eaa.iop.orgDOI: 10.1888/0333750888/2368 Bright Quasar 3C 273Thierry J-L Courvoisier FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 22:49:19 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsBright Quasar 3C 273ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSBright Quasar 3C 273QUASARS are the most luminous objects we know in theuniverse. 3C 273 is among the brightest quasars; it alsohappens to be one of the closest to the Earth. It is thereforebright in the night sky and hence has been the object ofintense studies since its discovery in 1963.Quasars are found in the center of some galaxies(collections of some hundred billion stars; theMILKYWAYis the galaxy in which our solar system isembedded). Quasars are very peculiar celestial objects,they outshine the galaxy that hosts them, althoughthey are barely as large as our solar system (whichis extremely small compared with the galaxy). Theyemit electromagnetic radiation that covers all the bandsaccessible to astronomical observations on the Earth andin orbit, from radio waves to gamma rays. Their energyradiation can vary in an important manner on time scalesthat may be as short as hundreds of seconds. The matterclose to the most central region of quasars moves atspeeds of several thousand kilometers per second. Theseproperties are most likely due to the presence in the centerof quasars of a black hole that may be as massive as manytens of millions of suns. This paradigm is the only one thatgives some hope of understanding the general propertiesof these objects.The discoveryQuasars were discovered in 1963 when it became possibleto localize with precisionRADIO SOURCES in the sky. 3C 273played a major role in the discovery of quasars. 3C 273is a bright radio source, it bears the number 273 inthe third Cambridge catalog of radio sources (hence thename). It was the second source that was identifiedwith a very peculiar ‘star’, a source of light that was notresolved as extended on photographic plates. The ‘star’,or better the source of optical light that was coincidentwith the radio source 3C 273, was soon seen to havepeculiar emission lines. These are the signature of thepresence of atoms that are excited and emit light at welldefined wavelengths. Stars normally don’t have emissionlines, and more surprisingly it was at first impossible torecognize the type of atoms that were responsible for theemission lines in 3C 273, as the wavelengths at which thelines were observed were not among those emitted byknown atoms. These very peculiar properties of the lightsource led to the name quasi stellar radio source that thenbecame QSO for quasi stellar object or quasar for quasistar.The mystery of the emission lines was solved when itwas noted that one could indeed recognize the wavelengthof the emission lines if one admitted that the lines areobserved not at the wavelength λ0typical of laboratorymeasurements of the atoms but at longer wavelengthsλ = λ0(1+z), where z is known as the redshift, equal to0.158 for 3C 273. Many galaxies showing the same effectwere already known in 1963, but with redshifts much lessthan that of 3C 273. In the universe this effect is a sign thatthe objects are moving apart from one another (expansion)and thus are at very large distances. At a redshift of 0.1583C 273 is thus found to be some 3 billion light years away.In 1963 3C 273 was the most distant object known (see alsoHIGH-REDSHIFT QUASARS).Being so far away and still easy to find onphotographic plates, 3C 273 is an object of 13th magnitude;3C 273 is clearly a very luminous object indeed. Its lightoutput corresponds to 1014times that of the Sun.Most quasars do not radiate substantially in the radiopart of the electromagnetic spectrum, and are called radioquiet quasars. A small fraction, of which 3C 273 is amember, do, however. These quasars are called radiobright quasars. They can be discovered as described aboveusing radio data. Radio quiet objects are found by lookingfor redshifted emission lines in the light of ‘stars’.It was also soon discovered that 3C 273 varies inintensity on time scales much less than 1 year. Since lightsources cannot vary on times scales much less than the timethe light needs to cross the source (this is due to a causalityargument: in order to significantly change the brightnessof an object a signal must reach all the parts of the object toconvey the instruction), it was also very quickly clear that3C 273 must be much smaller than 1 light year, a distancealready much less than that separating us from the closeststar in our Galaxy.The very unusual properties of 3C 273 spurred alarge set of observations using not only optical and radiotelescopes but also other instruments, both on the groundand on Earth orbiting satellites. It became clear that this(and other) quasar was the source not only of visual lightand radio waves, but also of infrared radiation, ultravioletlight, x-rays and gamma rays.Deep radio, optical and x-ray images in subsequentyears showed that the object is not really ‘star-like’ butthat a cigar shaped extension is associated with the mainsource (see figure 1 ofQUASISTELLAR OBJECTS: OVERVIEW). This‘jet’ extends from very small distances to some 20fromthe ‘core’. It is most prominent in the radio images. Verysurprisingly, the features making up the jet (often calledblobs, thus expressing our lack of understanding of thesefeatures) were seen to travel away from the core of theobject at speeds seemingly larger than the speed of light(superluminal motion). This superluminal motion is infact due to an aberration that takes place when objectsare moving in a direction close to the line of sight atvelocities close to (but not larger than) the speed of light.This showed that in addition to the already very unusualobserved properties of quasars one had to add the presenceof radio wave emitting matter moving at very high speedsalong a well defined direction, close to the line of sight.The source of energyThe only way known to us through which large quantitiesof energy can be released by matter in a very small volume,clearly necessary properties to understand the nature ofquasars, is to consider that the matter falls into a very deepgravitational potential well created by a very massive andCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS,