In the 1960s, astronomers found a strange celestial body in the vast sea of stars. It looks like a star from the picture, but it is certainly not a star. Its spectrum looks likePlanetary nebulaBut it is not a nebula, and the radio (i.e. radio wave) emitted by a galaxy is not a galaxy, so it is called a "quasar".Redshift: namelyBig Bang TheoryAll forms are spreading around and away from us;Due to the Doppler effect, the spectrum of light emitted by the stars leaving us moves towards the red spectrum.Red shift of quasars: that is, the process of the above stars leaving us.
A new type discovered in the 1960scelestial bodies, which is similar tofixed starIts spectrum has a huge red shift and emits strongradio waves。The remarkable feature of quasars is that they are moving away from us at a fast speed, so they have a greatred shift。Such celestial bodies are far away from us, about billions of light years away, or even farther, but their optical brightness is not weak.Radiation in visible light areapowerIt is hundreds of times more powerful than ordinary galaxies, and the radio power is one million times more powerful than ordinary galaxies.The discovery of quasars was one of the four major discoveries of astronomy in the 1960s.At first, people received radio waves, but did not know where they were emitted.In 1960, Matthews and Sandage found the radio source 3C48Optical counterpartIt looks like a star.Spectroscopic observation showed that there were many broad and strong emission lines in its spectrum, which could not be identified at that time.In 1963, radio source 3C273 was certified as a 13the magnitudeA celestial body resembling a star.M. Schmidt found that its spectrum was very similar to that of 3C48, and successfully identified the spectral line of 3C273.The results show that they are emission lines produced by some well-known elements on the earth, but their redshifts are large, reaching 0.158.The spectral line of 3C48 has also been confirmed, with a larger red shift of 0.367.Subsequently, a number of radio sources with properties similar to 3C48 and 3C273 were found in succession.They are all like stars on the photographic negative, so they are called quasi stellar radio sources.Optical observations show that the ultraviolet radiation of the quasar radio source is very strong.Later, some celestial bodies with optical properties similar to 3C48 and 3C273 were found, but they did not emit radio radiation.Such objects are called blue stars.Quasar radio sources and blue stars are collectively referred to as quasars.By 1979, more than 1000a quasar, whereQuasar radio sourceAbout 300.
Quasar characteristics
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① Quasars have similarfixed starThat means their angular diameter is less than 1 second.Very few quasars have weak nebular envelope, such as 3C48.Some quasars have jet like structures. ②There are many strong and wide emission lines in the spectrum of quasars, including allowable and forbidden lines.The most common is the spectral line of hydrogen, oxygen, carbon, magnesium and other elements. The helium line is very weak or does not appear, which can only be explained by the low abundance of helium.It is now generally believed that the emission lines of quasars are generated from a gas envelope, and the process of generation is similar to that of ordinaryGas nebulasimilar.The wide emission line of quasars indicates that there must be violent turbulent motion in the gas envelope.There are very sharp absorption lines in the spectra of some quasars, indicating that the speed of turbulent motion in the area where the absorption lines are generated is very small. ③Quasars emit strong ultraviolet radiation, so the color appears very blue.Optical radiation is polarized and non thermal.In addition, the infrared radiation of quasars is also very strong. ④The quasar radio source emits strong nonthermal radio radiation. ⑤Quasars generally have light changes, and the time scale is several years.The light of a few quasars changes violently, and the time scale is several months or days.From the light variable time scale, we can estimate the size of the area where the quasar emits optical radiation (from several light days to several light years).The radio radiation of quasar radio sources also changes frequently.It was also found that the two sub sources of several double source type stellar radio sources separated outward at a very high speed.No change in optical radiation and radio radiationPeriodicity。⑥The emission lines of quasars have a large red shift. ⑦Recent observations show that some quasars also emitX-rayradiation。
Characteristics of redshift
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Quasar redshift
Redshift is a common feature of extragalactic objects.Therefore, most astronomers believe that quasars are extragalactic stars.Redshift apparent magnitude relationThe results of the statistics show that Hubble's law is applicable to extragalactic galaxies.That is to say, their redshift isCosmological redshiftTheir distances are cosmological distances, and their redshifts are statistically related to apparent magnitude.However, for quasars, the redshift and apparent magnitudeStatistical correlationVery bad, which leads to two interrelated questions: whether the redshift of quasars is the cosmological redshift, and whether the distance of quasars is the cosmological distance.Most astronomers believe that the red shift of quasars is cosmological red shift.Therefore, the redshift reflects the retrogression of quasars and conforms to Hubble's law.According to this view, as a type of celestial body, quasars are the most distant celestial bodies observed by humans so far.Those who hold this view believe that the reason for the poor statistical correlation between the redshift and apparent magnitude of quasars is that the absolute magnitude dispersion of quasars is too large.If quasars are classified according to certain standards, and red shift apparent magnitude statistics are carried out for certain types of quasars, the correlation will be significantly improved.The observational facts supporting the cosmological redshift are as follows: three quasars have been found in threeGalaxy clusterThe redshift of these quasars is similar to that of galaxy clusters;Quasars and somePerturbed galaxyVery similar;BL Lacertae objects It is a kind of celestial body similar to stars in shape. It was previously thought that they wereGalaxyInternalVariable starIt has been confirmed that they are distant extragalactic objects.
