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Name: HCG 7, NGC 128 group
Description: Galaxy Cluster
Position (J2000): RA 00hr 39m 15.9s Dec +00° 53' 17"
Constellation: Cetus
Distance: 200 million light years
Image Credit: ESA/Hubble & NASA
Release date: August 8, 2011

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The NASA/ESA Hubble Space Telescope has imaged part of the Hickson Compact Group 7, or HCG 7 for short. This grouping is composed of NGC 197, a lenticular (lens-shaped) galaxy in the center and three spiral galaxies in close proximity. In this image, one of the spirals, NGC 192, dominates the foreground, with many more distant galaxies peppering the background. NGC 196 is on the top right and NGC 201 lies outside of the image. Observing tightly-knit galaxy groups like HCG 7 is important because they evolve in a different way from their more spaced-out counterparts in less crowded regions of the Universe.

A recent study using Hubble data analyzed the star clusters in HCG 7. Three hundred young clusters and 150 globular clusters were charted, and their ages and distributions measured. The results suggest that the rate of star formation has been fairly steady through time, although quite high in the central regions. Additional studies, including searches for material between the galaxies, hint that the stars in the HCG 7 galaxies formed by converting their gas without any gravitational influences caused by merging with other galaxies. This is puzzling, as the galaxies are depleting their supplies of gas at a rate that suggests that they have merged in the past.

This raises the question of whether the group really has evolved serenely, or if there are mysterious processes at work that are yet to be understood. The currently known information is contradictory and an encouragement for further studies to discover the real story behind HCG 7.

This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys. Images through a blue filter (F435W, colored blue), yellow-orange (F606W, colored green) and near-infrared (F814W, colored red) filters were combined. The total exposure times were 1710 s, 1230 s and 1065 s per filter, respectively, and the field of view is about 3.0 x 3.0 arcminutes.