Astronomy & Astrophysics 101: Quasar
Arthur T Knackerbracket has processed the following story:
Quasars are a subclass of active galactic nuclei (AGNs), extremely luminous galactic cores where gas and dust falling into a supermassive black hole emit electromagnetic radiation across the entire electromagnetic spectrum. The gas and dust become luminous as a result of the extreme gravitational and frictional forces exerted on them as they fall into the black hole.
Quasars are some of the most luminous objects in the known Universe, typically emitting thousands of times more light than the entire Milky Way. They are distinguished from other AGNs by their tremendous luminosity, and their enormous distances from Earth. As the speed of light is finite, objects observed from Earth are seen as they were when the light we see left them. The nearest quasars to Earth are still several hundred million light-years away, which means that they are observed now as they were several hundred million years ago. The absence of quasars closer to Earth does not mean that there were never quasars in our region of the Universe, but instead means that quasars existed when the universe was younger. The study of quasars provides fascinating insights into the evolution of the Universe.
[...] Hubble has also imaged quasar ghosts - ethereal green objects which mark the graves of these objects that flickered to life and then faded. These unusual structures orbit their host galaxies and glow in a bright and eerie green hue, and offer insights into the pasts of these galaxies.
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