Galaxies are not stable, on the contrary: they grow and move, that is, they are active. Some more and others not so much. Almost all galaxies have a black hole in their center.
While the black hole is active, it catches and engulfs all the matter around it, like a whirlpool.
When the black hole of an active galaxy no longer has the capacity to swallow more, matter continues to revolve around it, but it no longer falls inside. Galaxies whose black hole is still active are called active galaxies, as the Galaxy Centaurus A, NGC 5128.
Active galaxies are distinguished by their shape and the large amount of radiation they emit. The black hole in the core is surrounded by a bright disc of very hot matter, dust and gas. It is called accretion disk, and it spirals while emitting high energy radiation. From the poles, the black hole throws huge jets of particles into space, which can measure thousands of light years in length.
The Milky Way was also an active galaxy in the past.
Types of active galaxies
Four types of active galaxies are known: quasars, blazares, radiogalaxies and Seyfert galaxies.
The quasars and the blazares are the farthest and most energetic objects known. They are billions of light years from Earth. We see them as they were in the past, when galaxies were still forming. They are the brightest objects in the Universe, although they are so far away that their light reaches us very weakly. Almost all active galaxies that are known are quasars.
Radiogalaxies and Seyfert galaxies are closer and also very bright objects. They emit X-rays, infrared radiation and radio waves. Their radiation is so great that they are the main source of radio waves throughout the Cosmos.
In the following link we can see the X-ray image of a toroid in the nearby galaxy NGC 4388, of type Seyfert II.
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