High-energy radiation from outer stellar atmospheres

Abstract

Although the Sun had been known to be a source of X-ray and ultraviolet radiation since the late 1940s, it was only in the late 1970s that normal stars of nearly all spectral types and luminosity classes were recognized to be sources of high-energy radiation detectable from space (see Mewe 1996 for a historical perspective). The origin of this high-energy emission from normal (i.e. non-accreting) stars constitutes a fundamental and yet unsolved problem in stellar astrophysics, which space observations have helped to elucidate over the past 20 years. It was quite obvious from the very first observations that the generation of high-energy photons by thermal processes, such as the ones usually observed in normal stars, requires temperatures far in excess to those responsible for the stellar optical radiation. In particular, it requires that the temperature profile in stellar atmospheres, after reaching a minimum in the upper photosphere, rises again, attaining values of one to several million degrees in the corona. These outer atmospheric layers, where the temperature increases outwards through the chromosphere, transition region and corona, had been known and widely studied for a long time in the case of the Sun, but it was only much later with the advent of grazing incidence X-ray telescopes and sensitive X-ray detectors that their existence for the vast majority of stars was definitely proved.