The radio radiation emitted by normal galaxies is very weak indeed. However some of the most distant galaxies ever found have been discovered by radio astronomers because of the great intensity ‘of their radio emission. These are the strong extra-galactic radio sources described below. Many active galaxies radiate at radio wavelengths with intermediate strength. High-resolution radio telescopes enable astronomers to estimate the amount of energy needed to account for the radio emission. These amounts are not enough to sustain the phenomenon for more than a fraction of a galactic lifetime. This suggests that radio emission is either a transient phenomenon or that there may be hidden energy sources within radio-bright galaxies.

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