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When I find a faint object on the sky that looks like a star or a far away galaxy how do I know that it really is a quasar?

I guess that I could first compare the apparent magnitude (how bright it appears) and the redshift of spectral lines in light from the object.

Then if I interpret the redshift as Hubble redshift and that interpretation tells me that if the object in question shines brighter than some treshold value it must be a quasar?

Is there such a treshold value or how do astronomers know from observations what objects are quasars or not quasars?

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  • $\begingroup$ This seems the sort of thing that is easy to research for yourself. Then if you had specific questions about the identification of quasars you could ask here. $\endgroup$ – John Rennie Mar 27 at 15:53
  • $\begingroup$ This is what wikipedia says on "Active Galactic Nucleous": "Radio-quiet quasars/QSOs. These are essentially more luminous versions of Seyfert 1s: the distinction is arbitrary and is usually expressed in terms of a limiting optical magnitude. " $\endgroup$ – Agerhell Apr 4 at 14:39
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In addition to measuring the redshift and the apparent brightness of a faint object, astronomers also measure the spectrum of the light coming from it. These three things are used to identify whether or not a faint object really is a quasar, and the details of this identification are available on the web and in any good introductory text on astronomy.

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