The term active galaxy is used to describe a wide variety of galaxies that have unusual characteristics and that often have a peculiar, disturbed appearance. Whereas most of the energy radiated by a conventional galaxy like the Milky Way system is starlight – the combined output of the billions of stars and HII regions that it contains – an active galaxy radiates strongly over a wide range of wavelengths and is much brighter than an ordinary galaxy at radio, infrared, ultraviolet, and x-ray wavelengths. Their overall luminosities range from 1037 to more than 1040 watts (up to several thousand times that of the Milky Way Galaxy). Much of the energy radiated by an active galaxy is emitted by charged particles moving at high speeds in magnetic fields rather than by stars. An active galaxy contains a compact, highly luminous core that, in many cases, varies markedly and rapidly in brightness, and from which, in numerous cases, narrow jets of radiating material are being ejected. The seat of all this activity is known as an active galactic nucleus (AGN).

The principal types of active galaxies are radio galaxies, quasars, BL Lacertae objects (BL Lacs) or “blazars,” and Seyfert galaxies.

RADIO GALAXIES

Radio galaxies are so named because they are powerful sources of radio emission that radiate much more strongly at radio wavelengths than do conventional galaxies.