From Wikipedia, the free encyclopedia.
A quasar (from quasi-stellar radio source) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source), and has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubble's law, which implies that quasars must be very distant and must emit more energy than dozens of normal galaxies.
Some quasars display rapid changes in luminosity, which implies that they are small (an object cannot change faster than the time it takes light to travel from one end to the other). The highest redshift currently known for a quasar is 6.4 [1], which is significant because it implies a maximum distance—more distant quasars should be easily observable if they existed. This is taken to mean that the oldest observed quasars correspond to the beginning of galaxy formation.
Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest quasars clearly have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region but rather a spiky area known as the Lyman-alpha forest. This indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.
One other interesting characteristic of quasars is that they show evidence of elements heavier than helium. This is taken to mean that galaxies underwent a massive phase of star formation creating population III stars between the time of the Big Bang and the first observed quasars. However, this prediction has the problem in that, as of 2004, no evidence for such stars have been found, and it may seriously undermine our current views of the universe if no such stars are found in the next few years, and alternate mechanisms for producing heavy elements cannot be found.
One great topic of debate during the 1960s was whether quasars are nearby objects or distant objects as implied by their redshift. One strong argument against cosmologically distant quasars was that it implied energies that were far in excess of known energy conversion processes, including nuclear fusion. At this time, there were some suggestions that quasars were made of antimatter and that this accounts for their brightness. This objection was removed with the proposal of the accretion disc mechanism in the 1970s, and today the cosmological distance of quasars is accepted by almost all researchers.
Although most astrophysicists now believe that quasars are cosmological objects, there remain a few who cite evidence that they are nearby. For example, Y. P. Varshini has predicted that large redshifts attributed to quasars are a consequence of natural lasing on the emission spectra. Varshni and others also dispute the standard explanation of superluminal motion. Also, Halton Arp has stated that quasars are spawned by galaxies and has argued that quasars can be observed to be interacting with galaxies.
In the 1980s, unified models were developed in which quasars were viewed as simply a class of active galaxies, and a general consensus has emerged that in many cases it is simply the viewing angle that distinguishes them from other classes, such as (blazars and radio galaxies). The huge luminosity of quasars is believed to be a result of friction caused by gas and dust falling into the accretion disks of supermassive black holes, which can convert about half of the mass of an object into energy as compared to a few percent for nuclear fusion processes.
This mechanism is also believed to explain why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it. This means that it is possible that most galaxies, including our own Milky Way, have gone through a quasar stage and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.
Our best evidence suggests that a quasar is a supermassive black hole many millions or even billions times the mass of our Sun that emits intense radiation due to a large accretion disk of matter falling into the black hole.
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