ExtragalacticGalaxy, means atGalaxyIn addition, a large number of stars form galaxies.Because the distance is far away, it is vague in appearancelight spotTherefore, it is also called "extragalactic nebula".Extragalactic andGalaxyIt is also composed of a large number of stars, star clusters, nebulae andInterstellar matterform.
About one billion galaxies similar to the Milky Way have been observed.According to their shape and structure, they can be divided into:Spiral galaxy、Barred spiral galaxy、Elliptical galaxyandIrregular galaxy。It is estimated that the total number of extragalactic galaxies is more than 100 billion.The most common classification of extragalactic galaxies was proposed by Hubble in 1926.
The discovery of extragalactic galaxies extends human understanding beyond the Milky Way for the first time, which is an important milestone in the process of human exploration of the universe.
Since the invention of the telescope in the early 17th century, human vision has expanded to the deeper and deeper reaches of the universe. Astronomers have successively discovered some cloud like objects, known asNebula。Some nebulae are gaseous, and some are thought to be composed of many stars like the Milky WayCosmic IslandBecause it is too far away from the earth, the observation cannot distinguish the hazy objects composed of a large number of stars.
German philosopher in the 18th centuryKant The British astronomer Wright and others once conjectured that these nebulae were cosmic islands composed of star clusters like the Milky Way, and only because they were too far away to distinguish individual stars.
The discovery of extragalactic galaxies can be traced back to more than 200 years ago.At that time, the French astronomer Messier Charles compiledCatalogueThe nebula M31 is located in theHistory of AstronomyIt has an important position.On the night of early winter, people familiar with the stars can find it with the naked eye in Andromeda. A fuzzy spot is commonly known as the Andromeda Nebula[2]。
Since 1885, many new stars have been discovered in the Andromeda Nebula, which leads to the conclusion thatAndromeda NebulaNot the usual, passiveReflected lightDustGas cloudIt must be a system composed of many stars, and the number of stars must be very large, so it is possible to have so many new stars among them.Assuming that the brightness of these new stars at their brightest is the same as that of other new stars found in the Milky Way, it can be roughly inferred that the Andromeda Nebula is very far away from the Earth, far beyond the scope of the known Milky Way.But because the distance measured by nova is not very reliable, it has also caused controversy.
In 1917, American astronomer Richie photographed the nebulaNGCAt 6946, a new star was found in it.Later American astronomersCurtisThere are similar findings.Because the new stars in the nebula are extremely dim, they guess that the nebula should be very far away, and it is an extragalactic object.
In 1924, the American astronomer E. Hubble (1889-1953) used the largestAstronomical telescope——Mount Wilson ObservatoryObserving the Andromeda Nebula with a 2.5 meter diameter telescope, it is the first time to find that the nebula is actually composed of many starsCepheid variableThe Andromeda Nebula is located 700000 light-years away.This is far beyond the scope of the Milky Way, proving that it is a galaxy outside the Milky Way.Since then, Hubble has determined that the Triangle Nebula and NGC6822 are also located outside the Milky Way.
In 1924, Hubble used the largest telescope in the world at that time to find the so-called“Sky ruler"Cepheid variable ofPhotoperiodThe exact distance of the Andromeda Nebula can only be determined by the correspondence with the luminosity, which proves that it is indeed outside the Milky Way, that is, like the Milky Way, it is a huge and independent star group.Therefore, the Andromeda Nebula should be renamed Andromeda Galaxy.
The first extragalactic galaxy discovered is the Andromeda Nebula (M31).With the increase of telescope aperture and the progress of observation technology,hubble space telescope And variousSpace probeMore and more extragalactic galaxies have been discovered.At present, more than one billion galaxies can be observed within the observation range.Among these numerous extragalactic galaxies, only a few very bright ones have special names: some haveDiscovererSome are named after their constellations, such as the Hound Nebula.Most extragalactic galaxies are named after the number of a nebula or star cluster.
In March 2020, at a distance of 560 million light years from the earth“Makaryan 231”(Markarian 231) In the galaxy, Chinese and foreign scientists jointly discovered oxygen.This is the first time that humans have found oxygen outside the Milky Way, and it is also the most oxygen detected outside the solar system so far.[4]
physical property
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Galactic structure
E series is generally composed of nuclei and halos.The core is divided intoNucleosphereAnd core.Some dwarf E-series have no nuclei.The S series (including SB) is the most complex.
