Planetary nebula is actually some dyingfixed starThe diameter of the dust and gas shell thrown out is generally about one light year.It is composed of materials that are continuously ejected by stars whose masses are less than ten times of the sun at the end of their evolution after the hydrogen fuel in their cores is exhausted.
Planetary nebula refers to the nebula with a disk or ring shape and a dark extended view surface, belonging toEmission nebulaOne of.Seen through a telescope, it has the characteristics like Uranus andNeptuneSuch a slightly green round surface with clear edges.Planetary nebulae are round, oblate or annular, and some are similar to major planets, hence the name.
In February 2024, astronomers cracked the planetary nebulaThe secret of the disappearance of sulfur.[2]
In 1777,William Herschel After the discovery of such celestial bodies, they are called planetary nebulae.Observations with large telescopes show that the traveling star nebula has complex structures such as fibers, spots, air currents and small arcs.They are mainly distributed inGalactic surfaceNearby, subject toInterstellar extinctionA large number of planetary nebulae areDark nebulaIt is obscured and difficult to observe, and its central part has a very small core, which is the central star with high temperature.The gas shell of the planetary nebula is expanding at a speed of 10 to 50 kilometers per second.Its chemical composition is similar to that of stars, and its mass is generally 0.1 to 1Solar massThe density is between 100 and 10000 atoms [ions] per cubic centimeter, the temperature is between 6000K and 10000K, and the temperature of the central star is more than 30000K.The nebula absorbs its strong emissionUltravioletRadiation passageCascade transitionThe process is transformed intovisible light。It is estimated that the average lifetime of planetary nebulae is about 30000 years.The appearance of such nebulae indicates that the stars have reached their old age.
stayGalaxyDuring its existence [about 1 billion to 10 billion years], there are nearly 1 billion to 10 billion stars that have experienced the stage of planetary nebulae.Therefore, this kind of celestial body is probably a universal one.Most stars in the Milky Way are likely to "die" after passing through planetary nebulae.According to the distribution density near the sun (about per thousand cubic metersSecond gapThirty to fifty) estimates, the wholethe galaxyThere should be forty or fifty thousand in the system, of which only a small part is observed.
These nebulae are associated withDiffuse nebulaThey are completely different in nature. They are about the same mass as the sunStellar evolutionIn the late stage,nuclear reactionAfter stopping, it is the product of death.The volume of such nebula is expanding, and finally the gas gradually diffuses and disappears into interstellar space, leaving only one centerWhite dwarf。In the center of the planetary nebula, there is a high-temperature star, called the central star of the planetary nebula.This is a star that is evolving into a white dwarf star.
Planetary nebulae are the majorityStellar evolutionThe state to the end of the period.Our sun is a star of low massSolar massMany times more stars will be produced dramatically at the end of evolutionSupernovaExplosions, but for medium and low mass stars, will eventually develop into planetary nebulae.Stars whose mass is less than twice the mass of the sun spend most of their lives in the core to fuse hydrogen into heliumnuclear fusionReaction, the energy released by nuclear fusion blocks the collapse of the star's own gravity, making the star stable.After billions of years, the stars have used up hydrogen, and the energy released from the core will not be enough to generate enough pressure to support the outer shell of the stars, so the core will shrink to raise the temperature.Solar coreThe temperature of is close to 15 million K, but whenhydrogenWhen exhausted, shrinkage will raise the temperature to 100 million K.
fixed starDue to the rise of core temperature, the shell ofRed Giant。The core of the star continues to shrink and raise the temperature again. When the temperature reaches 100 million K, the helium in the core will begin to fuse into carbon and oxygen, which is the source of metals in the universe.The re ignited nuclear fusion reaction prevents the core from shrinking. The burning helium will generate carbon and oxygen cores inside, while the outside is surrounded by burning heliumHeliumnuclear fusionThe reaction is sensitive to the extreme temperature, which is proportional to the 40th power of temperature (T40), that is, as long as the temperature rises by less than 2%, the reaction rate will double, so as long as the temperature rises slightly, the reaction rate will rapidly increase, and then more energy will be released to further increase the temperature;Thus, the outward expansion rate of the shell increases and the temperature of the shell decreases.This makes the star very unstable, so a huge pulsation combination is generated, and the gas shell of the star is shrinking and expanding repeatedly, and will eventually be thrown into space.
