Astronomy Answers
AstronomyAnswerBook: Quasars

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This page answers questions about quasars. The questions are:

• [531] How were the black holes inside quasars formed?
• [9] What is een quasar?

[9]

1. Quasars

A quasar or QSO is a celestial body that at first sight looks like a star (i.e., as a point of light), but that turns out to be in the center of a galaxy. A quasar is much brighter than the rest of the galaxy, and can therefore be seen at much greater distances than the galaxy. The name "quasar" came from "quasi-stellar radio source", because the first quasars were discovered through their radio emissions (in 1963). Because quasars have now also been seen in many other kinds of electromagnetic radiation they are now also called "quasi-stellar objects" or QSOs.

Some quasars are very bright in radio emissions, but others are not. Those classes are called radio-quiet and radio-loud quasars.

Astronomers think that quasars are really large black holes that capture large amounts of matter from the surrounding galaxy, and that that matter radiates away a lot of energy while it falls towards the black hole, not just as visible light but also as, for example, radio waves and X-rays.

The intrinsically brightest quasar that I know of is called QSO B0827.9+5255 (but also has other names) and has an absolute bolometric magnitude of −32.5, which means it is as bright as 10¹⁵ or 10¹⁵ or 1,000,000,000,000,000 Suns, and about 63,000 times as bright as our whole Galaxy!

The dimmest quasars have absolute bolometric magnitudes of −23, which is still more than 10¹¹ or 10¹¹ or 100 thousand million times as bright as the Sun, and about 10 times as bright as our Galaxy. Anything dimmer than this is not considered to be a quasar. A Seyfert galaxy is a galaxy that has a quasar-like source in its center that is not bright enough to be called a quasar.

Quasars preferentially occur in the most massive galaxies. Radio-loud quasars seem to prefer elliptical galaxies, and radio-quiet quasars spiral galaxis. All quasars are far away, which means they existed only a long time ago. It seems that many massive galaxies contained quasars during their youth.

The professional journal article about QSO B0827.9+5255 is Astrophysical Journal, vol. 505, p. 529 (1998)

[531]

2. Black Holes inside Quasars

A quasar is the very active central part of a young galaxy. That central part is very bright in several kinds of radiation. It is assumed that the engine of a quasar is a supermassive black hole. Gas and other matter near the black hole that experience friction fall towards the black hole and get so very hot that it starts emitting lots of radiation (including light). This process will continue as long as enough gas remains around the black hole. After a thousand million years or so the gas around the black hole is mostly gone, and then the radiation declines accordingly. Then the central part of the galaxy loses its status as a quasar, but the supermassive black hole remains. Our Galaxy most likely contains such a supermassive black hole, too, and probably shone like a quasar for some time in its youth.

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Last updated: 2021-07-19