At first glance, there are only relatively few parameters of a star that are directly accessible to observation. In addition to the position in the sky, these are the distance, proper motion and radial velocity, the apparent brightness in different wavelength ranges, the degree of polarization of the radiation and the “star color”, expressed by a color index. In addition, there is the spectral type and, of course, the star spectrum itself. Some quantities such as apparent brightness, radial velocity (e.g. in pulsating stars), color index and spectral type can change over time, so that time series of these quantities represent another important source of information.