The ejected gas forms colored clouds near the star, leaving a bare core in the center.As more and more gas shells are thrown away from the stars, the exposed layers of the stars continue to penetrate into the core, and the surface temperature of the exposed parts becomes higher and higher.When the exposed surface temperature reaches about 30000K, there will be enough ultraviolet photons to dissociate atoms in the atmosphere, and then the gas begins to produceStimulated radiationPlanetary nebulae were born.
Life span
The gas in the planetary nebula drifts outward at the speed of thousands of kilometers per second. When the gas continues to expand outward, because the mass of the star is not enough to shrink the core to temperature, carbon and oxygen can be triggerednuclear fusionAt the required temperature, the central star will begin to cool gradually due to the stop of nuclear fusion reaction.Once the surface temperature of the core is too low to release enough ultraviolet light to make more and more distant gas glow, the cloud will no longer be seen, and the star will becomeWhite dwarfAnd the gas cloud will also be reorganized.It only takes about 10000 years for a typical planetary nebula to be born and reorganized.
main features
Planetary nebula IC418
Planetary nebulae are products of stars in their later years.Planetary nebulae are actually composed of gas ejected by dying stars.In the final stage of star life, stars rely on helium in the shell outside the coreFusion reactionProvide energy.This process is very unstable.A loose star that has expanded and combined with each other under the combined effect of internal turbulence and radiation pressureSurface layerIt is thrown into space, which forms a planetary nebula.There are a lot of materials thrown into space, moving at a high speed of 1000 kilometers per second, forming a strong "wind".Although the material that makes up the nebula is very thin, its mass is very high.stayGalaxyOn average, a new planetary nebula is born every year.Since the 18th century, astronomers have observed about 1500 images of planetary nebulae and catalogued them.In addition, there may be about 10000 planetary nebulae hidden in the denseDust cloudBack.
Planetary nebulae have various complex shapes, almost all of which have symmetry.It has colorful gas clouds and is one of the most magnificent landscapes in astronomy.The research on the formation and development process of nebula is continuing. There are many models, but none of them can correctly explain all the observations.
The original "interactive stellar wind hypothesis" model believes that when the high-speed stellar wind rushes into the low-speed stellar wind in front, a dense compressed gas circle will be formed at the junction of the two.This model gives a satisfactory explanation for the circular and nearly circular planetary nebulae.However, according to the observation, the circular planetary nebula only accounts for 10% of the total number, and more of them are flat and long.In the modified model of the "interactive stellar wind hypothesis", it is assumed that the low-speed stellar wind now forms a thick ring at the equator.Due to the influence of this ring, the high-speed stellar wind strongly deflects, forming an hourglass shape with mirror symmetry.In the computer simulation, this model satisfactorily explained all the shapes discovered up to 1993.
Intragalactic cycle
Planetary nebula
Planetary nebulae play an important role in the evolution of galaxies.In the early universe there was almost all hydrogen and helium.But stars can pass throughnuclear fusionproduceHeavy elementThe gas in planetary nebulae thus contains a large proportion of carbon, nitrogen and oxygen.And via expansion andInterstellar matterThey are mixed together, thus enriching the content of heavy elements.
Astronomers call this process metallization.Stars born later will have more heavy elements at the beginning.Even so, the proportion of heavy elements in stars is still very low, but it is enough to have a significant impact on the evolution of stars.In the early days of the universe,Heavy elementStars with lower content are called the secondAstral familyThe younger stars with more heavy elements are called the first star family.