Due to the improvement of observation technologyLenticular galaxyIt can still be seen that there areVortex structureIn fact, it should be Sa or SBa, but some SOs and SBOs cannot see any vortex structure.
Galaxy highlights
Extragalactic galaxies areArea light source, people can measure itsSurface brightnessTo study the change rule of surface brightness.Generally speaking, the greater the density of matter, the stronger the radiation will be. The change of luminosity on the galactic visual plane reflects the distribution of matter.Therefore, it is valuable to understand the structure of galaxies by studying the variation law of brightness. The surface brightness of different types of galaxies is different, and the brightness of elliptical galaxies, spiral galaxies, and lenticular galaxies are different.
If the distance of extragalactic galaxies is known, it can be obtained from the observed apparent star angleAbsolute magnitude, or luminosity.Observations show that the absolute magnitude of extragalactic galaxies is very diffuse.The absolute magnitude dispersion of elliptical galaxies is the largest, the brightest can reach - 22, and the darkest can reach below - 10.The absolute magnitude of spiral and irregular galaxies is relatively small.
Because the brightness of galaxies always darkens from the center to the edge, and the outer edge is not an obvious boundary, the results measured by different methods are often different.[1]
Galaxy size
Elliptical galaxyTheir diameters are generally between 3300 light years and 490000 light years;The diameter of spiral galaxies is generally between 16000 light-years and 160000 light-years;The diameter of irregular galaxies is generally between 6500 light-years and 29000 light-years.[1]
Galactic mass
Mass of galaxiesIt is generally between 1 million and 1 trillion times the mass of the sun.Elliptical galaxies differ greatly in mass, with the difference in size and mass reaching 100 million times.by comparison,Spiral galaxyThe mass is in the middle, and irregular galaxies are generally small.[1]
Galaxy spectrum
Extragalactic galaxy
Extragalactic galaxy is a very complex celestial system, and its light is the sum of the light emitted by its various components.Therefore, when the extragalactic galaxy is taken as a whole for spectroscopic research, the spectrum captured is the superposition of the spectra of all its orbital components.Obviously, the spectra of extragalactic galaxies are different with different components.The composition of extragalactic galaxies is related to their types, so different types ofCumulative spectrumIt is different.The cumulative spectral type of elliptical galaxies is the latest, roughly equivalent to K type.
From elliptical galaxies to irregular galaxies, cumulativeSpectral typeIt's getting earlier and earlier.The cumulative spectral type of IVr type is similar to that of Sc type, which is equivalent to type A or type F.Different types of spectra mean that their colors are different.From elliptical galaxies to irregular galaxies, the color index is getting smaller and smaller, that is, elliptical galaxies are the reddest and irregular galaxies are the bluest.yesSpiral galaxyFor example, the spectrum and color of the nucleosphere part and the spiral arm part are significantly different: the nucleosphere part is similar to the elliptical galaxy, with a late spectral type and a red color, while the spiral arm part has an earlier spectral type and a blue color.
The main components of galaxies are stars, and the cumulative spectrum is mainly similar to that of starsabsorption spectrum。There are also quite a number of galaxies, with emission lines in addition to absorption lines in the spectrum.Elliptical galaxies have the least emission lines.From elliptical galaxies to irregular galaxies, the proportion of galaxies with emission lines is increasing.For Sc series and Irr series, most of them have emission lines.The spectra of a few special extragalactic galaxies are mainly emission lines, with few absorption lines, and some even have no absorption lines at all.
