Stephan's Quintet, colliding galaxies. Combined infrared and optical image of Stephan's Quintet, a set of colliding galaxies that is 300 million light years away in the constellation Pegasus. The centres of the galaxies are bright yellow dots surrounded by a blue haze of stars. Two are seen at centre right, and the one at left of this pair is NGC 7318b. Collision speeds of millions of kilometres per hour have produced a shock wave (green) where hydrogen gas has been heated until it emits light. Infrared data from the orbiting Spitzer Space Telescope's IRAC sensor. Optical and near-infrared data from a ground-based telescope in Calar Alto, Spain.