A radio source is a source of extraterrestrial radio radiation. The strongest, apart from the Sun, is Cassiopeia A, followed by Cygnus A and the Crab Nebula (Taurus A). (The capital letters following the name of a constellation refer to the radio sources of the constellation, A being the strongest source, B the second strongest, and so on). Radio sources are divided into two main categories: Class I, which are those associated with our Galaxy, and Class II, which are extragalactic. Most radio sources are galaxies, supernova remnants, or H II regions.