Some galaxies are irregular—neither basically spiral nor elliptical in shape—and so do not fit conveniently into the Hubble classification of galaxies. Of these irregular galaxies, those that can be resolved into nebulae, stars, and clusters are classified as Irr 1; those that cannot be resolved in this way and appear to be amorphous are classified as Irr 2. Yet other galaxies are visually unremarkable, but are significant because they emit forms of radiation other than light, often radio waves. Included in this diverse group are quasars, the most enigmatic of cosmic objects. which is about 30,000 parsecs long. The tails contain stars and gas and were formed as the outlying stars of the two galaxies (originally normal spiral galaxies) were torn away during the interaction.
Most galaxies—regular and irregular—emit forms of radiation other than visible light, and many are sources of radio-wave emissions. Astronomers have detected weak radio signals from many spiral galaxies, including the center of our own, and stronger signals from several ellipticals. Some galaxies, however, are very strong emitters of radio waves, and these are classified as radio galaxies.
The first radio galaxy to be detected was Cygnus A, which emits about 1 million times more radio wavelength radiation than does the Milky Way. When these long-wavelength radiations are plotted on a map, and the resultant radio contours superimposed on a visual image of a radio galaxy, two very different pictures emerge: Cygnus A—a typical radio galaxy—appears visually as a roughly circular bright spot; the radio contours, however, reveal that the strongest radio emissions emanate from two regions (called lobes) situated one on each side of the visible galaxy.
At first, it was assumed that radio emissions were indicative of two galaxies colliding, but too many similar sources have since been discovered for this to be a realistic conclusion. In the case of Cygnus A, some astronomers have theorized that the radio emissions might indicate that a galaxy is undergoing the reverse process—splitting into two parts.
Since the discovery of Cygnus A, many other radio galaxies have been detected and their visible counterparts identified—Seyfert galaxies, for example, which have small but highly luminous nuclei, and are unusual sources of radiation, being a hundred times brighter in the infrared region than is our own galaxy. More recently, X-ray galaxies have been detected. These are optically irregular and contain a huge amount of dust and gas. Collisions in the hot gas are believed to be responsible for the X-ray emissions.
Quasars were discovered while radio astronomers were searching for the visible counterparts of radio sources. It was found that certain radio objects were accompanied by what looked like stars with unusual spectral lines; these starlike objects were called quasi-stellar radio sources, or quasars. Analysis of the spectral lines and their Doppler shifts indicates that quasars are fast-moving, extremely distant objects. They are also very compact and must be highly luminous. (They would not otherwise be visible at the vast distances at which they lie.) Quasars radiate up to a thousand times more energy than a normal galaxy. Most of the 1,300 or so quasars so far discovered are strong X-ray sources as well.
The most interesting question about quasars concerns the nature of the processes by which they generate huge amounts of energy in such compact regions of space. The most popular explanation for this phenomenon is the central black hole hypothesis. According to this, a quasar is like a normal galaxy, but has a central black hole from which it ultimately derives its energy. Gas from the stars in a quasar galaxy is gravitation-ally attracted to the center and accumulates in a ring (called an accretion ring) that circles the black hole, becoming increasingly hot as it does so and emitting X rays, ultraviolet radiation, and radio waves. Strong magnetic and electrical forces cause streams of electrons to be ejected along the axis of rotation. Ultimately, therefore, a quasar is powered by the energy generated by matter that is gravitation-ally attracted to the central black hole.
The origin and nature of the central black holes themselves, however, remains a mystery.