The stars in the galaxy are moving, and the body of the galaxy also rotates. The whole galaxy is also moving in space.The so-called redshift of galaxies is the displacement of a spectral line towards the red end in the spectral observation of galaxies.According to theDoppler effectThe red shift indicates that the observed object is moving away from the Earth in the direction of space line of sight.In 1929, Hubble found that the redshift of a galaxy is proportional to its distance from the Earth.The farther the distance, the greater the redshift.This relationship is calledHubble's law。This is the measured basis of Big Bang cosmology.[1]
Galaxy distribution
The Andromeda Nebula is an Sb type spiral galaxy[1]
Spiral galaxyThe symbol is S0, which has a core part called the nuclear sphere.Outside the nuclear sphere is a thin disk.There are two or more spiral arms extending outward from the outer edge of the nuclear sphere, and it is rarely found that there is one spiral arm.Some parts of the nuclear sphere are relatively round and some are relatively flat. E0-E7 can also be used to represent the shape of the nuclear sphere.[3]
The spiral galaxy can also be divided into SaSb, Sc, etc.The criteria for classification are the development degree of two spiral arms and the relative size of the nuclear sphere.The relative size of Sa type nuclear sphere is the largest, and the rotating arm is the most tightly wrapped;The relative size of Sc type nucleosphere is the smallest, and the rotating arm is the most developed.If the spiral galaxy is“Polar”That isAxis of symmetryIf it coincides with the line of sight, its vortex characteristics can be easily seen;If the axis of symmetry is perpendicular to the line of sight, the vortex shape is not easy to see.At the same time, due to the nearby symmetry plane of the galaxyInterstellar matterThe extinction effect ofScotoma。
Most spiral galaxies have two symmetrical spiral arms, such asBeagleSpiral galaxyM51、Triangular seatSpiral galaxy（M33）；Virgo River alien systemAlso known as the straw hat galaxy, it is a huge spiral galaxy. Seen from the side, the center is protruding and spherical, and the equatorial edge is disk-shaped with spiral arms around it.But generally speaking, the multi spiral arms often appear on the outer edge of galaxies, and they are very short and tightly wrapped.Some spiral galaxies have special shapes, for example, some have annular structures, and some have extremely irregular spiral arms, which are shaped like "V", etc.
Barred spiral galaxy
There is another type parallel to spiral galaxies, calledBarred spiral galaxyThe symbol is SBb barred spiral galaxy, which is characterized by a bar, with a nuclear sphere in the center of the bar, and the spiral arms extend outward from both ends of the bar.Like spiral galaxies, bar spiral galaxies can also be divided into SBaSBb, SBc, etc.
The classification basis is the same as that of spiral galaxies.The SBa type swivel arm is the least developed and looks likeGreek alphabet“θ”，Nucleospheremaximum.The SBc type swivel arm is most developed, like a capitallatin alphabet"S", the nuclear sphere is also the smallest.
Irregular galaxy
Irregular galaxyThe symbol is I or In.It has an irregular shape and is divided into two subtypes IrrI.There is no core in the center of the IrrI type irregular galaxy, so we can't see that there isRotational symmetryThe composition of its stars is similar to Sc, and the vortex structure can be seen faintly occasionally.Irr II is totally irregular and is a special celestial body, such as the famousM82。
Active galaxy
Active galaxyThese are some galaxies with very bright core and strong activity.The light emitted by the nucleus is often astrologicalTotal radiationMost of.It also includes many types such asGalaxy NSeyfert galaxy, etc.From starAccording to the distribution of coefficients by type, spiral galaxies (including bar galaxies) account for the largest proportion of galaxies, more than 60%, and irregular galaxies account for the smallest proportion, only about 2%.
Vandenberg classification
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According to the size of absolute magnitude, Vandenberg roughly divided the river extraterrestrial systems into five categories: supergiant system, bright giant system, giant system, sub giant system and dwarf system.These five categories are divided into Roman characters I, II, III, IVV means.
Based on this, Vandenberg proposed the binary classification of extragalactic galaxies, which is based on the Hubble type plus the luminosity type.This classification is very similar to the binary spectral classification of stars.
Distance measurement
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standard candle
E0 Elliptical.jpg[1]
Standard candleThe principle of the method is very simple.For some stars or star clusters in extragalactic galaxies, their apparent magnitude can be determined from observations.At the same time, if we know the absolute magnitude of these objects, specifically assuming that their absolute magnitude is equal to the absolute magnitude of the corresponding objects in the Milky Way, then we can calculate their distance according to the relationship between the apparent magnitude and the absolute magnitude, which is the distance of the alien system to which they belong.We can also take the extragalactic galaxy as a whole, calculate the cumulative apparent magnitude from the observation, and assume that its absolute magnitude is equal to a certainaverage valueTo find the distance.There are the following methods:
utilizeCepheid variable(includingLyra RR type variable）Find the distance.This is the most accurate and reliable method.Because of this, the determination of the zero point of the Cepheid variable's peripheral light relationship has become a very important task in astronomy.The disadvantage of this method is that it is only possible to distinguish Cepheid variable stars in the nearby extragalactic galaxies. For the farther extragalactic galaxies, this method cannot be used.
