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'Radio sources' are objects in outer space that emit strong radio waves. Radio emission comes from hot
gas, electrons spiralling in magnetic fields and specific wavelengths emitted by atoms and molecules in space.
In 1932, radio astronomer, Karl Jansky detected radio waves coming from an unknown source in the centre of our galaxy. Other strong 'radio sources' have been the remains of a supernova explosion in the Crab Nebula and pulsars. Short radio waves have come from complex molecules in dense clouds of gas where stars are giving birth. Searches have been taken to find out if there are other civilizations in our galaxy using radio waves, but have had no successes.
There are many sources of radio waves beyond our galaxy, most of which are found in other galaxies. A few examples include neutral hydrogen and carbon monoxide, mostly found in spiral galaxies, and quasars (short for "quasi-stellar radio source"), which are point-like sources of radio waves and other kinds of electromagnetic radiation that are thought to be powered by supermassive black holes in the centers of elliptical galaxies. Scientists have also discovered weak background radiation left over from the Big Bang (the rapid expanion, roughly 13.7 billion years ago, that was the beginning of our universe).