Figure 13.5: The radio source Cygnus A. The picture is a map at a wavelength of 22 cm of the radio source Cygnus A (3C 405)---one of the brightest radio sources on the sky and an example of a class of sources that are among the most energetic persistent sources in the universe. The double radio lobes are produced by jets of energetic particles emitted by the core object at the tiny dot between them. The lobes result when these jets are slowed down by thin intergalactic gas. The distance between the lobes is about 450,000 light-years. This is much larger than the size of a typical galaxy. The engine behind this powerful object is plausibly a rotating black hole roughly the diameter of our solar system and that is located in a galaxy midway between the lobes but not visible in the radio.