Use bright stars to find the distance.In every extragalactic galaxy where stars can be distinguished, there are always some brightest stars. They areO star, MorningB-type star, Flemish Rayet star, mid late typeSupergiantEtc.The average absolute magnitude of these stars is known.For example, the absolute star of the Wolf Rayet star is - 4 to - 6, and the average can be - 5;The brightest supergiant has an average absolute magnitude of - 9.This method is very convenient, because there are always some bright stars in the galaxy not too far away, especiallySupergiantThis method is better than Cepheid variableFaergyThe measured distance is longer, but the absolute magnitude dispersion is larger, so it is not as accurate as the latter.
Use nova to find distance.Many new stars have been found in some extragalactic galaxies.If new information can be obtained from observationstarlightThe apparent magnitude at maximum, and assuming that the absolute magnitude at maximum light is equal to the absolute magnitude at maximum galactic nova, then the distance can be calculated.The disadvantage of this method is that there are not many extragalactic galaxies to find new stars, so it cannot be widely used.
Find the distance with supernovae.The method is similar to nova.Due to the super new starlight, the absolute magnitude can reach - 17Wait until-19, so the distance of distant galaxies can be measured.However, due to the large dispersion of absolute magnitude when the light is maximum, the measured distance error is also large.
GlobularCumulative magnitudeFind the distance.The absolute magnitude of globular clusters is between - 5 and - 10, and the dispersion is too large.Obviously, if we use a single globular cluster to calculate the distance, the reliability is too poor.However, many globular clusters can be observed in the same galaxy, and the distance can be roughly estimated by averaging them.
It is also an important method to calculate the distance by using the accumulated magnitude of galaxies.The accumulated apparent magnitude Mr of the galaxy can be obtained from the observation. If the distance of the galaxy is calculated by other methods, then the accumulated absolute magnitude Mr of the galaxy can be obtained.Average all Mr obtained by this method to get an average value.For the galaxy whose distance is to be calculated, assume that its Mr is equal to this average value, and its Mr can be obtained from observation, so that the distance can be calculated.When using this method, it is often necessary to classify galaxies, not only according to elliptical galaxies, spiral galaxies, etc., but also to subdivide each type according to certain conditions, so that the dispersion of absolute magnitude is as small as possible, so as to obtain more reliable results.
Redshift method
Large Magellanic Cloud
red shift: Comparison of the same spectral line in the laboratory light source, celestial bodySpectral lineDisplacement towards the red end.It is indicated by the symbol Z.
In the 1920s, a phenomenon of great significance was found. Except for three galaxies (including Andromeda Nebula) close to people, the spectral types and lines of all galaxies shifted towards the red end;And the weaker the galaxy, the greater the number of redshifts.If you putSpectral line displacementIt is interpreted as Doppler effect, so it can be calculated from displacementApparent velocityVr。The distance can be obtained from other methods.
The observation results show that,Vr is approximately proportional to the distance r, that is, Vr=Hr where r isMillisecond gapIs the unit,Vr is in kilometers per second,Proportional constantH is calledHubble constant, in kilometers per second per million seconds.The formula is called Hubble's law.This formula except fora quasarThere is still some controversy, and it is generally applicable to the general extragalactic galaxies.For an unknown extragalactic galaxy, if we can get a sufficiently clear spectrum and calculate Vr, we can use the formula to calculate the distance.The advantage of this method is that it is generally applicable to all galaxies, especially for very distant extragalactic objects, and other methods cannot be used.The value of the Hubble constant H often changes.
Before 1930,The H value is set as 528 km/s · million second difference;It was changed to 180 in 1956;At present, it has changed to below 100, and tends to 50-60.This is not surprising, because as more and more data are accumulated,Observation accuracyHigher and higher,The H value becomes more and more accurate.
Basic Introduction
In the 17th century, people successively discovered some obscure celestial bodies, so they called them“Nebula”。Nebula contains planets andcometAlmost all extended celestial bodies outside.They are mainly composed of hydrogen, followed by helium, and contain a certain proportion ofmetallic elementandNonmetallic element。1990hubble space telescope Research since launch also found thatOrganic moleculeEtc.
In the 1920s, the American astronomer Hubble found a kind of celestial body called "Cepheid variable" in the Andromeda nebula, so he calculated the distance of the nebula, finally confirmed that it was a celestial system outside the Milky Way, and called them "extragalactic galaxies".More than 10 billion extragalactic galaxies have been discovered.
