The tendency of clusters of galaxies at greater distances (redshifts of about 0.4) to have a higher fraction of optically blue galaxies (mostly bright spirals and irregulars) than clusters of galaxies nearby. Although some astronomers suspect this may be only a selection effect, most believe the phenomenon is real and is probably related to the greater availability in the past of material from which new stars could form. The effect was first reported in 1978 by the American astronomers Harvey Raymond Butcher (1947–) and Augustus Oemler, Jr. (1954–).
Related category GALAXIES
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