On the basis of their optical spectra, compact objects are divided into 3 classes according to the probable nature of their energy sources, as follows:
(1)Non-thermal: This class includes Seyfert nuclei, most N-type radio galaxies, and also very luminous Zwicky compact objects. The existence of high-velocity clouds, and a wide variety of densities and temperatures appear to be common characteristics of these objects. QSOs are considered to be related, with similar but more extreme properties.(2)Hot, massive stars: This class includes some small galaxies, parts of galaxies or appendages to galaxies, which are usually much less luminous than class (1) objects.(3)Stellar: Some compact galaxies having only absorption line spectra fall into this class.A variety of morphological forms exists amongst class (1) and (2) objects.Emission line spectra of QSOs are reviewed, and related to the physical conditions in the emitting regions.
Optical Spectra of Radio-Loud and Radio-Quiet Active Galactic Nuclei