A main sequence for quasars

The last 15 years have seen a major step forward in the analysis of optical UV spectroscopic data of large quasar samples. Multivariate statistical approaches have proved to be especially helpful, as they have led to the definition of systematic trends (first and second eigenvector derived from Principal Component Analysis) in observed properties that are the basis of physical and dynamical modeling of quasar structure. In this talk I will discuss the empirical correlates of the so-called quasar “main sequence” associated with the quasar eigenvector 1, its governing physical parameters, as well as luminosity effects associated with the second eigenvector. I will also briefly discuss quasars in a segment of the main sequence which includes the strongest FeII emitters. These sources show a small dispersion around a well-defined Eddington ratio value, a property which makes them potential “Eddington standard candles”.