There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.
Hamlet, Act I, scene v
Conventional astronomy tells a curiously simple story about the Universe. Essentially, it is the story of stars, which take centre stage simply because they produce almost all the visible light which reaches us from space. Radio astronomy gives a radically different perspective. Normal stars produce almost no radio waves, and in their absence the exquisite sensitivity of radio telescopes reveals a radio sky that seems to belong to another universe. It is dominated by the great swath of our Galaxy, which far outshines the radio sun. In place of the stars, the radio sky is studded by small, bright clouds: the so-called radio sources. Some of the largest and brightest are Galactic nebulae which can also be seen with optical telescopes, but the majority lie far outside our Galaxy.
These extragalactic radio sources are associated with distant galaxies. Over the years, many different types of extragalactic radio source have been discovered, but we have gradually realized that nearly all of them fall into two great families. By far the most numerous in space are starburst sources like our own Galaxy, in which the radio waves come from the spiral arms and other regions containing massive young stars. The radio waves are produced in the interstellar gas as it is churned up by supernova explosions (the shockwaves radiating from old supernovae are visible in radio images as supernova remnants), and perhaps by other activity associated with massive stars. Altogether, this first family of extragalactic radio sources fits neatly into the traditional "star story". The second family is utterly different.
By a twist of fate, this second family, which we will call DRAGNs, are by far the most conspicuous of the extragalactic radio sources. In fact, the rare, extremely powerful DRAGNs make up more than 99% of the brightest hundred thousand radio sources in the sky. We see these objects because they are so luminous that they are easily visible from halfway across the observable universe, while the far larger number of starbursts at the same distance are so faint that they are detectable only in the deepest searches, or not at all.
Of all the discoveries of radio astronomy, DRAGNs have surely been the most unexpected. Without the view of the universe provided by radio telescopes, we would probably never have found them, for they are hardly detected in any other part of the electromagnetic spectrum. Even after half a century of study, some of the most obvious questions about them remain unanswered, and we only have the vaguest idea of how such things should come to exist.
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