<MPQA autoclass="obj" certainty="6.0">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.</MPQA>
<MPQA autoclass="obj" certainty="4.7">The quasar-starburst was found by a team of researchers from six institutions.</MPQA>
<MPQA autoclass="subj" certainty="43.7">A quasar (from quasi-stellar radio source) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source), and has a very high redshift.</MPQA>
<MPQA autoclass="subj" certainty="49.2">The general consensus is that this high redshift is cosmological, the result of Hubble's law, which implies that quasars must be very distant and must emit more energy than dozens of normal galaxies.</MPQA>
<MPQA autoclass="subj" certainty="28.4">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 J1819+3845 for another explanation).</MPQA>
<MPQA autoclass="subj" certainty="4.9">The highest redshift currently known for a quasar is 6.4 [1] (http://www.sdss.org/news/releases/20030109.quasar.html),</MPQA>
<MPQA autoclass="subj" certainty="6.4">which is significant because it implies a maximum distancemore distant quasars should be easily observable if they existed.</MPQA>
<MPQA autoclass="obj" certainty="13.5">This is taken to mean that the oldest observed quasars correspond to the beginning of galaxy formation.</MPQA>
<MPQA autoclass="obj" certainty="23.3">The first quasars marked the end of the dark age, the period after the emission of the cosmic microwave background radiation where there are no observable sources of radiation.</MPQA>
<MPQA autoclass="obj" certainty="24.5">1 Properties of Quasars 2 Quasar Emission Generation 3 History of quasar observation3.1</MPQA>
<MPQA autoclass="subj" certainty="12.7">See alsoProperties of QuasarsOf the several hundred quasars observed, all spectra have shown considerable redshifts, ranging from 0.06 to the recent maximum of 6.4.</MPQA>
<MPQA autoclass="subj" certainty="12.6">Therefore, all known quasars <MPQA autoclass="negative">lie</MPQA> at great distances from us.</MPQA>
<MPQA autoclass="obj" certainty="3.1">The closest being 240 Mpc away and the farthest being 5500 Mpc!</MPQA>
<MPQA autoclass="obj" certainty="17.5">However, most quasars are known to lie above 1000 Mpc in distance.</MPQA>
<MPQA autoclass="subj" certainty="24.8">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.</MPQA>
<MPQA autoclass="subj" certainty="26.9">Although <MPQA autoclass="negative">faint</MPQA> when seen optically, their high redshift and great distance imply that quasars are the brighest objects in the known universe.</MPQA>
<MPQA autoclass="subj" certainty="1.2">In a general sense, quasars range in luminosity from 10^38 W (the luminosity of the brightest radio galaxy) to 10^42 W. The average quasar is found to be 10^40 W, which is comparable to 1000 Milky Way galaxies, or 10 trillion Suns.</MPQA>
<MPQA autoclass="obj" certainty="14.3">When compared to active galaxies, quasars exhibit much of the same properties.</MPQA>
<MPQA autoclass="obj" certainty="17.5">Radiation is nonthermal and some are shown to have emission jets and lobes.</MPQA>
<MPQA autoclass="obj" certainty="17.9">Quasars can be observed in many parts of the electromagnetic spectrum including radio, infrared, optical, ultraviolet, X-ray and even gamma rays while most quasars are found to emit in the infrared.</MPQA>
<MPQA autoclass="obj" certainty="23.3">Quasars are also found to vary in luminosity in differing time periods.</MPQA>
<MPQA autoclass="obj" certainty="20.2">Some vary in brightness every few months, weeks, days, or hours.</MPQA>
<MPQA autoclass="obj" certainty="12.7">This recent evidence has allowed scientists to theorize that quasars exhibit energy in a very small region, since each part of the quasar would have to be in contact with other parts on such a timescale to coordinate the luminosity variations.</MPQA>
<MPQA autoclass="obj" certainty="13.1">As such, a quasar varying on the time scale of a few weeks cannot be larger than a few light weeks across.</MPQA>
