X-RAY VARIABILITY IN THE HOT SUPERGIANT ZETA-ORIONIS

Hot massive stars represent only a small fraction of the stellar popul ation of the galaxy, but their enormous luminosities make them visible over large distances. Therefore, they are ideal standard candles, use d to determine distances bf near galaxies. Their mass loss due to supe rsonic winds driven by radiation pressure contributes significantly to the interstellar medium and thus to the chemical evolution of galaxie s. All hot stars are soft x-ray sources; in contrast to the sun with i ts highly variable x-ray flux, long time scale x-ray variability is no t common among hot stars. An analysis is presented here of an unusual increase in x-ray flux observed with the roentgen observatory satellit e during a period of 2 days for the hot supergiant zeta Orionis, the o nly episode of x-ray variability that has been found in a hot star. Th ese observations provide the most direct evidence so far for the scena rio of shock-heated gas in the winds of hot stars.