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Characteristics of Active Galaxies

An active galaxy is one which emits more total energy than most galaxies of comparable size. Visually, these galaxies are not very different than their more ordinary neighbors. However, active galaxies emit enormous amounts of energy in wavelengths that the human eye can't see, but scientific instruments can easily detect. Active galaxies fall into one of three categories, though theoretically each category represents the same object simply with a different orientation to Earth.

Structure
- Astronomers believe that black holes are the engines of active galaxies.
  
  The most distinctive element of an active galaxy is its central core. This structure may be so luminous that the rest of the galaxy is totally obscured. At great distances, active galaxies may be mistaken for stars when in fact they are collections of billions of stars. A massive black hole is theorized to occupy the center of active galaxies. Jets of electromagnetic radiation and matter shoot out from both poles of the galactic center. Active galaxies are among the most mysterious objects in the universe.
Emissions
- Most of the energy produced by active galaxies is in the form of invisible radiation, such as radio waves, X-rays and gamma rays. The luminosity of an active galaxy -- that is, the total light produced at all wavelengths -- can be thousands of times higher than a galaxy like the Milky Way. The emissions of an active galaxy are highly variable; radiation type and amount can change for no apparent reason. This variability indicates that a dynamic process is under way at the core of such galaxies.
Seyfert Galaxies
- Seyfert galaxies are on the lower end of the energy scale. Because the central core and its jet are not pointed at Earth, these galaxies appear fairly ordinary. They are spiral disks with very bright centers, with much of their brightness in the ultraviolet and radio spectrum. Some Seyfert galaxies can substantially change brightness in a matter of days, indicating a small black hole at their centers. Studies of Seyfert galaxies have also revealed that they are moving at high velocities relative to most galaxies.
Quasars
- By any measure, quasars are the most powerful objects in the universe. They are also among the most distant that astronomers have been able to observe. A quasar is the result of an active galaxy having its jet of matter and radiation pointed almost perfectly in the direction of Earth. Quasars are believed to be responsible for gamma-ray bursts, a sudden energy spike that until recently was a complete mystery.
Blazars
- The most enigmatic active galaxies earn the designation of blazar. These objects are high energy, though not quite as luminous as a typical quasar. Their variability is remarkable, with emissions that can change power within minutes. Like quasars, blazars have an emission jet that is aimed in Earth's general direction. At least 60 of these objects have been cataloged. An explanation of their rapid variability has not been forthcoming.

Astronomy