History of observation
Planetary nebulae are usually faint objects, and none of them can be seen with the naked eye.The first planetary nebula discovered is located inFoxyOfDumbbell NebulaIn 1764Charles Messier Found and listed as No. 27 in its list（M27）。The resolution of telescopes used for early observation is very low,M27 and later discovered planetary nebulae look similar to gas planets, so the discoverer of UranusWilliam Herschel They are called planetary nebulae.Although we already know that they are completely different from planets, this name has become a proper noun and is still used today.
Planetary nebula
Until the 19th centurySpectroscopeAfter observing the spectrum of planetary nebula, its essence began to be understood.William Huggins He was one of the first astronomers to study the spectrum of celestial bodies. He used a prism to observe the spectrum.His observations show that the spectrum of the celestial bodyContinuous spectrumThere are many darkAbsorption lineLater, he discovered many objects that looked like nebulae, such as the Andromeda Nebula, also had similar spectra, and we know that some so-called nebulae at that time were actually galaxies.
However, when he observedCat's Eye NebulaHe found that the spectrum of the Cat's Eye Nebula was very different from others.There are only a few emission lines in the spectrum of Cat's Eye Nebula and similar objects.The most obvious ones are those with a wavelength of 500.7 nmSpectral lineHowever, it cannot be consistent with the spectral lines of any element known at that time.At first, he assumed that this was the spectral line of an unknown element and named it nebulium - as it led to the discovery in 1868Solar spectrumInHelium lineGuess.
However, helium was found on the earth shortly after it was found in the solar spectrum, but the hypothetical nebulium did not.In the early 20th century,Henry Norris RussellIt is proposed that it is not a new element. The 500.7 nm spectral line is a spectral line generated by a known element in an unfamiliar environment.
In the 1920s, physicists showed that under extremely low density of gas, electrons can stay in atoms or ions after being excitedMetastable stateAnd viatransitionGenerate spectral lines, but in the environment with high density, because of frequent collisions, theseEnergy levelThe electrons on the oxygen ion (O2+or OIII) are knocked off before the transitionMetastable transition500.7 nm spectral line can be generated when.Such spectral lines that can only be generated in very low density gases are calledForbidden line（forbidden lines）。Therefore,SpectroscopeThe observed spectral lines indicate that the nebula is composed of extremely thin gas.As further discussed below, the star at the center of the planetary nebula is very hot, but its brightness is very low, suggesting that it must be very small.Only when the star ran out of nuclear fuel could it collapse into such a small star, so planetary nebulae are considered asStellar evolutionThe final stage of.Spectral observations show that all planetary nebulae are expanding, so the appearance of planetary nebulae is the idea that stars throw gas shells into space before the end of life.
At the end of the 20th century, scientific and technological progress made us further understand planetary nebulae.Space telescopeAllow astronomers to studyvisible lightExternal electromagnetic waves.This is because the atmosphere only allows radio waves and visible light to pass through.Studying planetary nebulae with infrared and ultraviolet rays can more accurately measure their temperature, density andabundance。CCD technology can measure darker spectral lines that could not be measured in the past.The nebulae observed from the ground are simple in structure and regular in shape.But throughEarth's atmosphereAboveHubble Space TelescopeMany extremely complex nebula shapes and structures that have never been seen before have also been revealed.At Morgan KennerSpectral classificationPlanetary nebulae are classified as type - P under the system of "P", but such spectral labels are rarely used in practice.
research subject
A long-term problem in the study of planetary nebulae is that in most cases, their distances are not accurately determined.The closest planetary nebulae can be measured by their expansionparallaxDetermine their distance.High resolution observations with time differences of several years can show their vertical and line of sight expansion,Doppler effectThe spectral observations of the two can reveal their expansion in the line of sight direction.The distance of the nebula can be obtained by comparing the expansion of the opening angle with the calculated expansion speed.
