Department: Solar and Stellar Interiors

The MPS department "Solar and Stellar Interiors" was created in April 2011. It is a joint initiative of the Max Planck Society, the University of Göttingen, and the state of Niedersachsen. The joint appointment of Laurent Gizon as director at the MPS and a member of the University of Göttingen physics faculty has made possible the establishment of a teaching and research program at the Institute for Astrophysics at the university. The department research is driven by space-based (incl. SOHO, SO, SDO, Kepler, TESS ) and ground-based data (e.g., GONG). The observations are interpreted using a set of models, including solar and stellar structure models, helioseismic forward and inverse computations, simulations of solar magnetoconvection, and flux transport dynamo models.

One of the most important unsolved problems in solar and stellar physics today is the origin of the Sun’s eleven-year activity cycle. More generally, activity cycles in cool stars with convective envelopes are not understood, and these cycles have important influences on the surrounding interplanetary space out to the interstellar medium, including all of the planets within. These cycles are expected to be the result of the interplay between flows (e.g., rotation, meridional circulation, convection) and magnetic fields and are described by dynamo models. It is not clear, however, how and where the dynamo(s) operate, and what sets the amplitudes and periods of activity cycles. The reason for this is the absence of sufficient empirical information about subsurface dynamics in solar and stellar interiors.  

The tools used within the department are Helioseismology, the use of solar oscillations as probes of the interior of the Sun, and Asteroseismology, a generalization of helioseismology to the study of oscillations of other stars. These tools allow us to test and refine the theory of solar and stellar structure and evolution and to study thermal structures and flows in the interior of the Sun and stars, thereby bringing us closer to understanding internal transport mechanisms and large-scale magnetism. Further, asteroseismology allows us to place the Sun in the context of other similar stars and to study the possible future and past of the Sun. 

High spatial resolution data for helioseismology are currently available from the SOHO and SDO missions, and from the GONG network. The SDO/HMI helioseismic data, which automatically stream to MPS from Stanford University, play a central role in the activities of the Solar and Stellar Interiors Department as they enable us to infer global and local subsurface flows, study the global modes of rotating convection, or characterize the emergence of magnetic activity. One important discovery made is the observation and identification of the Sun’s inertial modes of oscillation. This breakthrough greatly extends the work on Rossby waves. Essentially all aspects of research in helioseismology are addressed by the department and ongoing efforts also dedicated to improving various helioseismology techniques.

Another important topic of research for the department is the analysis of the Solar Orbiter observations. Solar Orbiter’s orbit will reach up to about 30° out of the ecliptic plane and will allow us to study the polar regions of the Sun for the first time. The PHI instrument on Solar Orbiter will provide the necessary data to infer plasma flows using helioseismology and feature tracking methods. PHI will also carry out stereoscopic observations in combination with other instruments (e.g., HMI and GONG). 

Stars and Planetary Systems

Asteroseismology is now applied to a large number of stars thanks to high-precision photometric observations from the CoRoT, Kepler, and TESS space missions. The department uses these data sets, together with supporting numerical models and theoretical work, to study the interiors of Sun-like pulsating stars. Asteroseismology is expected to provide precise knowledge of stellar parameters (e.g., radius, mass, chemical composition, evolutionary stage, and rotation rate) which will have far-reaching implications for our understanding of stellar evolution and activity. One aspect of the work in the department is to build on knowledge from helioseismology to improve asteroseismic measurements and their interpretation.

PLATO Data Center

The department plays a key role in the PLATO mission ( ESA M3, 2026 launch, see Figure). The PLATO mission is the next generation planetary-transit experiment; its main objective is to characterize exoplanets and their host stars in the solar neighborhood. While it builds on the heritage from CoRoT and Kepler, the major breakthrough to be achieved by PLATO will come from combining high-precision photometry, asteroseismology, and ground-based follow-up spectroscopy for many tens of thousands of cool dwarf stars. PLATO’s strong focus on bright stars is intended to discover Earth-like planets for future programs aiming at the study of planet atmospheres and the search for biomarkers. Our department are working on the development of the PLATO Data Center infrastructure for the PLATO mission, which will support the calibration and analysis of the observations on the ground.