Dark nebula, yesInterstellar cloudOne kind of interstellar cloud is the cloud like objects formed by the non luminous diffuse matter in the Milky Way, whose density is enough to cover the objects from the backgroundEmission nebulaorReflection nebulaLight (like the Horsehead Nebula), or obscuring the backgroundfixed star(Like a coal bag).
AstronomicalextinctionUsually from largeMolecular cloudThe part with the lowest temperature and the highest densityInterstellar dustgrain.Large and complex dark nebula aggregates are often associated with huge molecular clouds. Small and lonely dark nebulas are calledBaoke Ball。
The formation of these dark nebulae is usually irregular: they have no well-defined shape and boundary, and sometimes form complex meandering shapes.The huge dark nebula can be seen with the naked eyethe galaxyA dark patch appears in.
Inside the dark nebula are places where important events occur, such asfixed starForm and shoot.There is no doubt that,Horsehead NebulaIs the most famous dark nebula.
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Dark nebula
Between starsspace。Interstellar spaceIt is not a vacuum without everything, but full of various substances.These substances includeInterstellar gas, dustParticle Flow、cosmic rayandInterstellar magnetic fieldEtc., collectively referred to asInterstellar matter。The distribution of these interstellar matter is uneven.In some places, gas and dust are relatively dense, forming various cloud like objects.These nebulous objects are called nebulae.
The term "nebula" has a history of only 200 years.At first, the observed diffuse cloud like objects were collectively called nebulae.laterAstronomical telescopeThe improvement of resolution divides these nebulae intocluster, galaxies and nebulae.GalaxyThe dense gas and dust in the nebula is called nebula;Galacticsphere systems , calledGalaxy。
The nebular matter in the Milky Way can be divided intoDiffuse nebula、Planetary nebulaandSupernova remnant material cloud；In terms of luminous properties, it can be divided intoEmission nebula, reflection nebula.Dark nebulae, also known as dark matter, are invisible in the universeMatter nebula。
Phenomenon characteristics
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Dark nebulae are cloud like objects formed by non luminous diffuse matter in the Milky Way.andBright nebulaSimilarly, their sizes and shapes are varied.The smaller one isSolar massFrom a few percent to a few thousandths, it appears on the background of some bright nebulaeSpheroid；The big ones have the mass of dozens to hundreds of suns, and some are even larger.Their internalMaterial densityThere is also a wide gap.
There are many such dark areas in the Milky Way, the most obvious being the dark area of the constellation Cygnus. The Milky Way is divided into two branches extending southward.Another example is Orion's famousHorsehead NebulaAnd Ophiuchus S-shaped dark nebula are also opaque dark nebulae.However, when the cloud layer is thin, some stars whose brightness has been greatly reduced can still be seen, so the stars seen in this sky area are much thinner than those in the sky area without dark nebulae.According to the polarization measurement of starlight passing through the dark nebulaDust particleIts diameter is about 10-5 cm.This is consistent with the situation in bright nebulae, indicating that there is no essential difference between dark nebulae and bright nebulae, but dark nebulae contain more dust.
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herschel Father and son first noticed some black spots and dark stripes in the bright Milky Way in 1784.At first they thought it was something in the galaxy that didn'tfixed starA hole or seam.Later photographic studies showed that this phenomenon was caused by some non luminous diffuse matter in front of stars.
There are many such dark areas in the Milky Way, the most obvious being the dark area of the constellation Cygnus. The Milky Way is divided into two branches extending southward.Another exampleOrionfamousHorsehead NebulaandOphiuchus S-shaped Dark NebulaIs also an opaque dark nebula.In the weaker part of the nebula, there are still some stars whose brightness has been weakened. It seems that the star density in these areas is very sparse.There is no essential difference between dark nebulae and bright nebulae, but dark nebulae contain relatively large dust, and many bright nebulae are actually part of a larger dark nebula.The globule is a small and dense globular dark nebula, also calledBarnard, which can only be observed with large telescopes.
