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In the following chapter I will discuss recent developments in the field of X-ray astronomy of cool stars, and try to highlight some of the basic observational findings obtained with the Einstein and ROSAT satellites. In particular, coronae appear to exist around all cool stars with outer convection zones with a minimum surface flux agreeing with the X-ray flux observed from solar coronal holes. The overall X-ray output of cool stars can vary enormously from star to star with a total range of four orders of magnitude. The factor most important in determining the actual X-ray emission level for a given star appears to be its rotation rate. For young stars one observes a significant evolution of rotation in the first 108 yrs of their main sequence life times, which leads to a corresponding evolution of X-ray luminosity. I argue that magnetic fields play a very central role for the development and evolution of coronae, and consequently the theory of magnetohydrodynamics (MHD) provides the appropriate theoretical framework to describe the dynamics of coronal plasma. I describe the fundamental assumptions of MHD, and present simple models of both magnetically closed and open regions as observed on the Sun, and — by analogy — presumed to exist also on other stars.
KeywordsCoronal Plasma Active Star Cool Star Angular Momentum Loss Stellar Corona
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