Butcher-Oemler Effect

As a Butcher - Oemler effect, the excess of blue galaxies in distant clusters of galaxies, compared with those referred to at a lower distance. It was discovered in 1978 by Harvey Butcher and Augustus Oemler, Jr. ..

Clusters of galaxies at great distances are observed due to the finite speed of light at a time when the universe was younger than today. The increasing proportion of blue galaxies with distance (or redshift ) thus corresponds to a temporal evolution of the composition of galaxy clusters in the sense of decreasing proportion of blue galaxies.

The blue color of galaxies is an indicator of active star formation, these are predominantly spiral galaxies and irregular galaxies. The majority of galaxies in clusters of galaxies are elliptical galaxies that contain little gas and no new stars form, while blue in the field, actively star -forming galaxies dominate. If now a galaxy from the surrounding field in a heap, it occurs in different ways in interaction with the intergalactic medium and the gravitational field of the cluster and the other galaxies in the cluster. This leads to the loss of interstellar gas and thus the suppression of star formation. The exact physical mechanisms which cause this transformation are the subject of active research. The observation of the Butcher - Oemler effect plays an important role.