Hubble Space Telescope's high resolution allows astronomers to classify galaxies in a cluster (CL 0939+4713) that existed four billion years ago, when the universe was two- thirds of its present age.

The galaxies in this mosaic are arranged according to the well-established system developed by American astronomer Edwin Hubble in the 1920s. Despite the cluster's great distance of four billion light-years, the Space Telescope pictures are sharp enough to distinguish between various forms galaxies.

The top three rows show familiar types of galaxies which are found today in nearby clusters: elliptical galaxies and lens shaped galaxies (SO) which may be transition objects between spiral and elliptical galaxies.

Rows 4 through 7 show spiral galaxies categorized by the openness of their pinwheel- shaped arms (Hubble classification Sa, Sb, Sc, Sd). Many of these have since disappeared through possibly a variety of processes: merger, disruption, and fading. In particular the spirals in row 7 (Sd) show peculiar morphologies.

The bottom row shows galaxies apparently merging into single systems.

Space Telescope reveals that star-forming galaxies were far more prevalent in the clusters of the younger universe than in modern clusters, a result having important implications for theories of galaxy evolution.

The image was taken with HST's Wide Field/Planetary Camera in Wide Field Camera mode, and required a six-hour exposure.

Photo Credit: Alan Dressier, Carnegie Institution, and NASA Co-investigators: Augustus Oemler (Yale Urnversfty), James E. Gunn (Princeton Universfty), Harvey Butcher (the Netherlands Foundation for Research in Astronomy).