Let us consider the Sun as a starting point of our discussion of X-ray stars. A whole armada of satellites has observed and is observing the corona of the Sun. With only a few hours delay, the obtained images are made available to the general scientific and nonscientific public through the internet (at the web site http://www.sec.noaa.gov/sxi). In Fig. 10.1 we show the X-ray morphology of the Sun as seen by the Japanese Yohkoh satellite through the solar cycle; note that the spectral sensitivity and angular resolution of Yohkoh is very similar to that of ROSAT or XMM-Newton. The solar X-ray corona appears far from being spherically symmetric, rather it shows an extremely high degree of spatial inhomogeneity and small-scale structure with vast brightness differences (>104) between adjacent features. Plasma confinement in numerous loop-like structures is readily apparent. Coronae of stars appear as point-like sources, and the solar example as shown in Fig. 10.1 leads us to expect lots of substructure below the instrumental resolution. For the X-ray Sun temporal variability is observed on essentially all time scales covered by the Yohkoh observations, starting from seconds/minutes during eruptions up to decades (cf., Fig. 10.1) manifesting the solar cycle. At solar maximum the appearance of the solar corona is dominated by so-called "active" regions, at minimum few or none of those regions are on the solar disk and the X-ray output is produced by
more diffuse, much fainter emission regions. As a consequence, in the course of its 11-year cycle the solar X-ray luminosity varies by up to two orders of magnitude, depending on the considered spectral band pass.
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