A type of spiral galaxy that has a bright nucleus and whose spectrum shows emission lines. Historically, N galaxies and Seyfert galaxies were defined by different astronomers on the basis of different information, and the difference between them is not always clear cut.
An energetic galaxy with an exceptionally bright core that emits non-thermal radiation. Carl Seyfert first observed these peculiar galaxies in the early 1940s. The majority of Seyfert galaxies have a spiral structure and vary in brightness over several months. This indicates a small energy source (perhaps a black hole) at the nucleus that is responsible for the non-thermal radiation. Seyfert galaxy luminosity varies between one-tenth and ten times the luminosity of our galaxy.
A spiral galaxy whose nucleus shows bright spectral emission lines in all wavelengths; a class of galaxies first described by C. Seyfert.