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Artist's rendering of the accretion disk in ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun.[1] Credit: ESO/M. Kornmesser

A quasar (/ˈkwzɑːr/) (also known as a QSO or quasi-stellar object) is an active galactic nucleus (AGN) of very high luminosity. Most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of Solar masses. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk, as gas in the accretion disk falls toward the black hole, energy is released in the form of electromagnetic radiation. Quasars emit energy across the electromagnetic spectrum and can be observed at radio, infrared, visible, ultraviolet, and X-ray wavelengths. The most powerful quasars have luminosities exceeding 1041 W, thousands of times greater than an ordinary large galaxy such as the Milky Way.[2]

The term "quasar" originated as a contraction of "quasi-stellar radio source", because quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin, and when identified in photographic images at visible wavelengths they resembled faint star-like points of light. High-resolution images of quasars, particularly from the Hubble Space Telescope, have demonstrated that quasars occur in the centers of galaxies, and that some quasar host galaxies are strongly interacting or merging galaxies.[3] As with other categories of AGN, the observed properties of a quasar depend on many factors including the mass of the black hole, the rate of gas accretion, the orientation of the accretion disk relative to the observer, the presence or absence of a jet, and the degree of obscuration by gas and dust within the host galaxy.

Quasars are found over a very broad range of distances (corresponding to redshifts ranging from z < 0.1 for the nearest quasars to z > 7 for the most distant known quasars), and quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, or approximately 10 billion years ago,[4] as of 2017, the most distant known quasar is ULAS J1342+0928 at redshift z=7.54; light observed from this quasar was emitted when the Universe was only 690 million years old. The supermassive black hole in this quasar is the most distant black hole identified to date, and is estimated to have a mass that is 800 million times the mass of our Sun.[5][6][7]


The term "quasar" was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, to describe certain astronomically-puzzling objects:

So far, the clumsily long name 'quasi-stellar radio sources' is used to describe these objects, because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form 'quasar' will be used throughout this paper.

History of observation and interpretation[edit]

Hubble images of quasar 3C 273. At right, a coronagraph is used to block the quasar's light, making it easier to detect the surrounding host galaxy.
Quasar QSO-160913+653228 is so distant its light has taken nine billion years to reach the telescope that took this photo, two thirds of the time that has elapsed since the Big Bang.[8]


Between 1917 and 1922, it became clear from work by Heber Curtis, Ernst Öpik and others, that some objects ("nebulae") seen by astronomers were in fact distant galaxies like our own. But when radio astronomy commenced in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation.

The objects emitted large amounts of radiation of many frequencies, but no source could be located optically, or in some cases only a faint and point-like object somewhat like a distant star, the spectral lines of these objects, which identify the chemical elements of which the object is composed, were also extremely strange and defied explanation. Some of them changed their luminosity very rapidly in the optical range and even more rapidly in the X-ray range, suggesting an upper limit on their size, perhaps no larger than our own Solar System,[9] this implies an extremely high power density.[10] Considerable discussion took place over what these objects might be, they were described as "quasi-stellar [meaning: star-like] radio sources", or "quasi-stellar objects", a name which reflected their controversial and unclear nature, and gradually this became shortened to "quasar". In 1966, Halton "Chip" Arp published his Atlas of Peculiar Galaxies, which included many quasars and other anomalous objects, fueling debate over their nature and how they should be classified.

In the 1950s and 1960s, the universe was not understood as well as it is now, it was known that more distant objects were moving away from us faster ("Hubble's Law"), but not yet confirmed that this was a result of space itself expanding. It was known that light from objects has distinct spectra but not fully appreciated how extreme the redshifting effect could be on an object's detected spectrum. Many astronomical objects now known to exist and studied in detail, were also not very well defined, or were considered theoretical or exotic ideas at the time, however it was understood that as a consequence of Special Relativity, light from a rapidly moving or receding source would be redshifted, meaning, its radiation would appear at lower frequencies than otherwise expected. It was also known that light from a very massive object would be redshifted due to the mass involved, as a result of general relativity.

