THE MORPHOLOGY OF DISTANT CLUSTER GALAXIES .1. HST OBSERVATIONS OF CL0939+47131

We have obtained Hubble Space Telescope (HST) WFC observations of CL 0 939+4713, a very rich cluster at z = 0.41 which exhibits an excess pop ulation of blue galaxies typical of clusters at that redshift. The ima ges are of sufficient quality, despite the HST spherical aberration pr oblems, to permit assignment of Hubble types to galaxies brighter than r approximately 24.0. Their colors, luminosity functions and structur e suggest that most of the galaxies are normal examples of their Hubbl e type. However, there is a significant number of objects which appear to be undergoing tidal interactions and/or mergers. These data sugges t that the excess blue galaxies seen in distant clusters are predomina ntly normal late-type spirals but also that dynamical interactions bet ween galaxies may be an important process in such clusters and may be responsible for those galaxies which exhibit spectroscopic sips of sta rbursts.