Origin of name
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There are many celestial bodies outside the Milky Way.There is a kind of celestial body in the sky, which is almost indistinguishable from the nebula of the Milky Way when viewed with a small telescope.If usingLarge telescopeLook, you will find that they are not diffuse gas and dust, but composed of stars that can be distinguished, and their shape is also like a vortex.They are celestial systems similar to the Milky Way, and their distances are beyond the scope of the Milky Way, so they are called "extragalactic galaxies".Andromeda galaxyIs located inandromedaAn extragalactic galaxy.
Extragalactic andGalaxySimilarly, it is also composed of a large number of stars, star clusters, nebulae and interstellar matter.There are as many as 100 billion galaxies observed, such as the Portuguese in 1518-1520Magellan circumnavigationIn the southern hemisphere, two large extragalactic nebulae (extragalactic galaxies) were found in the southern sky with the naked eye and named as:Large Magellanic CloudandMagellanic nebulaThey are the nearest extragalactic galaxy to the Milky Way, and are physically connected with the Milky Way, forming a triple galaxy.
Discovery history
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The discovery of extragalactic galaxies can be traced back to more than 200 years ago.In the catalog compiled by French astronomer Messier Charles at that time, the nebula M31 played an important role in the history of astronomy.On the night of early winter, people familiar with the stars can find it with the naked eye in Andromeda - a fuzzy spot, commonly known asandromedaOf the Great Nebula.
If we assume that the brightness of these new stars at their brightest is the same as that of other new stars found in the Milky Way, we can roughly infer that the Andromeda Nebula is very far away from us, far beyond the scope of our known Milky Way.But because the distance measured by nova is not very reliable, it has also caused controversy.It was not until 1924 that American astronomer Hubble found the Cepheid variable star called the "sky ruler" at the edge of the Andromeda Nebula with the world's largest 2.4-meter telescope at that time. The exact distance of the Andromeda Nebula could be determined by using the corresponding relationship between the light change period and luminosity of the Cepheid variable star, proving that it was indeed outside the Milky Way, which can also be said to be like the Milky Way,It is a huge and independent star group.Therefore, the Andromeda Nebula should be renamed Andromeda Galaxy.
The discovery of extragalactic galaxies can reflect on our Milky Way.It is just an ordinary galaxy, a member of the hundred billion galaxy family, an island in the ocean of the universeInfinite universeA very small part of.
Main categories
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Elliptical galaxy
The classification of galaxies was proposed by Hubble in 1926. It can be divided into:
Elliptical galaxy: It is round or oval in shape, with bright center and gradually dark edge.According to the shape, it can be divided into eight categories: E0 to E7Hypotype。Elliptical galaxy is a kind of extragalactic galaxy, which is spherical or ellipsoidal.The center area is the brightest, and the brightness decreases to the edge. For those close to the center, the member stars in the periphery can be distinguished with a large telescope.Elliptical galaxies according toHubble classification, pressEllipticityThe size is divided into E0There are eight subtypes of E1, E2, E3,..., E7,Type E0 is a circular galaxy,E7 is the flattest elliptical galaxy.
For the same type of extragalactic galaxies, there is a great difference in mass, which can be divided into giant and dwarf galaxies. Among them, the difference in mass of elliptical galaxies is the largest.Least massiveDwarf elliptical galaxyandGlobular clusterThe supergiant elliptical galaxy with the largest mass may be the largest in the universeStellar system，Quality rangeIt is about ten million to one million billion times of the sun, and its luminosity amplitude ranges from - 9 to - 23 absolute magnitudes.
The mass luminosity ratio of elliptical galaxies is about 50~100, while that of spiral galaxiesMass light ratioAbout 2~15.This shows that the productivity efficiency of elliptical galaxies is far lower than that of spiral galaxies.The diameter of elliptical galaxies ranges from 1 to 150Kilosecond gap。The total spectral type is K type, which isRed GiantOfSpectral characteristics。The color is redder than that of spiral galaxies, indicating that there are fewer young member stars than that of spiral galaxiesAstral familyII Celestial composition, no or only a fewInterstellar gasandInterstellar dustThere are no typical star family I objects in elliptical galaxiesBlue giant。
About the formation of elliptical galaxiesTheory of galaxy formationIt is believed that elliptical galaxies are formed by two spiral flat galaxies colliding, mixing and devouring each other.Astronomical observationDescription, the vortex is flatGalactic diskThe stars in the elliptical galaxy are all relatively young, while the stars in the elliptical galaxy are relatively old, that is, the spiral flat galaxy is formed first, and the two spiral flat galaxies meet and mix to form the elliptical galaxy.It's still usedcomputer simulationThe results show that under certain conditions, two flat galaxies can indeed develop into an elliptical galaxy after mixing.
