Spiral galaxy NGC 6946, composite radio-optical image. The bright star-filled central region of galaxy NGC 6946 is shown in optical light (blue), while the dense hydrogen of the spiral arms and galactic halo are seen in radio emissions (orange), as is the diffuse and extended field of hydrogen engulfing NGC 6946 and its companions (red). Radio data from the Green Bank Telescope (GBT) shows a faintly glowing hydrogen bridge between the larger galaxy and its smaller companions (upper right). The main radio data is from the Westerbork Synthesis Radio Telescope (WSRT), while the optical data is from the 48-inch Samuel Oschin Telescope. NGC 6946 is around 25 million light years from Earth, on the boundary between the constellations of Cepheus and Cygnus.