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Active Galactic Nuclei

Some galaxies have unusually bright cores. In fact they are so bright they cannot be explained by the presence of stars. The cores may be 100 times brighter than the rest of the galaxy! And all this light is produced in a very small area (compared to the rest of the galaxy). Usually this area is comparable in size with our solar system. Their size is sometimes hinted by the fact that the active galactic nuclei (AGN for short) vary quickly in brightness.
The brightness of the core is explained by the presence of an active black hole millions or even billions of times heavier than the sun. The supermassive black hole is being fed with gas or stars falling into it from the vicinity. The gas is heated to very high temperatures (millions of degrees) and thereby it emits enormous quantities of light, such as radio waves, x-rays and gamma rays.
Some of the matter that falls into the supermassive black hole is ejected as highly focused jets from its' rotational poles. These jets may have velocities close to the speed of light, and the jets may extend thousands of light years into space. Astronomers have discovered different kinds of AGN. They are:






Quasars: Quasars appear no different on the sky than stars, at first glance. That is why the are called Quasars (derived from "quasi-stellar"). However, when their spectra is analyzed quasars prove to be very different from stars. Their redshift is usually very high, which means they are far away, which means that they were much more common in the beginning of the universe. Being a quasar might be a natural state in the evolution of galaxies.
Quasars are the most luminous, powerful, and energetic objects known in the universe. Some shine stronger than 1 000 Milky Ways, (1012 Suns). For this luminosity to be sustained, the supermassive black hole, which powers (heats up) the quasar, has to swallow the equivalent of several solar masses (1 MSun = 2 x 1030Kg.) each year.
The largest known consumation of stars about 1800 MSun each year! When matter is no longer falling in to the supermassive black hole, the quasar (actually all AGNs) "shut down", meaning they no longer emit light.

Blazars: These objects are among the most violent phenomena we know of in the universe. They are like quasars, with the difference that the jets which are emitted from the black hole is pointed directly at us, on Earth.

Seyfert Galaxies: produce spectral lines from highly ionized gas and emit radio waves from synchrotron radiation in the jets. They also emit strong infrared, ultraviolet and x-rays. Almost all Seyfert galaxies are spiral galaxies.

Radio Galaxies: are almost always elliptical galaxies. There is no evidence which would explain why.
They produce enormous quantities of radio waves due to synchrotron emission.



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