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  A quasi-stellar radio source (quasar) is a powerfully energetic and distant galaxy with an active galactic nucleus. Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.

While there was initially some controversy over the nature of these objects as recently as the early 1980s, there was no clear consensus as to their nature there is now a scientific consensus that a quasar is a compact region 10-10,000 times the Schwarzschild radius of the central supermassive black hole of a galaxy, powered by its accretion disc.

Quasars show a very high redshift, which is an effect of the expansion of the universe between the quasar and the Earth. When combined with Hubble's law, the implication of the redshift is that the quasars are very distant -- and thus, it follows, very ancient. The most luminous quasars radiate at a rate that can exceed the output of average galaxies, equivalent to one trillion (1012) suns. This radiation is emitted across the spectrum, almost equally, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays. In early optical images, quasars looked like single points of light (i.e. point sources), indistinguishable from stars, except for their peculiar spectra. With infrared telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been identified in some cases.[1] These galaxies are normally too dim to be seen against the glare of the quasar, except with these special techniques. Most quasars cannot be seen with small telescopes, but 3C 273, with an average apparent magnitude of 12.9, is an exception. At a distance of 2.44 billion light-years, it is one of the most distant objects directly observable with amateur equipment.

Some quasars display changes in luminosity which are rapid in the optical range and even more rapid in the X-rays. This implies that they are small (Solar System sized or less) because an object cannot change faster than the time it takes light to travel from one end to the other; but relativistic beaming of jets pointed nearly directly toward us explains the most extreme cases. The highest redshift known for a quasar (as of December 2007[update]) is 6.43,[2] which corresponds (assuming the currently-accepted value of 71 for the Hubble Constant) to a distance of approximately 28 billion light-years. (N.B. there are some subtleties in distance definitions in cosmology, so that distances greater than 13.7 billion light-years, or even greater than 27.4 = 2*13.7 billion light-years, can occur.)

Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies. Large central masses (106 to 109 Solar masses) have been measured in quasars using 'reverberation mapping'. Several dozen nearby large galaxies, with no sign of a quasar nucleus, have been shown to contain a similar central black hole in their nuclei, so it is thought that all large galaxies have one, but only a small fraction emit powerful radiation and so are seen as quasars. The matter accreting onto the black hole is unlikely to fall directly in, but will have some angular momentum around the black hole that will cause the matter to collect in an accretion disc.
  Properties of Quasars

More than 200,000 quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.06 and 6.5. Applying Hubble's law to these redshifts, it can be shown that they are between 780 million and 28 billion light-years away. Because of the great distances to the furthest quasars and the finite velocity of light, we see them and their surrounding space as they existed in the very early universe.

Most quasars are known to be farther than three billion light-years away. Although quasars appear faint when viewed from Earth, the fact that they are visible from so far away means that quasars are the most luminous objects in the known universe. The quasar that appears brightest in the sky is 3C 273 in the constellation of Virgo. It has an average apparent magnitude of 12.8 (bright enough to be seen through a small telescope), but it has an absolute magnitude of −26.7. From a distance of about 33 light-years, this object would shine in the sky about as brightly as our sun. 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 revealed that this system is gravitationally lensed. A study of the gravitational lensing in this system suggests that it has been magnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273.

Quasars were much more common in the early universe. This discovery by Maarten Schmidt in 1967 was early strong evidence against the Steady State cosmology of Fred Hoyle, and in favor of the Big Bang cosmology. Quasars show where massive black holes are growing rapidly (via accretion). These black holes grow in step with the mass of stars in their host galaxy in a way not understood at present. One idea is that the jets, radiation and winds from quasars shut down the formation of new stars in the host galaxy, a process called 'feedback'. The jets that produce strong radio emission in some quasars at the centers of clusters of galaxies are known to have enough power to prevent the hot gas in these clusters from cooling and falling down onto the central galaxy.

Quasars are found to vary in luminosity on a variety of time scales. Some vary in brightness every few months, weeks, days, or hours. This means 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. As such, a quasar varying on the time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion which powers stars. The release of gravitational energy[citation needed] by matter falling towards a massive black hole is the only process known that can produce such high power continuously. (Stellar explosions - Supernovas and gamma-ray bursts - can do so, but only for a few weeks.) Black holes were considered too exotic by some astronomers in the 1960s, and they suggested that the redshifts arose from some other (unknown) process, so that the quasars were not really so distant as the Hubble law implied. This 'redshift controversy' lasted for many years. Many lines of evidence (seeing host galaxies, finding 'intervening' absorption lines, gravitational lensing) now demonstrate that the quasar redshifts are due to the Hubble expansion, and quasars are as powerful as first thought.

Quasars have all the same properties as active galaxies, but are more powerful: Their radiation is 'nonthermal' (i.e. not due to a black body), and some (~10%) are observed to also have jets and lobes like those of radio galaxies that also carry significant (but poorly known) amounts of energy in the form of high energy (i.e. rapidly moving, close to the speed of light) particles (either electrons and protons or electrons and positrons). Quasars can be detected over the entire observable 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 m), in the near infrared. A minority of quasars show strong radio emission, which originates from jets of matter moving close to the speed of light. When looked at down the jet, these appear as a blazar and often have regions that appear to move away from the center faster than the speed of light (superluminal expansion). This is an optical trick due to the properties of special relativity.

Quasar redshifts are measured from the strong spectral lines that dominate their optical and ultraviolet spectra. These lines are brighter than the continuous spectrum, so they are called 'emission' lines. They have widths of several percent of the speed of light, and these widths are due to Doppler shifts due to the high speeds of the gas emitting the lines. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the Lyman series and Balmer series), Helium, Carbon, Magnesium, Iron and Oxygen are the brightest lines. The atoms emitting these lines range from neutral to highly ionized, i.e. many of the electrons are stripped off the ion, leaving it highly charged. This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars, which cannot produce such a wide range of ionization
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