DR JEAN LORRE / SCIENCE PHOTO LIBRARY DR JEAN LORRE / SCIENCE PHOTO LIBRARY
Image-processed optical photograph of Stephan's Quartet group of galaxies and NGC 7320. The colour coding illustrates the different red shift values of the 4 members of the Quartet (coded red because of their higher red shift) & NGC 7320 (coded blue because of its lower red shift). The amount of red shift is a function of the velocity at which an object is receding relative to Earth, & therefore of its distance from Earth. Discovered in 1877 by Stephan, the group was known as Stephan's Quintet until the 1960s, when the anomalous red shift of NGC 7320 led to the conclusion that it lies in front of the Quartet. The central 1 of the Quartet group is in fact a pair of interacting galaxies.
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