In recent years, radio observations of dark nebulae have found that many bright nebulae are often contained in a larger dark nebula.Since 1946Bright diffuse nebulaSome spherical dark clouds - globules are found in the background.It is generally believed that the spheroids with high density of these substances are probably being formedProtostar, the predecessor of the star.
Research
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The dark nebula itself does not emit light, so the research using optical methods is very limited.Radio astronomy provides a powerful tool for the study of dark nebulae.This is mainly because the dark nebula itself has various radio radiation.Especially when they launchNeutral hydrogen 21 cm spectral lineSo that we can further study the size, structure and composition of a large number of low temperature dark nebulae, thus providing important data for the study of the structure and movement of the Milky Way Galaxy.The temperature in a typical dark nebula is about 5-10K.In addition, many organic molecules are found in the sky where dark nebulae are located, so some dark nebulae are also called interstellar molecular clouds.
Through millimeter wave observation, it is found thatcarbon monoxideThere are some "hot spots" with high temperature (15~50K) in the dark nebulainfrared radiation。Through infrared observation, it is also found that some infrared sources surrounded by dark nebulae are concentrated in the 2~20 μ m wave band, some of which are brighterInfrared sourceAnd the dark nebulaMicrowave sourceofThe observation also found some young celestial bodies such asHerbig emission star(about 105~106 years old), Hebig Aro objects, etc. are directly related to dark nebulae.The diameter of these dark nebulae is about 10 seconds apart, the average atomic density is about 5 × 103 per cubic centimeter, and the average temperature is about 10K.In the process of its evolution, the internal energy is reduced due to the loss of some radiation (such as millimeter wave), resulting in the collapse of the internal pressure less than its own gravity.
In the process of collapse, some lumps form a series of dense points under the action of gravity, which may be the original embryos forming stars or star clusters.According to the comparison between the star birth rate and the total mass of the dark nebula in the Milky Way, only a small part of the material (one thousandth to one hundredth) forms stars.
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Ophiuchus S-shaped Dark Nebula
Dark nebula
Where did the stars go?Previously it was thought to be a hole in the sky, but now astronomers know that it is darkMolecular cloud。In this molecular cloud, high-density dust and molecular gas almost completely absorb thevisible light。 The interior of the molecular cloud is probably the coldest and loneliest place in the universe.This dark nebula, B68 in Ophiuchus, is one of the most famous dark nebulae.No stars can be seen near the center of the nebula, indicating that B68 is not far from us. It is about 500 light-years away from us and about one light-year in size.It is not clear how nebulae like B68 formed, but we know that they are the possible birthplace of new stars.
Horsehead Nebula
Dark nebula
OrionThe Horsehead Nebula is one of the best recognized nebulae in the night sky. It is part of a large dark molecular cloud.This celestial body with unusual shape was found on the bottom plate of a picture at the end of the 18th century.The red glow of the nebula is mainly hydrogen irradiated by stars behind the nebula.The dark horse's head is mainly from the dense dust that obscures the light behind it, but the shadow on the left side of the bottom of the horse's neck is caused by the horse's neck.
The powerful magnetic field running through the nebula is forcing a large amount of gas to fly away from the nebula.The bright spots at the bottom of the Horsehead Nebula are young stars in the newborn stage.It will take about 1500 years for light to reach us from the Horsehead Nebula.
North America nebula
The name of this emission nebula isNorth America nebula, because it andNorth AmericaThe appearance of the mainland is very similar.The darker nebula to the right of the North American nebula is numbered NGC 7000, orpelican nebula 。The two emission nebulae are about 1500 light-years away from us, separated by a dark nebula, and they extend nearly 50 light-years in total.These two nebulae, located in the constellation Cygnus, are used in areas far away from urban light damageBinocularsWhen you find the bright star Tianjinsi, and then look to its northeast, it is not difficult to find these two beautiful nebulae.
Emission nebulae emit light, usually because the starlight excites and the hydrogen in the free nebula. But so far, astronomers have not yet known which stars excite these two nebulae to emit red glow.
