All these galaxies show the characteristic intense, point-like nucleus originally
remarked for this class by Carl Seyfert. The galaxies' Hubble types are also
representative for Seyfert galaxies as a group, dominated by early-type spirals
(Sa and Sb, with both barred and non-barred representatives) and S0 systems.
Spectra of Seyferts DID show normal lines from typical stars. However,
something in the spectrum made these galaxies really stand out: Emission
lines of Hydrogen and Helium seen in the nucleus.
Although in many cases the classes of active galaxies were originally thought to be distinct,
the modern view is that the different classes of active galaxies are related phenomena, and
that they all may be powered by the same thing: rotating, supermassive black holes at their centers.