![]() To explain this remarkable phenomenon, a remarkable explanation is required. It has to produce 10 11 or more times the power of our own Sun, but it has to do this in a region little larger than the Solar System. The engine that powers the AGN, the tiny nucleus of the active galaxy, is a great mystery. A further complication is that some nearby galaxies, including our own, show evidence of a low level of activity in their nuclei, but we shall concentrate in this course on the prominent and powerful active galaxies. At present we cannot tell which of these scenarios may be correct. So the fact that we observe a small percentage of galaxies in an active phase could mean that every galaxy becomes active for the same small percentage of its lifetime, but it could also mean that a small proportion of galaxies become active for a longer time. We have been aware of these objects only since the 1940s, and the galaxies have been around for at least 10 10 years. ![]() Whether every galaxy goes through an active phase in its lifetime, or whether active galaxies are a separate class of object is not clear. An active galaxy may be regarded as a normal galaxy plus an AGN with its attendant effects.Īctive galaxies seem to be quite rare in the nearby Universe. It is usually found that the tiny source region can be traced to the nucleus of the galaxy, so the origin of the excess radiation is attributed to the active galactic nucleus or AGN. There is also evidence in each case that a very large amount of energy is being released in a region that is tiny compared with the size of the galaxy, and so they are classified together. These types were discovered separately and at first seemed quite different, but they all have some form of spectral peculiarity. Active galaxies show extra emission of radiation, and this is most apparent from the spectra.Īctive galaxies come in a variety of types, including Seyfert galaxies, quasars, radio galaxies and blazars. Normal galaxies contain stars that are generally similar to those in our own Galaxy and spiral galaxies have additional similarities to the Milky Way in their gas and dust content. But if your telescope were equipped to examine the spectra of the galaxies, then the active galaxies would stand out. Even in images taken with the most modern equipment on a large telescope, it can be difficult to pick out the galaxies now known as 'active' from the other more normal galaxies.
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