An extremely distant, and thus old, celestial object whose power output is several thousand times that of our entire galaxy.
[quas(i-stellar) + (ST)AR.]
quasar
Dictionary:
qua·sar (kwā'zär', -sär', -zər, -sər)  |
n.
An extremely distant, and thus old, celestial object whose power output is several thousand times that of our entire galaxy.
An extremely distant, and thus old, celestial object whose power output is several thousand times that of our entire galaxy.
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An astronomical object that appears starlike on a photographic plate but possesses many other characteristics, such as a large redshift, that prove that it is not a star. The name quasar is a contraction of the term quasistellar object (QSO), which was originally applied to these objects for their photographic appearance. The objects appear starlike because their angular diameters are less than about 1 second of arc, which is the resolution limit of ground-based optical telescopes imposed by atmospheric effects.
Quasars were discovered in 1961 when it was noticed that very strong radio emission was coming from a localized direction in the sky that coincided with the position of a starlike object. When the positions of small-angular-diameter radio sources were accurately determined, the coincidence with starlike objects on optical photographs led to the discovery of a new, hitherto-unsuspected class of objects, the quasars. The full significance of the discovery was not appreciated until 1963, when it was noted that the hydrogen emission lines seen in the optical spectrum of the quasar 3C 273 were shifted by about 16% to the red from their normal laboratory wavelength. This redshift of the spectral lines is characteristic of galaxies whose spectra are redshifted because of the expansion of the universe and is not characteristic of stars in the Milky Way Galaxy.
The color of quasars is generally much bluer than that of most stars with the exception of white dwarf stars. The blueness of quasars as an identifying characteristic led to the discovery that many blue starlike objects have a large redshift and are therefore quasars. The quasistellar objects discovered this way turned out to emit little or no radio radiation and to be about 20 times more numerous than the radio-emitting quasistellar radio sources (QSSs). Why some should be strong radio emitters and most others not is unknown. Several orbiting x-ray satellites have found that most quasars also emit strongly at x-ray frequencies. Gamma rays have also been observed in many quasars. See also Gamma-ray astronomy; X-ray astronomy.
The emission from quasars varies with time. The shortest time scale of variability ranges from years to months at short radio wavelengths, to days at optical wavelengths, to hours at x-ray wavelengths. These different time scales suggest that the emissions from the different bands originate from different regions in the quasar. The rapid fluctuations indicate that there are some components in quasars that have diameters less than a light-hour or of the order of 109 km (109 mi), the size of the solar system. Very highly active quasars are sometimes referred to as optically violent variables (OVVs), blazars, or BL Lac's, after the prototype BL Lacertae, a well-known variable “star” that turned out to be a quasar. The optically violent variables have no or very weak emission lines in their optical spectrum.
The many similarities of the observed characteristics of quasars with radio galaxies, Seyfert galaxies, and BL Lacertae objects strongly suggest that quasars are active nuclei of galaxies. Quasars with large redshifts are spatially much more numerous than those with small redshifts. Because high-redshift objects are very distant and emitted their radiation at an earlier epoch, quasars must have been much more common in the universe about 1010 years ago. Observations with the Hubble Space Telescope have shown that this is the same epoch when galaxies are observed to be forming. Thus it is likely that quasars are associated with the birth of some galaxies.
More than 1053 J of energy are released in quasars over their approximately 106-year lifetime. Of the known energy sources, only gravitational potential energy associated with a mass about 109 times the mass of the Sun can provide this energy, but it is unknown how this gravitational energy produces jets of particles that are accelerated to very near the speed of light.
Several theories have been proposed for quasars. However, the most favored interpretation is that quasars are massive black holes surrounded by rapidly spinning disks of gas in the nuclei of some galaxies. The hot gas in the disk emits the x-ray and optical continuum, a heated halo around the disk produces the emission lines, and the relativistic radio jets are ejected along the rotation axis of the spinning disk. See also Astronomical spectroscopy; Astrophysics, high-energy; Black hole; Infrared astronomy; Neutron star; Radio astronomy.
| Britannica Concise Encyclopedia: quasar |
For more information on quasar, visit Britannica.com.
