A type of galaxy with a small, bright nucleus which shows broad, strong emission lines in its spectrum. The first galaxies of this type were described in 1943 by C. K. Seyfert. Nearly all known Seyfert galaxies are spirals or barred spirals, and Seyfert-type activity probably occurs in a small percentage of all spiral galaxies. Seyfert galaxies are classified according to the relative widths of the emission lines in their spectra. Type 1 Seyferts (such as NGC 5548) have broad emission lines of hydrogen but narrow forbidden lines of heavier elements. In Type 2 Seyferts (such as NGC 1068) the hydrogen lines and forbidden lines both have the same width, which is broader than the forbidden lines in Type 1 Seyferts but not as broad as the latter's hydrogen lines. Seyferts have active galactic nuclei that produce strong radiation, probably from an accretion disk around a massive black hole. The radiation excites gas around the central regions, giving rise to the observed emission lines. Seyferts are lower-luminosity examples of quasar activity.
Subjects: Astronomy and Astrophysics.