July 10, 1995 Abell 2218: A Galaxy Cluster Lens Picture Credit: NASA, HST, WFPC2, W. Couch (UNSW) Explanation: Sometimes one of the largest concentrations of mass known can act like a lens. Almost all of the bright objects in this image are galaxies in the cluster known as Abell 2218. The cluster is so massive and so compact that it bends light from galaxies that lie behind it, causing many of them to appear as stretched out arcs. Many dim, elongated arcs are visible on this photograph. This picture was taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. For more information about this picture see the Space Telescope Scientific Institute's press release. We keep an archive of Astronomy Pictures of the Day. Astronomy Picture of the Day is brought to you by Robert Nemiroff and Jerry Bonnell . Original material on this page is copyrighted to Robert J. Nemiroff and Jerry T. Bonnell.