Dynamics and internal structure of the Sun and stars
Stars, including the Sun, are often affected by oscillations around their equilibrium structure (eigen modes of oscillations) observed as periodic movements of their surface and a variation of star brightness at the same periods. The velocity of movement of the photosphere can be measured by Doppler effect, and the photometry accuracy allows to distinguish oscillations in brightness. These measurements are the basis of the observational heliosismology regarding the Sun and stars for asterosismology in general.
These oscillations are the results of two types of forces which tend to restore equilibrium after any disturbance: pressure, giving rise to sound waves, and the Archimedes force, which gives rise to gravity waves.
The study of eigen modes of vibrations of stars yields information essential to the understanding of their physical properties. Such modes depend on the internal structure of the star: variations in density, temperature, chemical composition (abundance of helium and other heavier elements), or the internal rotation profile of the star.
Aboard the satellite SOHO (ESA/NASA) observing the Sun from the Lagrange point, IAS is responsible for the scientific and technical aspects of the instrument GOLF (Global Oscillations at Low Frequencies), in observing the global solar oscillations velocity with a spectral sensitivity of about 1 mm/s. The LOI (Luminosity Oscillations Imager) of VIRGO/SOHO is an instrument that measures the oscillations in intensity with an imager made up of 12 pixels. IAS is also involved on the seismological aspects of the Picard mission (CNES) which observes the Sun from Earth orbit.
Concerning the study of the seismologic aspects of stars, IAS is deeply involved in the COROT mission (CNES) which is based on high precision photometric measurements of stars, and has major scientific goals for detecting and studying stellar vibrations and searching for extrasolar planets. This mission has expanded the possibilities of seismology in stars other than the Sun, some solar-like, others quite different such as red giants, (evolved stars).
IAS also participates in the analysis and interpretation of data from the Kepler mission (NASA) which has extended even further the number of stars for a seismic survey and therefore a better understanding of stellar interiors and their evolution.