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The year 1963 marks a watershed in extragalactic astronomy. The optical identification of radio sources 3C 273 and 3C 48 and the measurement of their redshifts demonstrated to astronomers the existence of a new class of energy sources that have a star-like appearance, yet produce luminous energy at a rate comparable to a galaxy of a hundred billion (1011) stars.

The quasi-stellar objects (QSOs) or ``quasars'' as these sources came to be called, arrived on astrophysicists' plates just about when they had digested the long-standing mystery of stellar energy. By the 1960s, the problems of stellar structure and evolution built on the pillars erected by Eddington, Milne, Chandrasekhar, Bethe, Lyttleton, Schwarzschild and Hoyle had been tackled successfully, thanks to the advent of fast electronic computers. The quasars, however, presented challenges of an altogether different nature. How could so much energy come with such rapid variability out of such a compact region and be distributed over such a wide range of wavelengths?

The classic book Quasi-Stellar Objects by Geoffrey and Margaret Burbidge, published in 1967, captured this early excitement and posed the numerous challenges of quasar astronomy very succinctly. Now, three decades later, we have the benefits of vast progress in the techniques of observational extragalactic astronomy and the intricate sophistication of ideas in high energy astrophysics. Yet it is fair to say that the understanding of quasars and the related field of active galactic nuclei (AGN) has not reached the same level of success that stellar studies had attained thirty years ago.

In this book we have attempted the admittedly difficult task of putting together the astronomical information on quasars and AGN and the interpretations given to it by theorists. Although a ``Standard Model'' is beginning to emerge, there are numerous details that still remain to be fitted. Many of the interpretations are tentative, either because the data is scarce or because there is so much of it that theorists have not yet been able to make much headway. However, there are some established paradigms, like accreting massive black holes as the central engine, accretion disks, relativistic beaming and, at a weaker level, the idea of unification. Much of the theoretical interpretation makes use of these basic ideas, which at least provide a convenient framework within which one may judge the successes and failures of the present limited understanding. We have also drawn the reader's attention to a series of anomalous findings by a minority of workers in the field, findings which perhaps deserve greater consideration than the dismissal usually accorded to them as being ``accidental'', ``insignificant'' or ``artifacts''.

Although we have individually worked on quasars and AGN over a number of years, neither of us can claim to be a pioneer in the field. In writing this book, therefore, we have not been motivated by the desire to champion a specific theory, scenario or paradigm, which perhaps gives us the advantage of being neutral and critical. Over the last decade, there has been a phenomenal increase in the range and quantity of data on quasars and AGN. Digesting, or even developing a superficial understanding of, these data has become a rather difficult task, especially for those who are away from the centres of activity and lack ready access to experts. Our motivation in writing the book has been to remedy this situation somewhat, by providing an overview with a strong pedagogic content.

The text itself is aimed at a typical reader who is familiar with astronomy and astrophysics up to the undergraduate level. It could serve as a teaching text at the advanced undergraduate or graduate level, or as a source book for the graduate student embarking on research in high energy astrophysics. The content of some of the chapters is more detailed than would be required in a graduate course, as we have also aimed at providing a review of the field for the research worker who is not an expert in the general area we address. The book is largely self-contained, but the reader is occasionally referred to a few well-known texts for details of derivations of some formulae, especially where these are a part of the general physics curriculum. We have tried to represent different points of view and cover many important areas of which we are aware, but, owing to the sheer magnitude of the work, we may have missed important facts and arguments. We have omitted detailed discussion on the physics of emission line regions, as this would make a book in itself, and several excellent treatments are available. Because of the vast range of the subject covered, and our own lack of expertise with many of the areas, we have had to depend heavily on published reviews and research articles by leaders in the field, as citations given in the text will testify. We may have inadvertently omitted to cite some important references, especially where the material has become a part of our teaching notes over the years, or is simply a part of the folklore. We gratefully acknowledge our debt to all these sources.

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