Our Survey of active galaxies and quasars looks rather like a zoo of various cosmic beasts .In each compartment we have a type of galaxy ,frequently named for its discoverer (e.g. lacertid) ,or the technique of discovery (e.g. radio galaxy).What systematic trends emerge when we study these species collectively ?One recurrent theme is that of active nuclei .Since the early 1960s it has been emphasized more and more that the central regions of galaxies are the seat of violent activity. What became clear over a period of two decades or so was the astonishing variety of this central activity.

Beginning with our own Milky Way, there is a powerful radio source, Sagittarius A at the galactic centre, and ample evidence that the central regions are expanding fast. Some astronomers interpret this as evidence for an explosion in recent times at the centre of the Galaxy. Not far away is another exploding galaxy, M82 and the galaxy with the optical jet, M87 . In the Seyfert galaxies the nuclei are certainly active and are responsible for the rich emission-line spectrum. The same applies to the optical properties of radio galaxies; opinion is firmly in favour of the idea that the clouds of plasma responsible for the radio structure must have emerged from galactic nuclei. Quasars are certainly small, as we expect for a galactic nucleus. Could it be that in quasars the nuclear emission is so intense that the light from any underlying galaxy is totally swamped? That this could be the case is strengthened by the facts that wisps are found round 3C48 and that a few quasars are members of clusters of galaxies. Even lacertids can be neatly accommodated into this unified picture: they may be galaxies with strong continuum emission from the nuclei but with no gas that can be excited to give the emissions that characterize quasars.

A unified model of all delinquent galaxies has great attractions for the theorist since it is one of the tasks of science to organize apparently unrelated facts into a rational sequence, and then to extract new facts from this ordering. It appears very possible that the ultimate source of energy in the excited nuclei is some form of highly-condensed object. If a great* deal of matter (millions of solar masses) is concentrated into a small volume or volumes of space (kilometres or planetary size), prodigious quantities of energy may be extracted – tens of times the energy available via nuclear reactions. The compact objects could be neutron stars or black holes, for the theoretical study of these has certainly demonstrated the possibilities of their producing vast supplies of energy, much of it in the exotic form (electrons moving at the velocity of light, for example) that is required. There is every hope that the variety of galaxies will one day be ordered and explained, just as the astrophysicists of the early twentieth century ultimately made sense of the apparently bewildering selection of stellar spectra and star types.

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