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 pale 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 - according to the standard theory of an expanding universe - of its distance from Earth. The group was known as Stephan's Quintet until the anomalous red shift of NGC 7320 showed that it lies in front of the other four. The two central members of the Quartet are interacting galaxies.
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