The nature of galaxies and clusters of galaxies at very large distances

On the basis of their particular characteristics, it is possible to distinguish galaxies are very large distances from stars and from nearby low-luminosity galaxies. Surveys for clusters of galaxies are discussed, taking into account a survey for distant galaxies conducted by Gunn et al. (1984). The considered survey has provided a catalog of about 425 clusters with redshifts ranging from z = 0.2 to over 0.9. It is pointed out that this catalog represents the limiting magnitude which can be reached using photographic techniques. The observation of more distant clusters can be based on the utilization of modern CCD technology. Attention is also given to the redshifts of clusters, evolutionary problems, cluster dynamics and superclusters, and supernovae.