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Naming[ edit ] The term “quasar” was first used in an article by astrophysicist Hong-Yee Chiu in May , 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 ] Sloan Digital Sky Survey image of quasar 3C , illustrating the object’s star-like appearance. The quasar’s jet can be seen extending downward and to the right from the quasar.
Hubble images of quasar 3C At right, a coronagraph is used to block the quasar’s light, making it easier to detect the surrounding host galaxy. But when radio astronomy commenced in the s, 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. They were described as “quasi-stellar [meaning: star-like] radio sources”, or “quasi-stellar objects” QSOs , a name which reflected their unknown nature, and this became shortened to “quasar”.
Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size. In , a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A. 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 Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to find a visible counterpart to the radio source and obtain an optical spectrum using the inch 5. This spectrum revealed the same strange emission lines. Schmidt was able to demonstrate that these were likely to be the ordinary spectral lines of hydrogen redshifted by Although it raised many questions, Schmidt’s discovery quickly revolutionized quasar observation.
Development of physical understanding s [ edit ] Main articles: Redshift , Metric expansion of space , and Universe 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. But if they were small and far away in space, their power output would have to be immense and difficult to explain. Equally, if they were very small and much closer to our galaxy, it would be easy to explain their apparent power output, but less easy to explain their redshifts and lack of detectable movement against the background of the universe. 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 this would support an interpretation of very distant objects with extraordinarily high luminosity and power output, far beyond any object seen to date. This extreme luminosity would also explain the large radio signal.
A major concern was the enormous amount of energy these objects would have to be radiating, if they were distant. In the s no commonly accepted mechanism could account for this. The currently accepted explanation, that it is due to matter in an accretion disc falling into a supermassive black hole , was only suggested in by Edwin Salpeter and Yakov Zel’dovich ,  and even then it was rejected by many astronomers, because in the s, the existence of black holes was still widely seen as theoretical and too exotic, and because it was not yet confirmed that many galaxies including our own have supermassive black holes at their center.
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.
Various explanations were proposed during the s and s, each with their own problems. It was suggested that quasars were nearby objects, and that their redshift was not due to the expansion of space special relativity but rather to light escaping a deep gravitational well general relativity.
This would require a massive object, which would also explain the high luminosities. However, a star of sufficient mass to produce the measured redshift would be unstable and in excess of the Hayashi limit.
One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion. There were suggestions that quasars were made of some hitherto unknown form of stable antimatter regions and that this might account for their brightness. The accretion-disc energy-production mechanism was finally modeled in the s, 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.
Modern observations s onward [ edit ] A cosmic mirage known as the Einstein Cross. Four apparent images are actually from the same quasar. Hence the name “QSO” quasi-stellar object is used in addition to “quasar” to refer to these objects, further categorised into the “radio-loud” and the “radio-quiet” classes.
The discovery of the quasar had large implications for the field of astronomy in the s, including drawing physics and astronomy closer together. 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.
Quasars are believed—and in many cases confirmed—to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, as suggested in by Edwin Salpeter and Yakov Zel’dovich. The energy produced by a quasar is generated outside the black hole, by gravitational stresses and immense friction within the material nearest to the black hole, as it orbits and falls inward.
Central masses of to solar masses have been measured in quasars by using reverberation mapping. Several dozen nearby large galaxies, including our own Milky Way galaxy, that do not have an active center and do not show any activity similar to a quasar, are confirmed to contain a similar supermassive black hole in their nuclei galactic center. Thus it is now thought that all large galaxies have a black hole of this kind, but only a small fraction have sufficient matter in the right kind of orbit at their center to become active and power radiation in such a way as to be seen as quasars.
Quasars may also be ignited or re-ignited when normal galaxies merge and the black hole is infused with a fresh source of matter. Properties[ edit ] Bright halos around 18 distant quasars  The Chandra X-ray image is of the quasar PKS , 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 arcseconds on a side. RA 11h 30m 7. Observation date: May 28, Instrument: ACIS.
All observed quasar spectra have redshifts between 0. Applying Hubble’s law to these redshifts, it can be shown that they are between million  and 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 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. It has an average apparent magnitude of Such quasars are called blazars. This discovery by Maarten Schmidt in was early strong evidence against Steady-state cosmology and in favor of the Big Bang cosmology. Quasars show the locations where massive black holes are growing rapidly by 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 would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. 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  Play media Animation shows the alignments between the spin axes of quasars and the large-scale structures that they inhabit. Since quasars exhibit all the properties common to other active galaxies such as Seyfert galaxies , the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes.
The brightest known quasars devour solar masses of material every year. The largest known is estimated to consume matter equivalent to 10 Earths per second. 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. Radiation from quasars is partially “nonthermal” i. 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.
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. 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. The intense production of ionizing ultraviolet radiation is also significant, as it would provide a mechanism for reionization to occur as galaxies form. Light from these stars may have been observed in using NASA ‘s Spitzer Space Telescope ,  although this observation remains to be confirmed. Quasar subtypes[ edit ] The taxonomy of quasars includes various subtypes representing subsets of the quasar population having distinct properties.
Radio-loud quasars are quasars with powerful jets that are strong sources of radio-wavelength emission. Type 2 or Type II quasars are quasars in which the accretion disk and broad emission lines are highly obscured by dense gas and dust. They are higher-luminosity counterparts of Type 2 Seyfert galaxies. Infrared surveys have demonstrated that red quasars make up a substantial fraction of the total quasar population.
OVV quasars are also considered to be a type of blazar.
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A compact, extremely bright celestial object whose power output can be hundreds to several thousand times that of the entire Milky Way galaxy. Quasars are among the most distant objects in the universe and are generally considered to be a form of active galactic nucleus. [From earlier quas (i-stell)ar (radio source).]
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noun Astronomy. one of over a thousand known extragalactic objects, starlike in appearance and having spectra with characteristically large redshifts, that are thought to be the most distant and most luminous objects in the universe.