  X-ray emission from stars has origins as diverse as the stars themselves:
accretion shocks, shocks generated in wind-wind collisions, or release of
magnetic energy. Although the scenarios responsible for X-ray emission are
thought to be known, the physical mechanisms operating are in many cases not
yet fully understood. Full testing of many of these mechanisms requires high
energy resolution, large effective area, and coverage of broad energy bands.
The loss of the X-ray calorimeter spectrometer on board ASTRO-E2 was a huge
blow to the field; it would have provided a large sample of high resolution
spectra of stars with high signal-to-noise ratio. Now, with the advent of the
ASTRO-H Soft X-ray Spectrometer and Hard X-ray Imager, we will be able to
examine some of the hot topics in stellar astrophysics and solve outstanding
mysteries.
