A group of galaxies that, to varying degrees, stand apart from the standard classification are those whose parts (Seyfert nuclei), or nearly the whole of their optical image (QSOs) are in a more highly condensed state than normal systems. As far as classification of their optical properties is concerned, they form a continuum which can be conveniently divided into three sections called Seyferts, N galaxies, and quasars. Although their radio properties were emphasized most strongly in the decade of the 1960s, radio emission appears to be superficial as regards the optical forms. Both radio-intense and radio-quiet examples exist in all three sections of the compact regime, and the radio-quiet (or radio-weak) objects form the majority of the sample per volume of space.
The degree of compactness varies along the sequence in the order Seyfert, N, QSO. Seyfert galaxies are almost normal in the Hubble sense, except for an intensely bright nucleus. Those Seyferts that are spirals (e.g., NGC 1068, NGC 4051, NGC 4151) have slightly abnormal outer arms that form nearly complete faint exterior ``rings'' beyond the inner spiral pattern (cf. Hodge 1968), but otherwise (and aside from their nuclei) they are easily placed within the Hubble sequence. N galaxies are dominated to a larger extent by their compact, nonthermal, central region, whereas most quasi-stellar objects (QSOs or quasars) are completely stellar (by definition) on photographs with a scale of 10" mm-1.
The continuity between Seyferts, N galaxies, and QSOs in their classification has been discussed by many authors, and will not again be reviewed here (cf. Sandage 1971, 1973). Early classical papers on the Seyferts alone include the original discussion by Seyfert (1943), and the renaissance of the subject by Woltjer (1959). The N systems of Morgan (1958, 1959) were reemphasized by Matthews, Morgan, and Schmidt (1964) via the route of radio sources, and the connections between Seyferts and QSOs were summarized by Burbidge, Burbidge, and Sandage (1963).
Discussions of the classification system as it existed in mid-1971 were given by Morgan (1971) in the Pontifical Academy volume devoted to this subject (O'Connell 1971). A useful summary of the Seyfert problem, of the group characteristics of these compact systems taken as a whole, and of the effect of different spatial resolutions on the classification of a given object can be found in the Proceedings of the Conference on Seyfert Galaxies and Related Objects edited by Pacholczyk and Weymann (1968).