| Columbia Encyclopedia: quasar |
quasar (kwā'sär), one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous objects in the universe; the name is shortened from quasi-stellar radio source (QSR). Quasars were discovered as the visible counterparts of certain discrete celestial sources of radio waves (see radio astronomy). Similar starlike objects that do not emit radio waves were subsequently discovered and named quasi-stellar objects (QSOs). Although their visible light is faint, the quasars are optically brighter than the galaxies with which radio sources had been identified before 1963. Before their spectra were studied carefully, it was believed that the quasars were stars in our galaxy. However, the lines in their spectra have enormous red shifts that seem to imply that they are receding from the Milky Way with speeds as great as 95% of the speed of light. Only shifts toward the red end of the spectrum have been observed for quasars; blue-shifted ones that would indicate a quasar approaching our galaxy have not yet been found. If quasars were simply objects being ejected from nearby galaxies at high speeds, and not the distant objects they appear to be, then some would have blue shifts. If Hubble's law for the expansion of the universe is extrapolated to include the quasars, they would be many billion light-years away and consequently as luminous intrinsically as 1,000 galaxies combined. To account for such brilliant light, astronomers believe that quasars are supermassive black holes in galactic nuclei, releasing energy by the accretion of matter through a rotating viscous disk (see cosmology).
Bibliography
See H. L. Shipman, Black Holes, Quasars, and the Universe (2d ed. 1980).
| Science Q&A: What are quasars? |
The name quasar originated as a contraction of "quasi-stellar" radio source. Quasars appear to be star like, but they have large red shifts in their spectra indicating that they are receding from the Earth at great speeds, some at up to 90 percent of the speed of light. Their exact nature is still unknown, but many believe quasars to be the cores of distant galaxies, the most distant objects yet seen. Quasars, also called quasi-stellar objects or QSOs, were first identified in 1963 by astronomers at the Palomar Observatory in California.
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| Science Dictionary: quasars |
| Essay: Quasars |
Many scientific discoveries have put an idea scientists had -- a theory -- on a stronger footing. Examples are the discovery of the cosmic background radiation that provided a confirmation of the big bang theory, and the discovery of pulsars, which made reality of the idea of neutron stars. The discovery of quasars, however, only caused bewilderment among astronomers. The consequence of this discovery was that one either had to question the validity of the yardstick of the astronomer, the redshift, or to agree that there are processes out there for which there was no explanation yet.
The quasar problem began this way: During the 1950s, radio astronomers discovered a number of very compact radio sources. Because radio telescopes at that time could not pinpoint celestial objects very accurately, it was difficult to find these objects with a telescope. One of these compact sources, known as 3C 273, was occulted by the Moon in 1962, however, and its exact position could be established. Photos taken with the Hale 5-m (200-in.) telescope at Mt. Palomar showed a starlike object at that position. However, its spectrum was unusual: It contained absorption lines that could not be identified. It, and later others like it, was called a quasistellar radio source (that is, identified by radio, it looks like a star, but is not a star), or quasar for short.
In 1963 Maarten Schmidt discovered that the absorption lines in the spectrum of quasar 3C 273 were common ones, but shifted toward the red end of the spectrum by an extraordinary amount. During the following years astronomers discovered a large number of quasars with large redshifts.
It was commonly held among astronomers that the radial redshifts (that is, directly away from Earth) observed in the spectra of extragalactic systems are Doppler shifts caused by velocity these systems have because they participate in the expansion of the universe. Redshifts caused by the expansion of the universe are called cosmological redshifts. If the redshifts of quasars are cosmological, quasars are the farthest objects ever observed in the telescope. Furthermore, if quasars can be observed over such distances, their energy output must be enormous.
Not all astronomers, however, believed at first that quasars have cosmological redshifts. American astronomer Halton Arp notably disputed this idea. But the more that was learned about quasars, the more it appeared that the redshifts are cosmological. For one thing, astronomers found that some quasar images are distorted by gravitational lenses, regions of high gravity between the quasars and Earth that act as lenses. Quasars must be great distances beyond the lenses, which are generally very far away themselves.
Accepting, then, that quasars produce great amounts of energy, what are they? The most popular belief is that quasars are massive black holes in the centers of distant galaxies, galaxies so far away that their stars and dust cannot be seen. Sometimes it is possible to observe a faint light from the galaxy, which helps verify this idea. Our own galaxy has such a black hole at its center. Perhaps if you were near 3C 273 and looked toward the Milky Way galaxy, you would see only the quasar at its center.
| Wikipedia: Quasar |
This article is about the astronomical object. For other uses, see Quasar (disambiguation).
A quasi-stellar radio source (quasar) is a very 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.
Contents |
Overview
Quasars show a very high redshift, which is an effect of the expansion of the universe between the quasar and the Earth. They are the most luminous, powerful, and energetic objects known in the universe. They inhabit the very centres of active young galaxies and can emit up to a thousand times the energy of our entire Milky Way. 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 = 2x13.7 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 partially '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 µm), 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 illusion 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. 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, 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
Iron Quasars show strong emission lines resulting from low ionization iron (FeII), such as IRAS 18508-7815.