Early observations (1960s and earlier)[edit]

The first quasars (3C 48 and 3C 273) were discovered in the late 1950s, as radio sources in all-sky radio surveys.[11][12][13][14] They were first noted as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size.[15] Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalogue as astronomers scanned the skies for their optical counterparts; in 1963, a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A. Matthews. Astronomers had detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum, which contained many unknown broad emission lines, the anomalous spectrum defied interpretation.

British-Australian astronomer John Bolton made many early observations of quasars, including a breakthrough in 1962. Another radio source, 3C 273, was predicted to undergo five occultations by the Moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to optically identify the object and obtain an optical spectrum using the 200-inch Hale Telescope on Mount Palomar. This spectrum revealed the same strange emission lines. Schmidt concluded that these were actually spectral lines of hydrogen redshifted at the rate of 15.8 percent. If this was correct, then 3C 273 was receding at an emormous velocity, around 47,000 km/s.[16] This discovery revolutionized quasar observation and other astronomers found similar redshifts in the emission lines from other quasars. 3C 48 itself was found to have a redshift of 37% of the speed of light. But the interpretation of these observations was heavily debated, and Bolton's suggestion that the radiation detected from quasars were ordinary spectral lines from highly redshifted sources was not widely accepted at the time.

Redshift controversy[edit]

One great topic of debate during the 1960s was how to interpret the redshifted light and comparatively small sizes seen with quasars.

An extreme redshift could imply great distance and velocity, but could also be due to extreme mass, or perhaps some other unknown laws of nature. Extreme velocity and distance would also imply immense power output, which lacked explanation, and conflicted with the traditional and predominant Steady State theory of the universe, the small sizes were confirmed by interferometry and by observing the speed with which the quasar as a whole varied in output, and by their inability to be seen in even the most powerful visible light telescopes as anything more than faint starlike points of light. But if they were small, the power output became harder to explain. If quasars were very small and nearer to our galaxy, it would be easy to explain the apparent power output, but less easy to explain their redshifts and lack of detectable movement against the background of the universe.

Various explanations were proposed over time, it was suggested, for example, that the redshift of quasars was not due to the expansion of space but rather to light escaping a deep gravitational well. However a star of sufficient mass to form such a well would be unstable and in excess of the Hayashi limit.[17] Quasars also show forbidden 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.[18] There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they 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.[citation needed] Others speculated that quasars were a white hole end of a wormhole.[19][20]

In 1963 Maarten Schmidt found a visible companion to the quasar 3C 273. Using the Hale telescope, Schmidt found the same odd spectra, but was able to demonstrate that it could be explained as the spectrum of hydrogen, shifted by a very large 15.8% If this was due to the physical motion of the "star", it would represent a speed of 47,000 km/s, far beyond the speed of any known star and defying an obvious explanation. Nor would this explain the huge radio emissions that had led to its original detection.

Schmidt noted that redshift is also associated with the expansion of the universe, as codified in Hubble's law. If the measured redshift was due to expansion, then the object in question would have to be very far away; in that case, it would have to have an extraordinarily high luminosity, equally beyond any object seen to date. This extreme luminosity would also explain the large radio signal. Schmidt concluded quasars are very distant, very luminous objects.[21]

Schmidt's explanation for the high redshift was not universally accepted at the time. Another explanation that was offered was that it was gravitational redshift that was being measured; this would require a massive object that would also explain the high luminosities. A star large enough to produce the measured redshift would be well beyond the Hayashi limit. Several other mechanisms were proposed as well, each with their own problems.

In 1967, Arp noted that one group of objects in his Atlas, numbers 1 through 101, were in many ways conventional galaxies that appeared to have small companion objects of unknown origin, he also observed that several of these objects appeared on the list of quasars. In some photographs a quasar is in the foreground of known galaxies, and in others there appeared to be matter bridging the two objects, implying they are very close in space. If they are, and the redshifts were due to Hubble expansion, then both objects should have similar redshifts, the galaxies had much smaller redshifts than the quasars. He also noted that quasars were not evenly spread over the sky, but tended to be more commonly found in positions of small angular separation from certain galaxies, this being the case, they might be in some way related to the galaxies.