<MPQA autoclass="obj" certainty="9.5">Quasar Emission GenerationSince quasars exhibit properties of all active galaxies, many scientists have compared the emissions from quasars to those of small active galaxies due to their likeness.</MPQA>
<MPQA autoclass="subj" certainty="3.2">The best explanation for quasars is that they are powered by supermassive black holes.</MPQA>
<MPQA autoclass="subj" certainty="19.9">Scientists theorize to create the luminosity of 10^40 W (average brightness of a quasar), a super-massive black hole would have to consume 10 stars per year.</MPQA>
<MPQA autoclass="subj" certainty="16.3">The brightest known quasars are <MPQA autoclass="speechDirectSubjective">thought</MPQA> to devour 1000 solar masses of material every year.</MPQA>
<MPQA autoclass="subj" certainty="13.8">Quasars are <MPQA autoclass="speechDirectSubjective">thought</MPQA> to 'turn on' and off depending on their surroundings.</MPQA>
<MPQA autoclass="subj" certainty="4.2">One implication is that a quasar would not, for example, continue to feed at that rate for 10 billion years, which nicely explains why there are no nearby quasars.</MPQA>
<MPQA autoclass="obj" certainty="15.2">In this framework, after a quasar finishes eating up gas and dust, it becomes an ordinary, normal galaxy.</MPQA>
<MPQA autoclass="obj" certainty="23.0">Quasars also provide some clues as to the end of the Big Bang's reionization.</MPQA>
<MPQA autoclass="subj" certainty="2.2">The oldest quasars (z 4) display a Gunn-Peterson trough and clearly have absorption regions in <MPQA autoclass="negative">front</MPQA> of them <MPQA autoclass="speechDirectSubjective">indicating</MPQA> that the intergalactic medium at that time was neutral gas.</MPQA>
<MPQA autoclass="subj" certainty="6.6">More recent quasars show no absorption region but rather their spectra contain a spiky area known as the Lyman-alpha forest.</MPQA>
<MPQA autoclass="subj" certainty="1.1">This <MPQA autoclass="speechDirectSubjective">indicates</MPQA> that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.</MPQA>
<MPQA autoclass="subj" certainty="0.6">One other interesting characteristic of quasars is that they show evidence of elements heavier than helium.</MPQA>
<MPQA autoclass="subj" certainty="27.1">This is taken to mean 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.</MPQA>
<MPQA autoclass="subj" certainty="30.6">However, this prediction has the <MPQA autoclass="negative">problem</MPQA> in that, as of 2004, no evidence for such stars have been found, and it may seriously <MPQA autoclass="negative">undermine</MPQA> our current views of the universe if no such stars are found in the next few years, and alternate mechanisms for producing heavy elements cannot be found.</MPQA>
<MPQA autoclass="subj" certainty="1.8">History of quasar observationThe first quasars were discovered with radio telescopes in the late 1950s.</MPQA>
<MPQA autoclass="obj" certainty="27.8">Many were recorded as radio sources with no corresponding visible object.</MPQA>
<MPQA autoclass="obj" certainty="26.4">Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalog as astronomers scanned the skies for the optical counterparts.</MPQA>
<MPQA autoclass="obj" certainty="5.8">In 1960 radio source 3C 48 was finally tied to an optical object.</MPQA>
<MPQA autoclass="subj" certainty="14.1">Astronomers detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum.</MPQA>
<MPQA autoclass="obj" certainty="0.5">Containing many unknown broad emission lines, the anomalous spectrum defied interpretation.</MPQA>
<MPQA autoclass="obj" certainty="8.4">In 1963 a breakthrough was achieved.</MPQA>
<MPQA autoclass="subj" certainty="13.3">Another radio source, 3C 273, that was tied with an optical object and exhibiting the same <MPQA autoclass="negative">strange</MPQA> emission lines.</MPQA>
<MPQA autoclass="subj" certainty="13.5">These were found not to be strange at all - but rather spectral lines of hydrogen found to be redshifted at the rate of 16 percent!</MPQA>
<MPQA autoclass="obj" certainty="8.7">This discovery by Maarten Schmidt found that 3C 273 was receding away at a rate of 44,000 km/s.</MPQA>
