7.2 Types of active galaxy

• All active galaxies have a compact, energetic nucleus - an AGN.

• Seyfert galaxies are spiral galaxies with bright, point-like nuclei which vary in brightness. They show excesses at far infrared and other wavelengths, and have strong, broad emission lines.

• Quasars resemble very distant Seyfert galaxies with very luminous nuclei. They are variable. About 10% are strong radio sources thought to be powered by jets of material moving at speeds close to the speed of light.

• Radio galaxies are distinguished by having giant radio lobes fed by one or two jets. They have a compact nucleus like Seyfert galaxies. The compact nucleus is variable, and its emission lines may be broad or narrow.

• Blazars exhibit a continuous spectrum across a wide range of wavelengths and emission lines, when present, are broad and weak. They are variable on very rapid timescales.