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Abstract

Magnetic fields are thought to play a significant role in various phenomena in the Universe, in particular in the evolution and the structure of stars. With asteroseismology, it is possible to probe the inner structure of stars. In this thesis, we investigate the properties of the oscillations of red giants that show signs of energy loss. We observationally constrained the mechanism causing this energy loss and found that it is consistent with the predicted effect of magnetic field in the core of the red giant. Using recent developments, we were also able to observe frequency shifts due to a magnetic field in the core of stars. We analysed stars with both magnetic frequency shifts and energy loss in the oscillations and showed that both phenomena can be explained by a magnetic field with the same field strength located in the core of the stars.