Many galaxies look almost like one of the Hubble classifications, but with some unusual feature. For example, imagine an elliptical galaxy that looks like someone sliced it through the center, pulled it apart a little bit, and displaced each half sideways. Hubble called these galaxies peculiar and added the designation, pec, to the classification. The galaxy described above might be an E0 pec galaxy. Whatever causes a galaxy to look as if it were ripped apart as described above would require large amounts of energy. Peculiar galaxies are therefore interesting because they often tend to be the active galaxies that emit large amounts of energy.
Active galaxies are galaxies that emit far more energy than normal galaxies. A galaxy is considered an active galaxy if it emits more than 100 times the energy of the Milky Way galaxy. Active galaxies often have a very compact central source of energy, much of which is emitted as radio waves rather than optical light. These radio waves are emitted by electrons moving in a helical path in a strong magnetic field at speeds near the speed of light. Active galaxies also often have a peculiar photographic appearance, which can include jets of material streaming out from the nucleus or the appearance of either explosions or implosions. They also tend to vary erratically in brightness on rapid time scales. There are a number of varieties of active galaxies, including: compact radio galaxies, extended radio galaxies, Seyfert galaxies, BL Lacertae objects, and quasars.
Compact radio galaxies appear photographically as ordinary giant elliptical galaxies. Radio telescopes however reveal a very energetic compact nucleus at the center. This radio nucleus is the source of most of the energy emitted by the galaxy.
Perhaps the best known compact radio galaxy is M87. This giant elliptical galaxy has both a very compact energetic radio source in the nucleus and a jet consisting of globs of material shooting out from the nucleus. Recent observations from the Hubble Space Telescope provide strong evidence that this core contains a supermassive black hole.
Extended radio galaxies consist of two giant lobes emitting radio waves. These lobes are on either side of a peculiar elliptical galaxy. The lobes can appear straight or curved as if the galaxy is moving through space. These lobes are the largest known galaxies and can stretch for millions of light years.
Seyfert galaxies look like spiral galaxies with a hyperactive nucleus. The spiral arms appear normal photographically, but they surround an abnormally bright nucleus. Seyfert galaxies also have evidence for hot turbulent interstellar gas.
BL Lacertae objects look like stars. In reality they are most likely to be very active nuclei of elliptical galaxies. However, BL Lacertae objects have sufficiently unusual behavior, including extremely rapid and erratic variations in observed properties, that their exact nature is not known for certain.
Quasars also look like stars, but they are perhaps the most distant and energetic objects in the universe known so far. Most astronomers consider them the very active nuclei of distant galaxies in the early stages of evolution. As for the other types of active galaxies they produce large amounts of energy in a very small volume. Most astronomers currently think that the energy source is a supermassive black hole.