This image is a composite of four deep ROSAT images of the Pleiades cluster showing an area of approximately 3 x 3 degrees (data taken from the ROSAT data archive; see Stauffer et al. 1994, ApJS 91, 625 for details). Several hundred X-ray sources have been detected in these observations.
Here comes a comparison of optical and X-ray images of the Pleiades. The upper image is the optical image and was taken with the UK Schmidt Telescope at the Anglo-Australian Observatory (Copyright Royal Observatory, Edinburgh/Anglo-Australian Observatory; more optical images taken with this telescope can be found following this link ). The lower image is a true color ROSAT image. The positions of the seven optically brightest stars are marked with green squares in the X-ray image.