Active galaxy

An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas.

Related Topics:
Galaxy - Star - Interstellar gas

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This energy, depending on the active galaxy type, can be emitted across most of the electromagnetic spectrum, as infrared, radio waves, UV, X-ray and gamma rays.

Related Topics:
Electromagnetic spectrum - Infrared - Radio waves - UV - X-ray - Gamma ray

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Frequently, the abbreviation AGN (Active Galactic Nuclei) is used, since most active galaxies appear to be powered by a compact region in the galactic centre. Some of these compact regions emit jets of matter that can extend for very large distances, powering extended structures (such as radio galaxies and radio-loud quasars). But in all cases the active nucleus or central engine is the fundamental source of energy.

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The standard theoretical model is that the energy is generated by matter falling onto a supermassive black hole of between 106 and 109 solar masses. As the material falls into the black hole, angular momentum causes the material to flatten into an accretion disk. Frictional heating causes the infalling material to turn into plasma, and this charged moving material produces a strong magnetic field via the alpha mechanism. Frequently, one observes jets emanating from the accretion disk, although the mechanism of formation of those jets is poorly understood. The accretion mechanism is highly efficient at turning matter into energy, and can convert almost 50% of the mass-energy of an object into energy as compared with only a few percent with nuclear fusion.

Related Topics:
Supermassive black hole - Accretion disk - Plasma - Magnetic field - Alpha mechanism - Nuclear fusion

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It is believed that when the black hole has eaten all of the gas and dust in its neighborhood that the active galactic nucleus ceases to emit large amounts of radiation and becomes a normal galaxy. This model is supported by what appears to be a quiet supermassive black hole in the center of the Milky Way, and in other nearby galaxies, and also nicely explains why quasars appear to have been much more common in the early universe, when more fuel was available.

Related Topics:
Milky Way - Quasar - Early universe

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This model also explains the different types of active galactic nuclei, which are believed to all be due to the same type of source, but can appear quite different depending on the angle the source makes to the earth, and the amount of gas and dust available to be fed into the black hole.

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