Published online by Cambridge University Press: 14 August 2015
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.