The cause of nebula shape diversity is a controversial topic.It is believed that the interaction between matter leaving the star at different speeds produces most of the observed shapes.However, some astronomers believe that central conjunctions are the cause of more complex and extreme planetary nebulae.Some planetary nebulae have been proved to have strong magnetic fields, as suggested by Grigor Gurzadyan in the 1960s.The magnetic interaction of ionized gases may be the reason for the shape of some planetary nebulae.
Identify the nebulaMetal abundanceThere are two ways.They have been compared with different types ofSpectral line——Compound lineAnd collision excitation lines.There are sometimes significant differences between the results of the two methods.Some astronomers explain this phenomenon through small temperature fluctuations inside planetary nebulae;Others thinktemperature effectIt could not lead to such a big difference, and a hypothesis was put forward that there was a low-temperature kink with very low hydrogen content.However, this kink has not yet been discovered.
Typical nebula
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Annular nebula
Annular planetary nebula
exceptSaturn RingOutside,Annular nebula(M57) is probably the most famous annular object in the sky.This simple and elegant planetary nebula may be the result of the projection of our line of sight from the Earth through the cylindrical cloud gas, which is thrown out by a dying central star.
Astronomers of the Haber family heirloom program, usingSpace telescopeSeveral images taken have produced this wonderful high resolution photo. The color used in the image is used to mark the temperature distribution of the star's shroud.The blue represents the hot gas near the hot central star region, which slowly changes to the green and yellow regions that are cooler outside, as well as the red gas that is also coldest at the extreme edge.In addition, many dark stripes can be seen near the edge of the nebula.
Annular nebulaLocated in the northern skyLyra（lyra）Inside, the size is about one light year, about 2000 away from usLight yearFar away.
Dumbbell Nebula
All dayPlanetary starIn the cloud,FoxyDumbbell NebulaUndoubtedly the most beautiful one, it is listed inmessier Cluster nebulaCatalogueNo. 27 of, so it is also calledM27Nebula.It is not the largest or brightest planetary nebula.Because the larger planetary nebulae are relatively dark, and the brightest planetary nebulae are very small, the Dumbbell Nebula in the constellation Fox becomes the most easily observed planetary nebula.It is easy to find M27 at 3 ° north of γ star in constellation Arrowhead.Even with a small telescope, it can be recognized at once.itsEquatorial coordinateIs: 19:59.6 right ascension;Declination+22 ° 43 '(2000.0).Angle size8 '* 4', distance 300Second gap，975Light year。
The Dumbbell Nebula in the constellation Fox is a very beautiful celestial body.Very bright,Apparent magnitude7.6, etc.It still stands out in the star sky background, which is full of stars. Its shape is like a dumbbell joined by two conical tops, so it is calledDumbbell Nebula。With a 6-inch telescope, it looks very clear and moving.When observing with a larger telescope, you can see the soft blue-greenHaloSurround the dumbbells.useLarge telescopePhotographic observations show thatazimuthThe nuclear star with 125 ° and 12 degree is obviously close to the western edge of the dumbbell shape. However, the astronomer Weibo noticed that there are several dark stars that have no physical connection with the nebula.It is difficult to recognize that 12th class nuclear star.In addition, at 25 'north of the Dumbbell Nebula, there is only one star of magnitude 5, which isFoxy14 stars.
Eskimo Nebula
Eskimo Nebula Also known as NGC 2392, it is an astronomerWilliam Herschel It was discovered in 1787 and got this nickname because it looked like a human head carrying an Eskimo fur hood from the ground.In 2000, the Hubble Space Telescope took a picture of the nebula, and found that the nebula has very complex cloud structures. The causes of these structures are still not completely clear.In any case, the Eskimo Nebula is an all embracing planetary nebula, and the cloud in the image is the outer gas shell thrown out by a star very like the sun 10000 years ago.The filamentous structure in the inner layer of the nebula clearly visible in the image is the central star material thrown out by the strong stellar wind, while the outer disk area has many strange orange fingers with a light year long.
