Comparing the magnetically active young star V889 Herculis to the Sun

This Master’s Thesis deals with stellar magnetic activity, both as it is seen in the Sun, our closest star, and in other, more extreme examples of stellar activity. For the Sun, much higher quality data is available, and for a much longer time, so it remains by far the best studied star, although stars, that are much more magnetically active, have been discovered. This thesis will review the most common forms of magnetic activity, as observed both in the Sun and in other stars. A special focus is given to BY Draconis -type stars. These are young stars, whose photometric brightness variations are caused by large, cool starspots, similar to the sunspots seen on the Sun. A particular BY Draconis -star, V889 Herculis, is analysed in detail, using spectroscopic and photometric methods. From spectroscopy, by the Doppler imaging method, a surface temperature map can be constructed for the star. From photometry, both short-term and long-term variations of the star can be studied. During the 20 years time span of the photometric data, the star seems to have gone through similar activity cycles as the well known 11 year solar cycle. The aim of this thesis is to compare solar and stellar activity in general, and specifically V889 Her to the Sun. Based on the analysis, the basic properties of V889 Her, seen in previous studies, are confirmed: a large polar spot is present on its surface, and there are clear changes in its activity. As the stellar parameters, like mass and surface temperature, of V889 Her are very similar to the Sun, and it is also a single star, the main difference is age. As a young star, V889 Her is still magnetically much more active than the Sun today, but probably it will continue to become more and more similar to the present Sun, losing its high activity with increasing age. Similarly, V889 Her shows us what the Sun most likely was like billions of years ago