Arp believed that the Big Bang theory—then in its infancy and not yet confirmed correct—was wrong, citing his research into these quasi-stellar objects. Instead, Arp supported a redshift quantization theory as an explanation of the redshifts of galaxies,[22] he argued that the redshift of these objects was not due to Hubble expansion or physical movement of the objects, but must have a non-cosmological or "intrinsic" origin. Instead, his hypothesis was that quasars were local objects ejected from the core of active galactic nuclei (AGN). Nearby galaxies with both strong radio emission and peculiar morphologies, particularly M87 and Centaurus A, appeared to support Arp's hypothesis.[23]

A major concern was the enormous amount of energy these objects would have to be radiating, if they were distant. No commonly-accepted mechanism could account for this, the correct explanation, that it was due to matter in an accretion disc falling into an supermassive black hole, was not accepted by many, because the existence of black holes was still seen as theoretical and too exotic by many astronomers in the 1960s, and because it was not yet confirmed that many galaxies (including our own) have supermassive black holes at their centers. The strange spectral lines in their radiation, and the speed of change seen in some quasars, also suggested to many astronomers and cosmologists that the objects were comparatively small and therefore perhaps bright, massive and not far away; accordingly that their redshifts were not due to distance or velocity, and must be due to some other reason or an unknown process, meaning that the quasars were not really powerful objects nor at extreme distances, as their redshifted light implied. A common alternative explanation was that the redshifts were caused by extreme mass (gravitational redshifting explained by general relativity) and not by extreme velocity (explained by special relativity). Other explanations were also suggested.[specify] This "redshift controversy" lasted for many years.

Eventually, many lines of evidence demonstrated that the quasar redshifts are genuine, and due to the Hubble metric expansion of space, and quasars are in fact as powerful and as distant as Schmidt and some other astronomers had suggested,[24] this included crucial evidence from optical viewing of quasar host galaxies, finding of 'intervening' absorption lines which explained various spectral anomalies, and observations from gravitational lensing.

The accretion disc energy-production mechanism was finally modeled in the 1970s, and black holes were also directly detected (including evidence showing that supermassive black holes could be found at the centers of our own and many other galaxies), which resolved the concern that quasars were too luminous to be a result of very distant objects or that a suitable mechanism could not be confirmed to exist in nature. Today the cosmological distance and energy output of quasars is accepted by almost all researchers.

Modern observations (1970s onward)[edit]

Picture shows a cosmic mirage known as the Einstein Cross. Four apparent images are actually from the same quasar.
Cloud of gas around the distant quasar SDSS J102009.99+104002.7, taken by MUSE.[25]

Later it was found that not all quasars have strong radio emission; in fact only about 10% are "radio-loud". Hence the name 'QSO' (quasi-stellar object) is used (in addition to "quasar") to refer to these objects, including the 'radio-loud' and the 'radio-quiet' classes, the discovery of the quasar had large implications for the field of astronomy in the 1960s, including drawing physics and astronomy closer together.[26]

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.[27]

Current understanding[edit]

It is now known that quasars are distant but extremely luminous objects, so any light which reaches the Earth is redshifted due to the metric expansion of space.[28] Quasars inhabit the center of active galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars, this radiation is emitted across the electromagnetic spectrum, almost uniformly, 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.

With high-resolution imaging from ground-based telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been detected in some cases,[29] these galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques. Most quasars, with the exception of 3C 273 whose average apparent magnitude is 12.9, cannot be seen with small telescopes.

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. Since light cannot escape the black holes, the escaping energy is actually generated outside the event horizon by gravitational stresses and immense friction on the incoming material.[30] Central masses of 105 to 109 solar masses have been measured in quasars by 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 are active (with enough accretion to power radiation), and it is the activity of these black holes that 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 into an accretion disc. Quasars may also be ignited or re-ignited when normal galaxies merge and the black hole is infused with a fresh source of matter; in fact, it has been suggested that a quasar could form when the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3–5 billion years.[30][31][32]

The mechanism of brightness changes probably involves relativistic beaming of astrophysical jets pointed nearly directly toward Earth, the highest redshift quasar known (as of June 2011) is ULAS J1120+0641, with a redshift of 7.085, which corresponds to a comoving distance of approximately 29 billion light-years from Earth (these distances are much larger than the distance light could travel in the universe's 13.8 billion year history because space itself has also been expanding).

