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most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. The light from the central nucleus varies in less than a year, which implies that the emitting region must be less than one light year across. They are named for the astronomer Carl Seyfert, who studied them extensively in the 1940s. They are a subclass of active galactic nuclei.

The Circinus Galaxy, a Seyfert Type 2 spiral galaxy.

Seyfert galaxies are characterized by extremely bright nuclei, and spectra which have very bright emission lines of hydrogen, helium, nitrogen, and oxygenOxygen is the chemical element in the periodic table that has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe. Molecular oxygen (O, often called free oxygen on Earth is thermodynamically un. These emission lines exhibit strong Doppler broadeningThe Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves, that propagate in a wave medium, the velocity of the observer and t, which implies velocitiesVelocity (symbol: v is a vector measurement of the rate and direction of motion. The scalar absolute value ( magnitude) of velocity is speed. Velocity can also be defined as rate of change of displacement or just as the rate of displacement, depending on from 500 to 4000 km/s, and are believed to originate in an accretion disk surrounding the central black hole. Each part of the accretion disk has a different velocity relative to our line of sight, and the faster the gas is rotating about the black hole, the broader the line will be. The narrow lines are believed to originate from the outer part of the disk where the rotational velocity is lower, while the broad lines originate closer to the black hole. This is confirmed by the fact that the narrow lines do not vary detectably, which implies that the emitting region is large, contrary to the broad lines which can vary on relatively short timescales. Seyfert galaxies also show strong emission in the radio, infraredInfrared IR radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. The name means "below red" (from the Latin infra "below"), red being the color of visible light of longest wavelength., ultravioletNote: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. Ultraviolet UV radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. It can be subdivided into, and X-rayIn the NATO phonetic alphabet, X-ray represents the letter X. Rontgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz to 60 EHz). X-rays parts of the spectrum.

Seyferts were first classified as Type 1 or 2, depending upon whether the spectra show both narrow and broad emission lines, or only narrow lines. They are now given a fractional classification depending upon the relative strengths of the narrow and broad components (e.g. Type 1.5 or Type 1.9). The narrow and broad components are believed to both originate from the accretion disk, but in Type 2 Seyferts it is believed that the broad component is obscured by dust and/or by our viewing angle on the galaxy. In some Type 2 Seyfert galaxies, the broad component can be observed in polarized light; it is believed that light from the broad-line region is scattered by a hot, gaseous halo surrounding the nucleus, allowing us to view it indirectly. This effect was first discovered in the Type 2 Seyfert NGC 1068.