What makes this nebula special is that its structure is the most complex of almost all recorded nebulae.Images taken from the Hubble Space Telescope show that,Cat's Eye NebulaIt has knots, spray columns, arcs and other structures in various shapes.This nebula is one of the most widely studied nebulaeApparent magnitudeIt is+8.1 and has high surface luminosity.Its declination and declination are respectively 17h 58.6m and+66 ° 38 ', and its high declination represents that observers in the Northern Hemisphere can easily see it.Many large telescopes are located in the northern hemisphere. Because the nebula is near the north yellow pole, it should not be difficult to find it if you look near the yellow pole in good weather.
The brighter part of the inner nebula is about 20 angular seconds in diameter, and its expanding nebulaHalo substanceThe diameter is about 386 angular seconds (6.4 angular minutes).Its nebula halo material is originalStellar evolutionbyRed GiantIt is ejected during the stage.According to the observation results, the density of the main body of the nebula is about 5000 particles per cubic centimeter, and the temperature is about 8000 K1. The temperature of the outer nebula halo is higher, up to 15000 K, while the density is lower than that of the interior.In the center of the nebula is an O-shaped star, whose temperature is about 80000 K, luminosity is about 10000 times that of the sun, and radius is 0.65 times that of the sun.according tospectroscopyAccording to the analysis, due to the influence of stellar wind, the mass of the central star is losing at the rate of 20 megatons per second, equal to 3.2 × 10 ^ - 7 per yearSolar massThe wind speed of stellar wind is 1900 kilometers per second.According to the calculation results, the mass of the central star is about the same as that of the sun, about one solar massStellar massIt is estimated to be about five times the size of the sun.
Nascent nebula
Planetary nebula
NASAutilizehubble space telescope A planetary nebula was found deep in the universe, USANASAThe space agency named it Hen 3-1475.There are many beautiful and fascinating objects in the universe. Recently, NASA revealed that they found a planetary nebula in the deep universe, which is gradually forming. This object is one of the most beautiful and fascinating objects found in the universe.
The reason why it is called a planetary nebula is that its shape resemblesplanetIt is very beautiful. When NASA first found this nebula, it was rapidly forming and expanding. This nebula has its own energy and light source. It uses the radiation of its own core to generate heat and light. But like the sun, it is also rapidly consuming its own "life" and removing the gas "shell",The objects inside the nebula will have enough energy to produce light.
Each planetary nebula has a complex structure,Hen 3-1475 is no exception, because scientists have completely observed the formation process of the entire planetary nebula, which will help people understand the past and present life of this planetary nebula.Can be accessed viahubble space telescope It was found that the shell of the star did not completely peel off, because there was not enough heat to achieve this result, so the shell of ionized gas was formed, becoming a shell that was not bright enough.If it must be foreignReflection of lightAt this time, the shell of the star is intact, so the nebula will shine, and eventually form a planetary nebula.
Hen 3-1475 atSagittariusAbout 18000Light yearAnd away from the earth.The brightness of the star in the center of this planetary nebula is 12000 times more than that of the sun in the solar system.This planetary nebula has a very obvious feature, that is, there are a large number of stars in its center and two S-shaped poles, leading to many dust rings around the central star.The speed of these dust rings is as high as hundreds of kilometers per second.
bipolar jets It will take a long time for the formation of the new moon, and this problem has been puzzling American astronomers.A spherefixed starHow did these complex structures come into being?Recent research shows that the formation of the bipolar jet is mainly caused by the central star, which causes the ejected gas to move in the opposite direction and change once every thousand years.Finally, the form of bipolar jet is formed.
other
The following is a list of some planetary nebulae that have been found:
Wang Yi, a special correspondent of the Global Times, reported that according to the US Space Network on the 11th, the second generation wide-angle and planetary camera of NASA's Hubble Space Telescope recently took a group of pictures, one of which clearly shows a planetary nebula resembling a "cosmic giant eye".This bright nebula is actually composed of bright gas and dust. It is called Kohut 4-55 planetary nebula (K4-55 for short).This picture shows the Hubble Space TelescopeUS Eastern TimeShot and released on the 10th.