In the 1980s, unified models were developed in which quasars were classified as a particular kind of active galaxy, and a consensus emerged that in many cases it is simply the viewing angle that distinguishes them from other classes, such as blazars and radio galaxies.[33] The huge luminosity of quasars results from the accretion discs of central supermassive black holes, which can convert on the order of 10% of the mass of an object into energy as compared to 0.7% for the p-p chain nuclear fusion process that dominates the energy production in Sun-like stars.

Bright halos around 18 distant quasars.[34]

This mechanism also explains 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 the Milky Way, have gone through an active stage, appearing as a quasar or some other class of active galaxy that depended on the black hole mass and the accretion rate, and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.


The Chandra X-ray image is of the quasar PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light years from Earth. An enormous X-ray jet extends at least a million light years from the quasar. Image is 60 arcsec on a side. RA 11h 30m 7.10s Dec −14° 49' 27" in Crater. Observation date: May 28, 2000. Instrument: ACIS.

More than 200,000 quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.056 and 7.085. Applying Hubble's law to these redshifts, it can be shown that they are between 600 million[35] and 28.85 billion light-years away (in terms of comoving distance). Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.

The power of quasars originates from supermassive black holes that are believed to exist at the core of most galaxies, the Doppler shifts of stars near the cores of galaxies indicate that they are rotating around tremendous masses with very steep gravity gradients, suggesting black holes.

Although quasars appear faint when viewed from Earth, they are visible from extreme distances, being the most luminous objects in the known universe, the brightest quasar 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 medium-size amateur telescope), but it has an absolute magnitude of −26.7.[36] 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 4 trillion (4 × 1012) times that of the Sun, or about 100 times that of the total light of giant galaxies like the Milky Way.[36] This assumes the quasar is radiating energy in all directions, but the active galactic nucleus is believed to be radiating preferentially in the direction of its jet; in a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward the Earth, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at the Earth.

The hyperluminous quasar APM 08279+5255 was, when discovered in 1998, given an absolute magnitude of −32.2. 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 of this system suggests that the light emitted 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 than they are today, 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 the locations 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 jets, radiation and winds created by the 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 those clusters from cooling and falling onto the central galaxy.

Quasars' luminosities are variable, with time scales that range from months to 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 as to allow the coordination of the luminosity variations. This would mean that a quasar varying on a 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 that powers stars. The conversion of gravitational potential energy to radiation by infalling to a black hole converts between 6% and 32% of the mass to energy, compared to 0.7% for the conversion of mass to energy in a star like our sun.[37] It is the only process known that can produce such high power over a very long term. (Stellar explosions such as supernovas and gamma-ray bursts, and direct matter-antimatter annihilation, can also produce very high power output, but supernovae only last for days, and the universe does not appear to have had large amounts of antimatter at the relevant times).

Gravitationally lensed quasar HE 1104-1805.[38]
Animation shows the alignments between the spin axes of quasars and the large-scale structures that they inhabit.

Quasars have all the properties of other active galaxies such as Seyfert galaxies, but are more powerful: their radiation is partially 'nonthermal' (i.e., not due to black body radiation), and approximately 10 percent are observed to also have jets and lobes like those of radio galaxies that also carry significant (but poorly understood) amounts of energy in the form of particles moving at relativistic speeds. Extremely high energies might be explained by several mechanisms (see Fermi acceleration and Centrifugal mechanism of acceleration). Quasars can be detected over the entire observable electromagnetic spectrum including radio, infrared, visible light, ultraviolet, X-ray and even gamma rays. Most quasars are brightest in their rest-frame near-ultraviolet wavelength of 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 900.0 nm, in the near infrared. A minority of quasars show strong radio emission, which is generated by jets of matter moving close to the speed of light. When viewed downward, these appear as blazars and often have regions that seem to move away from the center faster than the speed of light (superluminal expansion), this is an optical illusion due to the properties of special relativity.

