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Stellar activity is a collective term used to describe phenomena related to the presence of magnetic fields in cool stars, i.e. stars of spectral types from late A to late M at the low-mass end of the main-sequence. Magnetic activity effects all atmospheric layers of a star ranging from the cool photosphere up to the hot coronal layers at temperatures of a few million degrees. Typical activity phenomena observed on the Sun and stars therefore include sun and star spots, i.e., dark areas on their surfaces which we know to be the sites of intense magnetic fields on the Sun, as well as the highly structured coronal regions of the Sun and the stars. Since the plasma temperatures in these regions exceed one million K (reaching tens of millions K in stars), the UV and X-ray domain is ideal to investigate stellar coronae and their underlying physics. Images obtained at these wavelengths reveal the extremely complex structure of the solar corona; some examples are shown on this page. By analogy stellar coronae are thought to be similarly structured as the Sun's corona, however, stars are usually observed without any spatial resolution. One of the challenges faced by our group is to infer what kind of (magnetic) structures are underlying stellar coronae.



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Stellar activity is directly linked to the existence of strong magnetic fields thought to be generated by dynamo processes in the stellar interior, with the power of the dynamo strongly depending on the rotation and interior structure of the star. The presence and changing configurations of magnetic fields are also responsible for the structure and dynamical appearance of coronae and the launching of the solar and stellar winds. Transient phenomena are present on all observable timescales and include burst-like forms of energy release in flares up to gradual changes of the stellar magnetic configuration that is the foundation for the 11-year solar activity cycle. Our group is engaged in studying and analyzing stellar chromospheric and coronal variability on various timescales ranging from short duration flares on time scales of seconds to cyclic variability on time scales of years.

The study of stellar activity provides important diagnostics on the structure and evolution of stars and their atmospheres, on stellar magnetic fields and their generating mechanism or on the energy release mechanism in hot plasma. High-energy X-ray and UV radiation as well as particle flux also effect the stellar environment and thus influence circumstellar matter from the birth of a star to the current Earth's environment. One of the major recognitions of stellar activity studies is the fact that young stars are more "active" than the present-day Sun by many orders of magnitude. As a consequence, the high-energy environment, that the Young Sun provided for its planets, was very much different from the environment we are living in today.