Abstract :
Chandra and XMM-Newton have opened new windows to X-ray diagnostics of stellar X-ray sources. Coronae of cool stars are ideal laboratories to investigate hot, magnetically trapped plasma and its dynamics, such as coronal mass motions, temperature and emission measure changes during flares, electron densities in various coronal temperature regimes, optical depth effects, coronal abundances and abundance variations, etc. This paper summarizes a few outstanding physical problems in high-energy astrophysics of stars, with emphasis on the role of energy release by flares. First results from XMM-Newton and Chandra spectroscopy of cool stars are discussed.
