Information AboutQuasar |
| CATEGORIES ABOUT QUASAR | |
| quasars | |
| radio astronomy | |
| active galaxy types | |
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A quasar (contraction of '''QUASi-stellAR radio source''') is an extremely bright and distant Active Nucleus of a young Galaxy . They were first identified as being high- Redshift sources of Electromagnetic Energy , including Radio Waves and Visible Light that were point-like, similar to Star s, rather than extended sources similar to galaxies. While there was initially some controversy over the nature of these objects, there is now a Scientific Consensus that a quasar is a compact halo of Matter surrounding the central Supermassive Black Hole of a young galaxy. OVERVIEW Quasars show a very high Redshift which is an effect of the Expansion Of The Universe between quasars and the Earth. When combined with Hubble's Law , the implication is that the quasars are very distant. To be observable at that distance, the energy output of quasars must dwarf that of almost every known astrophysical phenomenon in a galaxy, excepting comparatively short-lived events like Supernova e and Gamma-ray Bursts . Quasars may readily release energy in levels equal to the output of hundreds of average Galaxies combined. The output of light is equivalent to one trillion suns. In optical Telescope s, quasars look like single points of light (i.e. Point Source ) although many have had their "host galaxies" identified. The galaxies themselves are often too dim to be seen with all but the largest telescopes. 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 Year s, it is one of the most distant objects directly observable with amateur equipment. Some quasars display rapid changes in Luminosity , which implies that they are small (an object cannot change faster than the time it takes light to travel from one end to the other; but see Quasar J1819+3845 for another explanation). The highest Redshift currently known for a quasar is 6.4.1 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 . No other currently known mechanism appears able to explain the vast energy output and rapid variability. Knowledge of quasars is advancing rapidly. As recently as the 1980s, there was no clear consensus as to their origin. PROPERTIES OF QUASARS More than 100,000 quasars are known. All observed spectra have shown considerable redshifts, ranging from 0.06 to the recent maximum of 6.4. Therefore, all known quasars lie at great distances from us, the closest being 240 Mpc (780 million Ly ) away and the farthest being 4 Gpc (13 billion ly) away. Most quasars are known to lie above 1.0 Gpc in distance; since light takes such a long time to cover these great distances, we are seeing quasars as they existed long ago — the universe as it was in the distant past. Although faint when seen optically, their high Redshift implies that these objects lie at a great distance from us, making quasars the most luminous objects in the known universe. The quasar which appears brightest in our sky is the ultraluminous 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. So from a distance of 10 Parsec s (about 33 Light-year s), this object would shine in the sky about as bright 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. HS 1946+7658 was thought to have an absolute magnitude of −30.3, but this too was magnified by the Gravitational Lensing effect. 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 evidence has allowed scientists to theorize 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. Quasars exhibit many of the same properties as active galaxies: Radiation is nonthermal and some are observed to have jets and lobes like those of Radio Galaxies . Quasars can be observed in many parts of the Electromagnetic Spectrum including Radio , Infrared , Optical , Ultraviolet , X-ray and even Gamma Ray s. 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. ''Iron Quasars'' show strong emission lines resulting from ionized Iron , such as IRAS 18508-7815 . QUASAR EMISSION GENERATION ever seen in such a combination. The quasar-starburst was found by a team of researchers from six institutions]] 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 Hole s. To create a luminosity of 1040 W (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. 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 (z > 4) 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 , 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.2 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. |
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