CanadaAstronomer Cormendy found in his observations that some elliptical galaxies are much more massiveGiant elliptical galaxyThe central part of theLuminance distributionAbnormal, as if there is another one in the centerSmall nucleus。His explanation is the result of a very small elliptical galaxy being swallowed by a giant elliptical galaxy.However, the density of galaxies in the universe is very low after all, and they have very little chance of colliding with each other. It is very difficult to find that two galaxies are in the collision and phagocytosis stage from the observation.Therefore, this formation theory needs further exploration.
spiral galaxy
The classification of galaxies was proposed by Hubble in 1926. It can be divided into:
solar systemThe Milky Way is aspiral galaxy , mainly composed of hundreds of billions of stars andInterstellar medium(gas and dust).Most of them are densely distributed near the symmetry plane of the Milky Way, formingSilver plateAnd the rest are scattered on the silver plate, which is nearly sphericalSilver haloLi.Stars and interstellar media are notuniform distributionBut more densely distributed in several areas stretching out from the center of the Milky WaySpiralIn the rotating arm, it is in strips.
General distributionStars in the spiral arms are young and rich in metal, and most of them areIonized hydrogen cloudSuch interstellar media.The stars in the silver halo are old andLean metalOf.The oldestStellar ageFor 15 billion years, some stars have already aged and passedSupernova explosionThe internal composite containsHeavy elementThe pieces and ashes fall onto the silver plate.
Lenticular galaxy
The classification of galaxies was proposed by Hubble in 1926. It can be divided into:
In elliptical galaxies, galaxies that are more flat than E7 type and begin to appear vortex characteristics are calledLenticular galaxy。Lens galaxy is a transitional type galaxy when elliptical galaxy transits to spiral galaxy or elliptical galaxy transits to rod spiral galaxy.
Irregular galaxy
Irregular shape, no obvious core and spiral arm, no disk like symmetry structure or no rotation symmetry galaxy can be seen, which is represented by the letter Irr.At the end of the dayBright galaxyAmong them, irregular galaxies only account for 5%.According to the classification of galaxies, irregular galaxies can be divided into Irr I type and Irr II type.Type I is a typical irregular galaxy. In addition to the above general characteristics, some have indistinctly visible irregular bar structure.They areDwarf galaxyThe mass is 100 million to 1 billion times that of the sun, and there are some that can reach 10 billion times that of the sun.They are small in size, and their long diameter ranges from 2 to 9Kilosecond gap。
Star family composition and Sc typespiral galaxySimilar: O-B-type star、Ionized hydrogen region, gas and dust and other young star family I objects account for a large proportion.Type II has an amorphous appearance, can not distinguish the composition of stars and star clusters, and often has obviousDust belt。 Some type II irregular galaxies may be exploding or post exploding galaxies, while others may be affected byCompanion galaxyOfGravitational disturbanceAnd distorted galaxies.So the origin of type I and type II irregular galaxies may be completely different.
Main features
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size
Elliptical galaxies vary greatly in size, with diameters ranging from more than 3300 light-years to 490000 light-years;The diameter of spiral galaxies is generally between 16000 light-years and 160000 light-years;The diameter of irregular galaxies is generally between 6500 light-years and 29000 light-years.Of course, because the brightness of galaxies always darkens from the center to the edge, and there is no obvious boundary on the outer edge, the results measured by different methods are often different.
quality
The mass of galaxies is generallySolar massBetween 1 million and 1 trillion times.Elliptical galacticQuality differenceIt is very big, and the size and quality are 100 million times worse.In contrast, spiral galaxies have a medium mass and irregular galaxies are generally smaller.
motion
The stars in the galaxy are moving, the galaxy itself rotates, and the whole galaxy is also moving in space.GalacticRedshift phenomenonSo-called galacticred shiftPhenomenon is that in the spectral observation of galaxiesSpectral lineThe displacement towards the red end.Why is there such displacement?What does this displacement phenomenon show?According to theDoppler effectThe redshift indicates that the observed objects are moving away from us in the direction of space line of sight.In 1929, Hubble found that the redshift of a galaxy is proportional to the distance from the galaxy to us.The farther the distance, the greater the redshift.This relationship is calledHubble's law。This isBig Bang The measured basis of.
