A distant, compact object far beyond our Galaxy, which looks starlike on a photograph, but has a redshift characteristic of an extremely remote object. The word is a contraction of quasi-stellar object. The distinctive features of quasars are an extremely compact structure and high redshift corresponding to velocities approaching the speed of light. Implied distances run into thousand millions of parsecs, making them the most distant and luminous objects in the universe, millions of times brighter than normal galaxies. They are thought to be young galaxies with massive black holes at their centres into which gas and stars are falling.
A quasar (contraction of QUASi-stellAR radio source) is an astronomical source of electromagnetic energy, including light, which shows a very high redshift. To be observable at that distance, the energy output of quasars must dwarf that of almost every known astrophysical phenomenon with the exception of comparatively short-lived supernovae and gamma-ray bursts.
In optical telescopes, most quasars look like single points of light (i.e.
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.
Quasars are believed to be powered by accretion of material onto supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies.
Knowledge of quasars is advancing rapidly.
Properties of quasars
More than 100,000 quasars are known. Therefore, all known quasars lie at great distances from us, the closest being 240 Mpc (780 million ly) away and the farthest being 4 Gpc (13 billion ly) away. 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 implies that these objects lie at a great distance from us, making quasars the brightest objects in the known universe. This quasar's luminosity is, therefore, about 2 trillion (2 × 1012) times that of our sun, or about 100 times that of the total light of average giant galaxies like our Milky Way.
The hyperluminous quasar APM 08279+5255 was, when discovered in 1998, given an absolute magnitude of −32.2, although high resolution imaging with the Hubble Space Telescope and the 10 m Keck Telescope reveal that this system is gravitationally lensed.
Quasars are found to vary in luminosity on a variety of time scales. This evidence has allowed scientists to theorize that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale to coordinate the luminosity variations.
Quasars exhibit many of the same properties as active galaxies: Radiation is nonthermal and some are observed to have jets and lobes like those of radio galaxies. Quasars can be observed in many parts of the electromagnetic spectrum including radio, infrared, optical, ultraviolet, X-ray and even gamma rays. Most quasars are brightest in their rest-frame near-ultraviolet (near the 1216 angstrom (121.6 nm) Lyman-alpha emission line of hydrogen), but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 9000 angstroms (900 nm or 0.9 µm), in the near infrared.
Quasar emission generation
Since quasars exhibit properties common to all active galaxies, many scientists have compared the emissions from quasars to those of small active galaxies due to their similarity. To create a luminosity of 1040 W (the typical brightness of a quasar), a super-massive black hole would have to consume the material equivalent of 10 stars per year. The brightest known quasars are thought to devour 1000 solar masses of material every year. 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.
Quasars also provide some clues as to the end of the Big Bang's reionization.
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.
History of quasar observation
The first quasars were discovered with radio telescopes in the late 1950s. This discovery revolutionized quasar observation and allowed other astronomers to find redshifts from the emission lines from other radio sources.
The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in 1964, in Physics Today, to describe these puzzling objects:
So far, the clumsily long name ‘quasi-stellar radio sources’ is used to describe these objects.
—Hong-Yee Chiu in Physics Today, May, 1964
Later it was found that not all (actually only 10% or so) quasars have strong radio emission (are 'radio-loud').
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. Quasars also show unusual spectral emission lines which were previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well . 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.
In 1979 the gravitational lens effect predicted by Einstein's General Theory of Relativity was confirmed observationally for the first time with images of the double quasar 0957+561.
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 an active stage (appearing as a quasar or some other class of active galaxy depending on black hole mass and accretion rate) 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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