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Artist's rendering of 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

Quasars (/ˈkwzɑːr/) or quasi-stellar radio sources are the most energetic and distant members of a class of objects called active galactic nuclei (AGN). Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies. Their spectra contain very broad emission lines, unlike any known from stars, hence the name "quasi-stellar." Their luminosity can be 100 times greater than that of the Milky Way.[2] Most quasars were formed approximately 12 billion years ago, and they are normally caused by collisions of galaxies, with the galaxies' central black holes merging to form either a supermassive black hole[3] or a binary black hole system.

Although the true nature of these objects was controversial until the early 1980s, there is now a scientific consensus that a quasar is a compact region in the center of a massive galaxy surrounding a central supermassive black hole.[4] Its size is 10–10,000 times the Schwarzschild radius of the enclosed black hole. The energy emitted by a quasar derives from mass falling onto the accretion disc around the black hole.


Quasars show a very high redshift, which is an effect of the metric expansion of space between the quasar and the Earth.[5] When the observed redshift of quasars is interpreted in terms of Hubble's law, it is inferred that quasars are very distant objects. Quasars inhabit the very center of active, young 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.

A Hubble picture showing a quasar core
Quasar QSO-160913+653228 is so distant its light has taken nine billion years to reach us, two thirds of the time that has elapsed since the Big Bang.[6]

In early optical images, quasars appeared as 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 detected in some cases.[7] 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.

The luminosity of some quasars changes rapidly in the optical range and even more rapidly in the X-ray range. Because these changes occur very rapidly they define an upper limit on the volume of a quasar; quasars are not much larger than the Solar System.[8] This implies an astonishingly high power density.[9] 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 (see more discussion of how cosmological distances can be greater than the light-travel time at metric expansion of space).

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.[10] 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 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 into an accretion disc. Quasars may also be ignited or re-ignited from normal galaxies when they merge and the black hole is infused with a fresh source of matter. In fact, it has been suggested that a quasar could form as the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3–5 billion years.[10][11][12]


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[13] 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, we see them and their surrounding space 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 all galaxies. The Doppler shifts of stars near the cores of galaxies which indicates 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 because 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 medium-size amateur telescope), but it has an absolute magnitude of −26.7.[2] 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 our Sun, or about 100 times that of the total light of giant galaxies like our Milky Way.[2] 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 us, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at us.


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 release of gravitational energy[14] 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 likewise, but only for a few weeks. Black holes were considered too exotic by some astronomers in the 1960s. They also 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 (optical viewing of host galaxies, finding 'intervening' absorption lines, gravitational lensing) now demonstrate that the quasar redshifts are due to the Hubble expansion, and quasars are in fact as powerful as first thought.[15]

Gravitationally lensed quasar HE 1104-1805.[16]

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 looked at down the jet, these appear as blazars and often have regions that appear 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 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. These widths are due to Doppler shifts caused by 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, 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.


Emission generation[edit]






History of observation[edit]






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.

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 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.[25] 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.[26] 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.[27][28] However, when accretion disc energy-production mechanisms were successfully modeled in the 1970s, the argument that quasars were too luminous became moot and today the cosmological distance of quasars is accepted by almost all researchers.


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.[30] 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.


Role in celestial reference systems[edit]


Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky.[32] 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]


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.[34] 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.[35] The components are separated by an estimated 30–50 kpc, which is typical of interacting galaxies.[36] An example of a triple quasar that is formed by lensing is PG1115 +08.[37]

Quasars in interacting galaxies.[38]

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).[39][40]

The first quadruple quasar was discovered in 2015.[41]


See also[edit]


  3. ^ "HubbleSite - NewsCenter - Hubble Sees the 'Teenage Years' of Quasars (06/18/2015) - Introduction". hubblesite.org. Retrieved 2015-06-19. 
  4. ^ Irion, Robert. "A Quasar in Every Galaxy?" (PDF). Sky and Telescope. New Track Media. Retrieved 20 February 2014. 
  5. ^ Grupen, Claus; Cowan, Glen (2005). Astroparticle physics. Springer. pp. 11–12. ISBN 3-540-25312-2. 
  6. ^ "A cross-section of the Universe". ESA/Hubble Press Release. Retrieved 29 April 2014. 
  9. ^ "7. HIGH-ENERGY ASTROPHYSICS ELECTROMAGNETIC RADIATION". Neutrino.aquaphoenix.com. Retrieved 2011-07-01. 
  10. ^ a b Thomsen, D. E. (Jun 20, 1987). "End of the World: You Won't Feel a Thing". Science News. Society for Science & the Public. 131 (25): 391. doi:10.2307/3971408. JSTOR 3971408. 
  16. ^ "http://www.spacetelescope.org/images/heic1116a/". ESA/Hubble Press Release. Retrieved 4 November 2011.  External link in |title= (help)
  17. ^ "NASA Goddard Space Flight Center: News of light that may be from population III stars". Nasa.gov. Retrieved 2011-07-01. 
  19. ^ Shields, Gregory A. (1999). "A BRIEF HISTORY OF AGN". The Publications of the Astronomical Society of the Pacific. 111 (760): 661–678. arXiv:astro-ph/9903401free to read. Bibcode:1999PASP..111..661S. doi:10.1086/316378. Retrieved 3 October 2014. 
  20. ^ "Our Activities". European Space Agency. Retrieved 3 October 2014. 
  22. ^ Wallace, Philip Russell (1991). Physics: Imagination and Reality. ISBN 9789971509293. Retrieved 8 May 2013. 
  28. ^ Santilli, Ruggero Maria (2006). Isodual theory of antimatter : with applications to antigravity, grand unification and cosmology. Dordrecht: Springer. p. 304. ISBN 9781402045172. Retrieved 4 June 2015. 
  29. ^ "Active Galaxies and Quasars – Double Quasar 0957+561". Astr.ua.edu. Retrieved 2011-07-01. 
  30. ^ "Peter J. Barthel, Is every Quasar beamed?, The Astrophysical Journal, 336:606-611, 1989". Adsabs.harvard.edu. Retrieved 2011-07-01. 
  35. ^ "Triple quasar QQQ 1429-008". ESO. Retrieved 2009-04-23. 
  36. ^ Djorgovski, S. G.; Courbin, F.; Meylan, G.; Sluse, D.; et al. (2007). "Discovery of a Probable Physical Triple Quasar". The Astrophysical Journal. 662 (1): L1–L5. arXiv:astro-ph/0701155free to read. Bibcode:2007ApJ...662L...1D. doi:10.1086/519162. 
  38. ^ "Quasars in interacting galaxies". Retrieved 19 June 2015. 
  39. ^ "Extremely rare triple quasar found". phys.org. Retrieved 2013-03-12. 
  40. ^ Farina, E.P; et al. (2013). "Caught in the Act: Discovery of a Physical Quasar Triplet". Monthly Notices of the Royal Astronomical Society. 431 (2): 1019–1025. arXiv:1302.0849v1free to read. Bibcode:2013MNRAS.431.1019F. doi:10.1093/mnras/stt209. 
  41. ^ "Surprise Quasar Quartet Defies Explanation". news.discovery.com. Retrieved 2015-05-17. 
  42. ^ Peacock, John A. (1998). Cosmological physics. Cambridge Astrophysics series. Cambridge University Press. ISBN 0521422701. Retrieved 9 February 2012. 

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