physical characteristics
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Typical size
Planetary nebulae are typically about one light year in size and contain extremely thin gas, with a density of about 1000 particles per cubic centimeterparticle, justEarth's atmosphereOne tenth of a trillion (1024) of density.The density of young planetary nebulae will be relatively high, reaching 100000 particles per cubic centimeter.As clouds grow, their expansion will lead to a decrease in density.
form
Generally speaking, planetary nebulae are symmetrical and almost spherical, but there are still various shapes and very complex forms.About 10% of planetary nebulae have strongDipolarity, and a few are asymmetric, and even one is rectangular.The origin of various shapes has not yet been fully understood, but it may be the gravitational interaction caused by the central star being a binary star.Another possibility is that the planets have disturbedStar formationNebular matterJet。In January 2005, astronomers announced that magnetic fields had been detected in the stars at the center of two planetary nebulae, and assumed thatMagnetic energyPartially or completely explain their special shape.
temperature
Stellarradiant energyHeat the cloud to 10000K.Different from intuition, the farther away from the center, the higher the cloud temperature is, because the higher the energy, the less easily the photons are absorbed.Therefore, the photons with lower energy will be absorbed first, and the photons with higher energy can almost reach the periphery. The higher the energy, the higher the gas temperature.
structure
Nebula can also be described by material boundary or radiation boundary. According to this intuitionistic term, the former does not have enough material in the cloud to absorbStellar radiationThe visible part is full of ions;The latter is that there are not enough ultraviolet photons from the central star, so that the front edge surrounding the star diffusion is free, and the gas outside becomes neutral atoms.
Because the gas in the planetary nebula is free plasma, the role of the magnetic field will have a significant impact on the plasma andFibre structureBecomes unstable.
Quantity distribution
In ourGalaxyAbout 1500 planetary nebulae are known to exist among 200 billion stars.Because the life span is very short compared with the life span of stars, it is very rare.The planetary nebulae found are all distributed on the plane of the Milky Way, and a large number of them are concentrated near the center of the Milky Way.The number of star clusters found is very small, with only one or two known examples.
In modern astronomy,CCD has almost completely replaced photographic negatives. In the last sky survey using Kodak TP 2415 negatives, with high-quality color filters, it is the most obvious radiation line in almost all planetary nebulae, that is, screening with the brightest emission spectrum line of hydrogen, and found many planetary nebulae.
Research questions
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The distance of planetary nebulae is usually difficult to measure[1]。The distance of the planetary nebula near by can be measured by measuring its expansion speed.High resolution observations separated by several years can show the expansion of the nebula in the vertical line of sight direction, and the Doppler shift of the observed spectrum can tell the speed in the line of sight direction.Comparing the angle of expansion and the speed of expansion can reveal the distance to the nebula.
The problem is how to produce a wide range of planetary nebulae with different shapes, which is still a controversial topic.In theory, the interaction between substances leaving the star at different speeds can produce various shapes observed.However, some astronomers have assumed that the more complex and extreme planetary nebulae should be caused by close conjoined stars.Several show strong magnetic fields, and their interaction with ionized gas can explain the shape of some planetary nebulae.
There are two main methods to measure the metal abundance in nebulae, both of which rely on the spectral lines excited by recombination lines and collisions.However, the results of these two methods sometimes differ greatly.This may be explained by the existence of some temperature disturbances in planetary nebulae;But some differences are too big to be explained by temperature.Some hypothesizes exist very small hydrogen condensation nodes to explain the observed phenomena.However, such nodes have not yet been observed.
In February 2024, astronomers cracked the planetary nebulaThe secret of "missing" sulfur element:The "missing" anomaly of sulfur element is related to the mass of the host star of planetary nebula.Relevant research results were published online in Astrophysical Journal Letters.[2]