formation
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The radio radiation of energy and particle accelerated quasars is undoubtedly synchrotron radiation, and the optical radiation is also probably synchrotron radiation.As for the infrared radiation of quasars, further research is needed, but at least part of it may still be synchrotron radiation.If the red shift of quasars is cosmological red shift, their luminosity (including radio, infrared, visible light and even X-ray) is the object with the largest radiation power observed so far.The lifetime of quasars is estimated to be about 106 years.Therefore, the total energy of high-energy electrons and magnetic fields will reach 1062 ergs.It is now generally believed that high-energy electrons originate from the central region of quasars.However, the size of the optical radiation area of quasars estimated from the light variation data is only a few light days to a few light years, that is, 1015~1017 cm.The high-energy electron source must be smaller.
problem
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① Why can such a small area emit such huge energy? ②What is the mechanism of high-energy electron production?In order to explain these problems, many theoretical models have been proposed.One model is star collision. It is believed that the space density of stars in the center of quasars is very high, and they often collide, thus releasing energy.But how the energy released by collision is converted into high-energy electronsenergyThis is not clear.Because supernovae release a large number of high-energy electrons when they explode, there is a view that stars will stick together after collision to formqualityBigger and bigger stars.Massive stars quickly evolve into supernovae, and then explode, releasing high-energy electrons.The star collision model requires a large star number density, which is a serious difficulty for it.The other model is about 108sunlightA massive star of mass.The luminosity of such stars can be very strong, but the energy spectrum will be thermal radiation, which cannot explain the observed situation.In addition, such stars are also very unstable.Later, a mass star model with magnetic field and rotating is proposed, which is called magnetic rotor.The magnetic rotor is stable and has high luminosity.At the same time, due to rotation, the magnetic lines of force will kink, and eventually produce a neutral line or neutral sheet, leading to an explosion.This can be used to explain the light variation of quasars.However, the light change should be periodic, which contradicts the observation results.In additionBlack hole accretion, Baidongmaterial-Antimatter annihilation model.So far, there is no satisfactoryModel。
influence
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Quasar redshift
Quasars andActive galaxyActive galaxies (also called stimulated galaxies) all have a nucleus in a state of intense activity.Active galactic nucleusIt is similar to quasars in many aspects: small in size;There are strong emission lines in the spectrum;It emits non thermal radiation from radio band to X-ray band;There are often light changes and eruptions.Therefore, quasars may be some kind ofActive galaxy, the observed quasars are likeGalactic nucleusActivities of.Of course, if the quasar is locatedcosmologyDistance, then, their activities will be more intense and more powerful than ordinary active galaxies.The radio properties of quasar radio sources are similar to radio galaxies and N galaxies.The latter two are generallyGiant elliptical galaxy。Therefore, some astronomers speculate that quasars are far awayGiant elliptical galaxy。In terms of optical properties, quasars are similar to type I Seyfert galaxies.Therefore, it is more likely that quasars are far awaySeyfert galaxies This view.There may be two reasons for the generation of the absorption line of a quasar. One is that the absorption line is generated from the gas cloud near the quasar, which is thrown from the quasar.The other is that the absorption lines are generated from some extragalactic objects between quasars and observers.These extragalactic objects may not be related to the same kind of stars.
phenomenon
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It has been found that the two dense sub sources of several star like radio sources such as 3C345 are separated at a very high speed.If the quasar is located at the cosmological distance, the expansion speed of the two sub sources will exceed the speed of light, and the maximum speed can reach 10 times of the speed of light.Some people think that quasars are not located at the cosmological distance, so there will be no superluminal phenomenon at all.But observation shows that there is oneRadio galaxySimilar superluminal phenomena also exist, and radio star systems undoubtedly lie at cosmological distances.It can be seen thatevidenceNot enough.Another view is that superluminal phenomenon exists.However, in order not to contradict the theory of relativity, it is considered that this phenomenon does not reflect the real movement of particles, but rather some "illusion", so it is "vision"Superluminal expansion。At present, several models have been proposed to explain the phenomenon of apparent superluminal velocity, but none of them can completely solve the problem.[1]
