The Core of the HR Diagram

How Stars are formed and evolve is an important problem for astrophysics and cosmology. Our knoweldge of the processes involved has dramatically developed during the last hunderd years. The age of Stars, star clusters and of structures in our Galaxy, the formation of chemical elements and the chemical evolution of our Galaxy and the Universe critically depend an stellar processes. State of the art theories and instruments are being continuously applied to understand these processes. Despite all the efforts we have not yet reached the goal - but certainly are much closer to it than a few generations ago. Nature still provides us with surprises. The solar neutrino flux, for example, is about a factor of these lower than predicted by theory. A critical review of the standard solar model and taking into account the infonnation which can be deduced from solar oscillation clearly indicates that the model correctly describes ihe nuclear processes in the solar core. As a consequence, partiele physisists had to abandon the concept of a massless neutrino which seems to oscillate between various forms. All that provided a real surprise and a change of concepts in partiele physics. Stars are laboratories free of charge, in which processes can be studied under conditions impossible to provide an earth. This is another argument why astrophysics is indispensible for testing physical concepts in a wide parameter range. Only alter a theory has susccessfully passed all possible tests, we may assume that the underlying physical processes are truly understood. Convection, interaction with a magnetic field, diffusion, and rotation are still poorly understood processes waiting for a proper treatment. The research proposed in this application shall contribute in reducing this deficit. Pulsation of Stars with otherwise similar surface properties, but different evolutionary status allow to efficiently study (and test) models of stellar structure and evolution. Another example may be given here: the obviously very successful standard solar model fails to correctly predict the pulsation characteristics of hotter counterparts of our Sun. The space photometer MOST has clearly shown that Procyon, a hotter solar twin, does not pulsate in luminosity an a level of a few ppm. The model, however, predicted amplitudes of some ten ppm! Obviously, some physical concepts are not yet correctly implemented in the Standard solar model, which rises doubts in the reliability of extrapolating further concepts of stellar structure - with implications to what was written at the beginning.