures can be probed with asteroseismology because oscillations of different frequencies penetrate to different depths. The next step was to exploit the scientific potential of these observations and estimate the stars' masses and ages to test stellar evolution theory.

For this purpose, X-ray observations from NASA's Chandra X-ray Observatory (CXO) were analysed to derive a set of constraints on stars' properties and the pulsations' energies. The findings have been published in peer-reviewed journals and presented at international conferences, raising the visibility of European research within the global scientific community.