One of the earliest discoveries made in radio astronomy was the existence of unusual objects now known as radio galaxies. The first of these, a strong radio source named Cygnus A, was detected by Grote Reber in 1940 using a homemade antenna in his backyard. Cygnus A emits about a million times as much energy in the radio region of the electromagnetic spectrum as does our own galaxy in all regions of the spectrum. Powerful radio-emitting sources like Cygnus A are now known as radio galaxies.
Radio galaxies also emit optical (visible) light, but they tend to look quite different from the more familiar optical galaxies with which astronomers had long been familiar. For example, Cygnus A looks as if two galaxies are colliding with each other, an explanation that had been adopted by some astronomers before Reber's discovery. Another radio galaxy, Centaurus A, looks as if it has a dark band running almost completely through its center. Still another radio galaxy, known as M87, seems to have a large jet exploding from one side of its central body.
In most cases, the radio image of a radio galaxy is very different from the optical image. In the case of Cygnus A, for example, the radio image consists of two large lobe-shaped structures extending to very large distances on either side of the central optical image. Studies have shown that these radio-emitting segments are very much younger (about 3 million years old) compared with the central optical structures (about 10 billion years old).
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