Fifty Years of Quasars: Physical Insights and Potential for Cosmology
  Last year (2013) was more or less the 50th anniversary of the discovery of
quasars. It is an interesting time to review what we know (and don't know)
about them both empirically and theoretically. These compact sources involving
line emitting plasma show extraordinary luminosities extending to one thousand
times that of our Milky Way in emitting volumes of a few solar system diameters
(bolometric luminosity log L$_{bol} \sim $ 44-48 [erg s$^{-1}$]: D=1-3 light
months $\sim$ $10^3$ - $10^4$ gravitational radii). The advent of 8-10 meter
class telescopes enables us to study them spectroscopically in ever greater
detail.
  In 2000 we introduced a 4D Eigenvector 1 parameters space involving optical,
UV and X-Ray measures designed to serve as a 4D equivalent of the 2D
Hertzsprung-Russell diagram so important for depicting the diversity of stellar
types and evolutionary states. This diagram has revealed a principal sequence
of quasars distinguished by Eddington ratio (proportional to the accretion rate
per unit mass). Thus while stellar differences are primarily driven by the mass
of a star, quasar differences are apparently driven by the ratio of
luminosity-to-mass.
  Out of this work has emerged the concept of two quasars populations A and B
separated at Eddington ratio around 0.2 which maximizes quasar multispectral
differences. The mysterious 8% of quasars that are radio-loud belong to
population B which are the lowest accretors with the largest black hole masses.
Finally we consider the most extreme population A quasars which are the highest
accretors and in some cases are among the youngest quasars. We describe how
these sources might be exploited as standard candles for cosmology.
