M81 galaxy cluster, composite radio-optical image. This view shows clouds of hydrogen gas (red and green) surrounding the galaxies, discovered in two sets of radio observations. Initial observations by the Very Large Array (VLA) are in green, with additional observations by the Green Bank Telescope (GBT) in red. The gas clouds are thought to be remnants of earlier interactions among the galaxies forming this cluster. The optical data is from the Schmidt telescope. M81 is 11.8 million light years from Earth, in the constellation of Ursa Major. The radio observations were made in 1992 (VLA) and 2003 (GBT).