Introduction

It should be clear from the discussions in the preceding chapters that an overwhelming amount of information is now available for describing quasar properties. Observationally, the study of quasars has been a great success. Also, it should be no surprise that, as the data have accumulated, it becomes more difficult to produce models that can explain everything. As might be expected for the most energetic objects in the universe, quasars are complex. This should not be a source of discouragement. It is not necessary to understand all details of the solar surface to know why the Sun shines. It is not necessary to understand all sedimentary rocks to know why continents drift. It is not necessary to memorize the taxonomy of all living creatures to realize why evolution occurs. When we are after the fundamental understanding of why something happens, all of the details are not required. In the study of quasars, we are still struggling to the point of knowing which details can be safely ignored, and it is for guidance in this regard that existing models are most useful.

The single most significant observational datum about quasars is that their spectra are so extraordinarily similar, even over ranges of 107 in luminosity, for objects separated by more than ten billion light years in the universe.