ASK spectroscopic sequence of galaxies

Considering galaxy spectra as multidimensional vectors, the majority of the spectral classes are distributed along a well-defined curve going from the earliest to the latest types, suggesting that the optical spectra of normal galaxies can be described in terms of a single affine parameter.

Optically bright active galaxies, however, appear as an independent, roughly orthogonal branch that intersects the main sequence exactly at the transition between early and late types.