Radio
galaxies are a broad class of galaxies that typically exhibit one
thousand to one million times more radio emission
("brightness") than a normal galaxy such as our Milky Way. About one in a
thousand galaxies is a radio galaxy with a brightness a thousand times that of a normal galaxy, while only one in ten million will have a brightness a million times normal. The brightest radio galaxies have about the same radio intensity as the quasars, objects that may themselves be an extreme case of a radio galaxy. It has not been possible to find an optical counterpart to every detected radio galaxy. In many cases the optical photographs, taken even through the largest telescopes, show no object in the position of the radio galaxy that could be the source of the radio
emission. Where optical identifications have been made, the radio galaxy has often been found to be a giant elliptical galaxy, the brightest objects in the universe, with the exception of the quasars. Very often the galaxy has some peculiarity, such as dust lanes, which are unusual in elliptical galaxies. The most common arrangement of the radio-emitting regions in radio galaxies takes the form of two very large clouds of radio-emitting material symmetrically situated around the center of the galaxy itself. These two clouds usually lie far beyond the optical boundaries of the galaxy and are usually much larger than the galaxy itself. Thus they may lie as far as several hundred thousand light-years from the core of the galaxy and may be thousands of light-years in diameter. Another common arrangement of the radio-emitting regions is in the form of a very compact source, probably connected with the nucleus of the galaxy. In some cases this source is surrounded by a halo of much fainter radio emission. The radio emission from radio galaxies is very highly polarized, indicating that its origin is the radio radiation of very energetic electrons moving at nearly the speed of light and spiraling in the weak magnetic field of the galaxy. These energetic electrons are the result of some very violent event, still not understood, in which an amount of energy equivalent to the total annihilation of up to ten million stars was released. Calculations indicate that the typical radio galaxy will be a brilliant radio emitter for one hundred million to one billion years. Radio galaxies, BL Lacertae objects, and quasars are likely all manifestations of the same basic phenomenon (see radio astronomy). Many observers believe they are powered by giant explosions in the nucleus of a galaxy, caused by an event not yet understood but possibly involving the infall of matter into a massive black hole.