<MPQA autoclass="obj" certainty="27.6">This discovery revolutionized Quasar observation and allowed for other astronomers to find redshifts from the emission lines from other radio sources.</MPQA>
<MPQA autoclass="obj" certainty="29.9">3C 48 was found to have a redshift of 37 percent - 1/3 the speed of light.</MPQA>
<MPQA autoclass="obj" certainty="28.5">With these discoveries, the name "Quasar" came into light.</MPQA>
<MPQA autoclass="obj" certainty="0.8">The term quasi-stellar object came to be used as a term for these puzzling objects to astronomers.</MPQA>
<MPQA autoclass="subj" certainty="4.3">Later it was found that not all (actually only 10% or so) quasars have strong radio emission (are 'radio-loud').</MPQA>
<MPQA autoclass="obj" certainty="16.3">The name 'QSO' (quasi-stellar object) is sometimes given to the radio-quiet class.</MPQA>
<MPQA autoclass="obj" certainty="7.1">Others use the terms 'radio-loud' and 'radio-quiet quasars'.</MPQA>
<MPQA autoclass="subj" certainty="17.5">One great topic of debate during the 1960s was whether quasars are nearby objects or distant objects as implied by their redshift.</MPQA>
<MPQA autoclass="subj" certainty="27.4">It was suggested, for example, that the redshift of quasars was not due to the Doppler effect but rather to light escaping a deep gravitational well.</MPQA>
<MPQA autoclass="subj" certainty="11.9">One strong argument against cosmologically distant quasars was that it implied energies that were far in <MPQA autoclass="negative">excess</MPQA> of known energy conversion processes, including nuclear fusion.</MPQA>
<MPQA autoclass="subj" certainty="0.9">At this time, there were some suggestions that quasars were made of antimatter and that this accounts for their brightness.</MPQA>
<MPQA autoclass="obj" certainty="0.0">This objection was removed with the proposal of the accretion disc mechanism in the 1970s, and today the cosmological distance of quasars is accepted by almost all researchers.</MPQA>
<MPQA autoclass="subj" certainty="5.9">Although most astrophysicists now <MPQA autoclass="speechDirectSubjective">believe</MPQA> that quasars are cosmological objects, there remain a few who cite evidence that they are nearby.</MPQA>
<MPQA autoclass="subj" certainty="3.5">For example, Y. P. Varshni has predicted that large redshifts attributed to quasars are a consequence of natural lasing on the emission spectra.</MPQA>
<MPQA autoclass="subj" certainty="0.1">Varshni and others also <MPQA autoclass="negative">dispute</MPQA> the standard explanation of superluminal motion.</MPQA>
<MPQA autoclass="subj" certainty="17.8">Also, Halton Arp has stated that quasars are spawned by galaxies and has argued that quasars can be observed to be interacting with galaxies.</MPQA>
<MPQA autoclass="subj" certainty="47.4">In the 1980s, unified models were developed in which quasars were viewed as simply a class of active galaxies, and a general consensus has emerged that in many cases it is simply the viewing angle that distinguishes them from other classes, such as (blazars and radio galaxies).</MPQA>
<MPQA autoclass="subj" certainty="43.4">The huge luminosity of quasars is <MPQA autoclass="speechDirectSubjective">believed</MPQA> to be a result of friction caused by gas and dust <MPQA autoclass="negative">falling</MPQA> into the accretion disks of supermassive black holes, which can convert about half of the mass of an object into energy as compared to a few percent for nuclear fusion processes.</MPQA>
<MPQA autoclass="subj" certainty="24.9">This mechanism is also <MPQA autoclass="speechDirectSubjective">believed</MPQA> to explain 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.</MPQA>
<MPQA autoclass="obj" certainty="9.4">This means that it is possible that most galaxies, including our own Milky Way, have gone through a quasar stage and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.</MPQA>
<MPQA autoclass="obj" certainty="21.1">See alsoList of quasarsRetrieved from "http://en.wikipedia.org/wiki/Quasar"</MPQA>
<MPQA autoclass="obj" certainty="30.9">Categories: Astronomy | Celestial bodies | Galaxies | Quasars</MPQA>