We know from studies of normal galaxies that regions of recent star formation are particularly luminous. It is possible to account for the high nuclear luminosity of certain galaxies by supposing the galaxy to be very young. This would mean that we were seeing its initial burst of star formation. Such an explanation undoubtedly accounts for some of the COMPACT GALAXIES and is confirmed by their spectra which show them to be similar to giant ionized hydrogen regions. They also are found to contain large amounts of neutral hydrogen; this implies that they are, indeed, fairly young so that only a small fraction of the total gas content of these galaxies has yet been able to collapse to form stars. Apart from the young galaxies, there are also objects which have abnormally luminous nuclei but which are believed to have a considerable age. These include (lie normal or supergiant elliptical galaxies, found in rich regular clusters of galaxies. Giant elliptical galaxies with active nuclei are often associated with strong radio emission and their origins may well be related.

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