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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; but see J1819+3845 for another explanation). The highest redshift currently known for a quasar is 6.4 .
Properties of quasars
Of the several hundred quasars observed, all spectra have shown considerable redshifts, ranging from 0.06 to the recent maximum of 6.4. Therefore, all known quasars lie at great distances from us, the closest being 240 Mpc away and the farthest being 5500 Mpc away. Most quasars are known to lie above 1000 Mpc in distance; since light takes such a long time to cover these great distances, we are seeing quasars as they existed long ago - the universe as it was in the distant past.
Although faint when seen optically, their high redshift and great distance imply that quasars are the brightest objects in the known universe. In a general sense, quasars range in luminosity from 1038 W (the luminosity of the brightest radio galaxy) to 1042 W. The average quasar is found to be 1040 W, which is comparable to 1000 Milky Way galaxies, or 10 trillion Suns. Quasars are also found to vary in luminosity in differing time periods. Some vary in brightness every few months, weeks, days, or hours. This recent evidence has allowed scientists to theorize that quasars exhibit energy in a very small region, since each part of the quasar would have to be in contact with other parts on such a timescale to coordinate the luminosity variations. As such, a quasar varying on the time scale of a few weeks cannot be larger than a few light weeks across.
When compared to active galaxies, quasars exhibit much of the same properties. Radiation is nonthermal and some are shown to have emission jets and lobes. Quasars can be observed in many parts of the electromagnetic spectrum including radio, infrared, optical, ultraviolet, X-ray and even gamma rays while most quasars are found to emit in the infrared.
Quasar emission generation
Since quasars exhibit properties of all active galaxies, many scientists have compared the emissions from quasars to those of small active galaxies due to their likeness. The best explanation for quasars is that they are powered by supermassive black holes. Scientists theorize to create the luminosity of 10^40 W (average brightness of a quasar), a super-massive black hole would have to consume 10 stars per year. The brightest known quasars are thought to devour 1000 solar masses of material every year. Quasars are thought to 'turn on' and off depending on their surroundings. One implication is that a quasar would not, for example, continue to feed at that rate for 10 billion years, which nicely explains why there are no nearby quasars. In this framework, after a quasar finishes eating up gas and dust, it becomes an ordinary, normal galaxy.
Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest quasars (z > 4) display a Gunn-Peterson trough and 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 their spectra contain 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.
History of quasar observation
The first quasars were discovered with radio telescopes in the late 1950s. Many were recorded as radio sources with no corresponding visible object. Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalog as astronomers scanned the skies for the optical counterparts. In 1960 radio source 3C 48 was finally tied to an optical object. Astronomers detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum. Containing many unknown broad emission lines, the anomalous spectrum defied interpretation. In 1963 a breakthrough was achieved. Another radio source, 3C 273, that was tied with an optical object and exhibiting the same strange emission lines. These were found not to be strange at all - but rather spectral lines of hydrogen found to be redshifted at the rate of 16 percent! This discovery by Maarten Schmidt found that 3C 273 was receding at a rate of 44,000 km/s. This discovery revolutionized quasar observation and allowed other astronomers to find redshifts from the emission lines from other radio sources. 3C 48 was found to have a redshift of 37 percent - 1/3 the speed of light.
The neologism "quasar", derived from quasi-star or a contraction of the more formal term quasi-stellar object, came to be used as a descriptor for these puzzling objects. Later it was found that not all (actually only 10% or so) quasars have strong radio emission (are 'radio-loud'). The name 'QSO' (quasi-stellar object) is sometimes given to the radio-quiet class. Others use the terms 'radio-loud' and 'radio-quiet quasars'.
One great topic of debate during the 1960s was whether quasars were nearby objects or distant objects as implied by their redshift. It was suggested, for example, that the redshift of quasars was not due to the Doppler effect but rather to light escaping a deep gravitational well. 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 some hitherto unknown form of stable antimatter and that this might account 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. Varshni has predicted that large redshifts attributed to quasars are a consequence of natural lasing on the emission spectra  – see laser star model of quasars. 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 discs 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.
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