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Ancient Galaxy Cluster
Looking back in time to when the universe was in its formative youth, the Advanced Camera for Surveys (ACS) aboard NASA's Hubble Space Telescope captured this revealing image of an ancient galaxy cluster. It shows mature galaxies in a massive cluster that existed when the cosmos was 5 billion years old. The cluster, called RDCS 1252.9-2927, is as massive as 300 trillion suns and is the most massive known cluster for its epoch. This Hubble image shows only the core of the cluster. Dominating the core are a pair of large, orange elliptical galaxies [near center of image]. Their orange color indicates an older population of stars. Most of the stars are at least one billion years old. The two galaxies appear to be interacting and may eventually merge to form a larger galaxy that is comparable to the brightest galaxies seen in present-day clusters.
The orange galaxies surrounding the central pair are also cluster members. The cluster probably contains many thousands of galaxies, but only about 50 can be seen in this image. Many of the other galaxies in the image, including several of the blue galaxies, are foreground galaxies. The color-composite image was assembled from observations taken between May and June 2002 by the ACS Wide Field Camera.