Quasar emission generation
Since quasars exhibit properties common to all active galaxies, the emissions from quasars can be readily compared to those of small active galaxies powered by supermassive black holes. To create a luminosity of 1040 W, or Joules per second, (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. Quasars 'turn on' and off depending on their surroundings, and since quasars cannot continue to feed at high rates for 10 billion years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.
Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest 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.
One other interesting characteristic of quasars is that they 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,[3] although this observation remains to be confirmed.
History of quasar observation
The first quasars were discovered with radio telescopes in the late 1950s. Many were recorded 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.[4] Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalogue as astronomers scanned the skies for the optical counterparts. In 1960, radio source 3C 48 was finally tied to an optical object. Astronomers detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum. Containing many unknown broad emission lines, the anomalous spectrum defied interpretation — a claim by John Bolton of a large redshift was not generally accepted.
In 1962 a breakthrough was achieved. 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 realized that these were actually spectral lines of hydrogen redshifted at the rate of 15.8 percent. This discovery showed that 3C 273 was receding at a rate of 47,000 km/s.[5] This discovery revolutionized quasar observation and allowed other astronomers to find redshifts from the emission lines from other radio sources. As predicted earlier by Bolton, 3C 48 was found to have a redshift of 37% the speed of light.
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. 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.
– 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'). 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.[6] 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.[7] 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. Others speculated that quasars were a white hole end of a wormhole. 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 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.[8]
In the 1980s, unified models were developed in which quasars were classified as a particular kind of active galaxy, and a general consensus 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 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.
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 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.
Further reading
- Melia, Fulvio, The Edge of Infinity. Supermassive Black Holes in the Universe 2003, Cambridge University Press, ISBN 978-0-521-81405-8 (Cloth)
See also
References
- ^ Hubble Surveys the "Homes" of Quasars Hubblesite News Archive, 1996-35
- ^ Chris J. Willott et al. (2007). "Four Quasars above Redshift 6 Discovered by the Canada-France High-z Quasar Survey". The Astronomical Journal 134: 2435–2450. doi:. http://www.iop.org/EJ/abstract/1538-3881/134/6/2435.
- ^ NASA Goddard Space Flight Center: News of light that may be from population III stars
- ^ "The MKI and the discovery of Quasars". Jodrell Bank Observatory. http://www.jb.man.ac.uk/public/story/mk1quasars.html. Retrieved 2006-11-23.
- ^ Schmidt Maarten (1963). "3C 273: a star-like object with large red-shift". Nature 197: 1040–1040. doi:. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1963Natur.197.1040S&db_key=AST&data_type=HTML&format=&high=4521318e0232118.
- ^ S. Chandrasekhar (1964). "The Dynamic Instability of Gaseous Masses Approaching the Schwarzschild Limit in General Relativity". Astrophysical Journal 140 (2): 417–433. doi:.
- ^ J. Greenstein and M. Schmidt (1964). "The Quasi-Stellar Radio Sources 3C 48 and 3C ". Astrophysical Journal 140 (1): 1–34. doi:.
- ^ Active Galaxies and Quasars - Double Quasar 0957+561
External links
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Wikimedia Commons has media related to: Quasars |
- 3C 273, the brightest quasar
- 3C 273: Variable Star Of The Season
- SKY-MAP.ORG SDSS image of quasar 3C 273
- Expanding Gallery of Hires Quasar Images
- Gallery of Quasar Spectra from SDSS
- SDSS Advanced Student Projects: Quasars
- Black Holes: Gravity's Relentless Pull Award-winning interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
- Research Sheds New Light On Quasars (SpaceDaily) July 26, 2006
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
| Translations: Quasar |
Ελληνική (Greek)
n. - (αστρον.) κβάζαρ
Português (Portuguese)
n. - quasar (m) (Astr.)
Svenska (Swedish)
n. - kvasar (astron.)
中文（简体）(Chinese (Simplified))
恒星状球体, 类星球体, 半星球体
中文（繁體）(Chinese (Traditional))
n. - 恆星狀球體, 類星球體, 半星球體
한국어 (Korean)
n. - 의사 성체, 항성상 천체
العربيه (Arabic)
(الاسم) كويزار, في علم الفلك نوع من الكوكبه قلبها لامع تشيع موجات الراديو
עברית (Hebrew)
n. - קוואזאר - גרם שמיימי הפולט גלי רדיו
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