Quasar redshifts are measured from the strong spectral lines that dominate their visible and ultraviolet emission spectra, these lines are brighter than the continuous spectrum. They exhibit Doppler broadening corresponding to mean speed of several percent of the speed of light. 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, 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.

Iron quasars show strong emission lines resulting from low ionization iron (FeII), such as IRAS 18508-7815.

Emission generation[edit]

This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination.

Since quasars exhibit properties common to all active galaxies, the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes. To create a luminosity of 1040 watts (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 devour 1000 solar masses of material every year, the largest known is estimated to consume matter equivalent to 600 Earths per minute. Quasar luminosities can vary considerably over time, depending on their surroundings, since it is difficult to fuel quasars for many billions of years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.

Spectrum from quasar HE0940-1050 after it has travelled through intergalactic medium.

Quasars also provide some clues as to the end of the Big Bang's reionization, the oldest known quasars (redshift = 6) display a Gunn-Peterson trough and 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.

Quasars show evidence of elements heavier than helium, indicating 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. Light from these stars may have been observed in 2005 using NASA's Spitzer Space Telescope,[39] although this observation remains to be confirmed.

Like all (unobscured) active galaxies, quasars can be strong X-ray sources. Radio-loud quasars can also produce X-rays and gamma rays by inverse Compton scattering of lower-energy photons by the radio-emitting electrons in the jet.[40]

Role in celestial reference systems[edit]

The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation.[41]

Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky,[42] the International Celestial Reference System (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by very-long-baseline interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better, which is orders of magnitude more precise than the best optical measurements.

Multiple quasars[edit]

A multiple-image quasar is a quasar whose light undergoes gravitational lensing, resulting in double, triple or quadruple images of the same quasar, the first such gravitational lens to be discovered was the double-imaged quasar Q0957+561 (or Twin Quasar) in 1979.[43] A grouping of two or more quasars can result from a chance alignment, physical proximity, actual close physical interaction, or effects of gravity bending the light of a single quasar into two or more images.

As quasars are rare objects, the probability of three or more separate quasars being found near the same location is very low, the first true triple quasar was found in 2007 by observations at the W. M. Keck Observatory Mauna Kea, Hawaii.[44] LBQS 1429-008 (or QQQ J1432-0106) was first observed in 1989 and was found to be a double quasar; itself a rare occurrence. When astronomers discovered the third member, they confirmed that the sources were separate and not the result of gravitational lensing, this triple quasar has a red shift of z = 2.076, which is equivalent to 10.5 billion light years.[45] The components are separated by an estimated 30–50 kpc, which is typical of interacting galaxies.[46] An example of a triple quasar that is formed by lensing is PG1115 +08.[47]

Quasars in interacting galaxies.[48]

In 2013, the second true triplet quasars QQQ J1519+0627 was found with redshift z = 1.51 (approx 9 billion light years) by an international team of astronomers led by Farina of the University of Insubria, the whole system is well accommodated within 25 (i.e., 200 kpc in projected distance). The team accessed data from observations collected at the La Silla Observatory with the New Technology Telescope (NTT) of the European Southern Observatory (ESO) and at the Calar Alto Observatory with the 3.5m telescope of the Centro Astronómico Hispano Alemán (CAHA).[49][50]

The first quadruple quasar was discovered in 2015.[51]

When two quasars are so nearly in the same direction as seen from Earth that they appear to be a single quasar but may be separated by the use of telescopes, they are referred to as a "double quasar", such as the Twin Quasar,[52] these are two different quasars, and not the same quasar that is gravitationally lensed. This configuration is similar to the optical double star. Two quasars, a "quasar pair", may be closely related in time and space, and be gravitationally bound to one another, these may take the form of two quasars in the same galaxy cluster. This configuration is similar to two prominent stars in a star cluster. A "binary quasar", may be closely linked gravitationally and form a pair of interacting galaxies, this configuration is similar to that of a binary star system.

See also[edit]


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