Stellar Coronae

The vast majority of “normal” hydrogen-burning stars are of mass equal to or lower than that of our Sun, and they share with our host star the property of producing X-ray emission from a hot outer atmospheric layer, the corona. Whether a star can form a corona is rooted in its interior structure which determines if and what kind of magnetic field can be generated and provide the energy to heat the outer atmosphere to the point of producing (thermal) X-rays. Other atmospheric processes manifest at different wavelengths are intimately linked to coronal X-ray emission, and all these phenomena are commonly united under the term “magnetic activity” as they are triggered by the star’s dynamic and changing magnetic field. At closer inspection, these “normal” stars are divided into a variety of objects, each with their own idiosyncratic magnetic activity behavior. In this review, we discuss the basic physical processes that drive and characterize stellar coronae, and we present an up-to-date summary of our knowledge about the X-ray properties of the individual types of stars that populate the magnetically active regime and its boundaries at high and low stellar mass. We develop thus a coherent picture of stellar activity, using the solar-stellar analogy to examine similarities and differences between our Sun (discussed in the article by Testa & Reale in this handbook) and other coronally active stars. We omit only the pre-main sequence, which is treated in the accompanying article by Sciortino.