As a hugesphere systems For example, galaxies also have an evolutionary process of formation, development and decay.According to the morphological sequence, there are elliptical galaxies, spiral galaxies and irregular galaxies.Does this morphological difference represent their different evolutionary stages?Who is young?Who is middle-aged?Who is old?It is still under exploration.
Irregular galaxies are not structures.E series is generally composed of nuclei and halos.The core is divided intoNucleosphereAnd core.Some dwarf E-series have no nuclei.The S-series (including SB) is the most complex, with a core, a nuclear sphere, a disc and a halo, and a rotating arm in the disc.The main difference between S0 system and E system is that the SO system has disks,The difference between SO system and S system is that SO system has no rotating arm.
spectrum
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Extragalactic galaxy is a very complex celestial system, and its light is the sum of the light emitted by its various components.Therefore, when we take extragalactic galaxies as a wholeSpectroscopicDuring the research, the spectrum captured is the superposition of all its components.Obviously, different components lead to different spectra of extragalactic galaxies.The composition of extragalactic galaxies is related to their types, so different types of cumulative spectra are different.
The cumulative spectral type of elliptical galaxies is the latest, roughly equivalent to K type.From elliptical galaxies to irregular galaxies, the cumulative spectral type is getting earlier and earlier.The cumulative spectral type of Ivr type is similar to that of Sc type, which is equivalent to Type A or Type F.The difference of different types of spectra means that their colors are also different.From elliptical galaxies to irregular galaxies,Color indexSmaller and smaller, that is, elliptical galaxies are the reddest, and irregular galaxies are the bluest.For spiral galaxies, the nucleosphere andSwivel armThe spectrum and color of some parts are significantly different: the nuclear sphere part is similar to the elliptical galaxy, with a later spectral type and a red color, while the spiral arm part has an earlier spectral type and a blue color.
The main component of galaxies is stars, and the cumulative spectrum is mainly similar to the absorption spectrum of stars.However, there are quite a few galaxies in the spectrum exceptAbsorption lineThere are also some transmission lines.Elliptical galaxies have the least emission lines.From elliptical galaxies to irregular galaxies, the proportion of galaxies with emission lines is increasing.For Sc series and Irr series, about or even most of them have emission lines.The spectra of a few special extragalactic galaxies are mainly emission lines, with few absorption lines, and some even have no absorption lines at all.There are also some extragalactic galaxies with only continuous spectra, and no spectral lines can be seen.
brightness
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Absolute magnitude.If the distance of extragalactic galaxies is knownApparent magnitudeThe absolute magnitude can be obtained.Observations show that the absolute magnitude of extragalactic galaxies is very diffuse.The absolute magnitude of elliptical galaxies is the brightest, the brightest can be as bright as - 22, and the darkest can be as dark as-10Etc.The absolute magnitude of spiral and irregular galaxies is relatively small.
Vandenberg has five types of extragalactic galaxies: supergiant, bright giant, giant, sub giant and dwarf.These five categories are divided intoRoman charactersI, II, III, IV, V indicate.On this basis, Vandenberg proposed the binary classification of extragalactic galaxies, that is, the Hubble type plus the luminosity type.This classification is related to the binary of starsSpectral classificationVery similar.
Surface brightness.Extragalactic galaxy is a surface light source. We can measure its surface brightness and study its variation law.Generally, surface brightnessthe magnitude/Angular second2 indicates.Generally speaking, the greater the density of matter, the stronger the radiation. The change of luminosity on the galactic view plane can reflect the distribution of matter.Therefore, it is very valuable to study the variation law of brightness to understand the structure of galaxies. The surface brightness of different types of galaxies is very different, and the brightness of elliptical galaxies, spiral galaxies, and lenticular galaxies are different.
