An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

3 AGN reside at the centres of galaxies

3.1 AGNs

It is clear that the objects studied by Fath and Seyfert, such as those shown in Figure 1 and Figure 2, are bright nuclei at the centres of apparently otherwise normal galaxies. For many AGN this fact is not immediately obvious. One of the two bright objects in Figure 3 is an AGN, the other is a foreground star in our own Galaxy. It is impossible to tell from the image which is which. Like many AGN, this one appears as a point source of light, just as stars do. Hence their discoverers called these objects quasi-stellar objects or QSOs, which have been contracted to quasars. Seyferts and quasars are among the subclasses of AGN.

Initially only AGN which were detected as radio sources were called quasars as opposed to QSOs. Now most astronomers use the terms interchangeably.

Figure 3
Figure 3 This image, taken by the Hubble Space Telescope, shows two very bright objects which appear to be point sources of light. Both these bright objects show diagonal spikes which are caused by diffraction of light within the telescope. The bright object at the centre of the image is an AGN, the other bright object (to the right of the AGN) is a forground star in our own Galaxy. Also apparent is a faint elliptical galaxy just above the AGN, a distorted spiral galaxy towards the top of the image, and a scattering of other galaxies
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