Butcher-Oemler effect

The Butcher-Oemler effect (sometimes abbreviated BOE) is an observed distribution of the characteristics galaxies, that at middle-to-high redshifts (z = 0.3 up to perhaps z = 1) there is a higher percentage of blue galaxies than found nearer the present time. Blue indicates early stars, a sign of very recent star formation, so the observed distribution suggests a higher star formation rate in that era than at present. The effect was published in 1978 and is still of research interest.