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 civilisations in our galaxy using radio waves, but have had no successes.

Strong sources of radio waves beyond our galaxy are other galaxies, which emit radio waves and quasars. Scientists have also discovered weak backround radiation left over from the Big Bang Explosion , thought to be the beginning of the universe.