X-ray emission from stars

Data from the Einstein Observatory are examined to discover the processes which lead to the appearance of stellar surface activity. Previous astrophysical observations are reviewed, including stellar X ray, UV, Ca II, H, and K emissions observations, and monitoring of the solar corona. All stars have been observed to emit X rays at one time or another, and the Einstein spacecraft has furnished data on X ray emission and stellar rotation, Ca II, H, and K emission from late-type stars, X ray emission from early-type stars and pre-main-sequence stars, and has provided evidence that X rays are not emitted by stellar winds. Stellar coronae have been identified as the source of the X ray emission in pre-main-sequence stars, and correlations have been found between the level of X ray emission and the rotation rate in late-type stars. Further attention is given to the capture of the energy of infalling and outgassing material by the stellar magnetic fields, and purposes of the AXAF orbiting instrument to be launched by the Shuttle are discussed, specifically for stellar X ray spectrographic observations.