Stellar composition
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The most direct way to study the star composition of extragalactic galaxies is to use large telescopes to decompose galaxies into stars as much as possible.Indeed, a large number of stars of various types have been observed in nearby galaxies, such asOB star, mid late typeSupergiant、RR Lyrae variable 、Classical Cepheid Variable, NovaSupernova、long periodVariable stars, etc.We also observed manyOpen clusterAnd globular clusters.But this method is very limited, because extragalactic galaxies are too far away from us after all.Even for the nearest galaxy, only the high luminosity stars in it can be observed.For example, Andromeda is bigNebulaIf you use a 5m telescope to observe, take itsLimiting magnitudeIt is 23, and only stars with absolute magnitude of - 1.4 can be observed, such as the solar typedwarf starIt can't be observed at all.If the distance between galaxies is more than oneMillisecond gapEven if there is a supernova explosion inside, we cannot observe it.Generally speaking, we can study the star composition of galaxies by studying their spectra and colors.
The nuclear spheres of elliptical galaxies and spiral galaxies are very similar in spectrum, color index, etc., indicating that their star compositions are very similar.Relatively speaking, the spectral type of the spiral arm is earlier and the color is blue, indicating that the star composition of the spiral arm is different from that of the nuclear sphere.It was based on the study of the Milky Way and extragalactic galaxies that Bader proposed the concept of two star families.The nuclear spheres of elliptical and spiral galaxies are mainly composed ofStar Family IIform;The spiral arms and irregular galaxies are mainly composed ofStar Family Iform.However, it should be pointed out that every galaxy, including elliptical galaxies and irregular galaxies, by no means includes only stars of one star family.For example, the spectrum of elliptical galaxies often contains some spectral lines of heavy elements.The intensity of these lines indicates that theContent ratioextremeStar family IIHigh.Therefore, elliptical galaxies may also contain some disk family stars.On the contrary, irregular galaxies may also contain some family II stars, such asLarge and small Magellanic cloudsI found a lot ofLyraGuanidine typeVariable starAnd globular clusters, which are the stars of Extreme Group II.
Gas and dust content
Many galaxies have spectra similar toGalactic nebulaThe emission lines of the stars indicate that they have interstellar gas.The observation of 21 cm spectral line of neutral hydrogen also confirmed this point.Few elliptical galaxies have emission lines;In addition, except for an elliptical galaxy, the rest have not been observed so farNeutral hydrogen21 cm line.These indicate that there is no or very little gas in elliptical galaxies.However, strong emission lines have been observed in the core of some elliptical galaxies, including manyForbidden lineTherefore, there should be gas in the core.
Elliptical and irregular galaxies must have interstellar gas and dust.In fact, in some nearby spiral galaxies and irregular galaxies, we can directly see manyGas nebula。Observation shows that from Sa to IrrGas contentGradually increase,The gas content in Irr is more than 20%.The gas and dust are mainly concentrated near the symmetry plane.In some spiral galaxies facing us, dark absorption bands caused by dust extinction can be clearly seen.
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Through scientists' unremitting exploration, more than 1 billion extragalactic galaxies have been discovered, with a distance of 36 billion light-years.What are the extragalactic galaxies closer to our Milky Way?
Large, Magellanic Galaxy
Large Magellanic Galaxy(Large Magellanic Cloud), 160000 light years away from us;The second is the Little Magellanic Galaxy, 190000 light-years away from us.In terms of shape, they are all irregular galaxies.They are subsidiary galaxies of the Milky Way and can only be seen in the southern hemisphere.Visible to the naked eye.Magellan discovered them when he sailed to the southern hemisphere, hence the name.
Andromeda galaxyThe Andromeda Galaxy is a galaxy closer to our Milky Way.She is a typicalSpiral galaxy(Spiral Galaxy), but it is larger than the Milky Way.
Since human beings live in the Milky Way, they cannot observe the whole picture of the Milky Way. In the past, the Milky Way was considered as a spiral galaxy like the Andromeda Galaxy, but the latest research shows that the Milky Way should be aBarred spiral galaxy。The Andromeda Galaxy, the Milky Way and more than 30 other galaxies together form a larger galaxy group, the Local Group Galaxy Cluster.
Andromeda galaxy ranked 31st in the list of distant fuzzy objects by Charles Messier, a French astronomer in the 18th century, so it is also called M31.It is about 2 million light years away from the Earth, with a diameter of 160000 light years (100000 light years in the Milky Way), a mass of no less than 3.1 × 1011 solar masses, and more than 200 million stars, making it the largest one in the galaxy cluster.
