The term "stellar activity" summarizes a number of phenomena on the stellar surface and in the stellar atmosphere. The origin of many of these phenomena are changes in the structure of the stellar magnetic field. While it is possible to directly observe different forms of activity on the Sun in great detail, this remains impossible for other stars. However, by analyzing the spectra of these stars, information about their activity can be obtained. This thesis addresses two aspects of activity: First, the X-ray emission from the positions of A-type stars and a possible mechanism to produce these X-rays and second, the chromospheric activity in the spectral range from late A- to late K-type stars. Since the dynamo, which is responsible for the activity of solar-like stars, emerges in the range of late A- to early F-type stars, this spectral range is of special interest for the understanding of the activity phenomena. To study the X-ray activity of A-type stars, their optical positions as given in the Bright Star Catalogue were compared with the positions of the X-ray sources listed in the ROSAT catalogs. The matching criteria for the ROSAT All-Sky Survey data were 90 arcseconds, 36 arcseconds for the pointing observations with the PSPC and 18 arcseconds for HRI data. Those stars which could be associated with X-ray sources were tested for indications of hidden late-type companions. Variations in the radial velocity, the proper motion, and the light curve were interpreted as signs for binarity...

thesis