A bright nebula consisting mainly of gas in a galactic nebula, includingPlanetary nebulaandEmission nebulaEtc.In the 1960s, infrared observations of planetary nebulae revealed that there were also a considerable amount of dust in these nebulae, which were always considered to be composed entirely of gas.Nebula[1]）It is an interstellar cloud of dust, hydrogen, helium, and other ionized gases.
Further research shows that these dusty materials, like gas, existed in the early stage of nebula formation.In the gas nebula, there are dust particles that cannot be ignored, but they are less than other nebulae.Most of the diffuse gas nebulae in the Milky Way are concentrated near the galactic plane, with an average silver latitude of about 2 °, belonging to the flat subsystem (see the Milky Way subsystem).Their mass ranges from a fraction to several thousand times the mass of the sun, but most of them are about ten times the mass of the sun.All gas nebulae have emission spectra.The temperature of diffuse gas nebulae shining on stars is generally lower thanPlanetary nebulaThe temperature of the central star, so the excitability of the diffuse gas nebula is also lower than that of the planetary nebula.The strongest spectral lines in the spectrum are the Palmer (H α) line of hydrogen and the primary ionized oxygen (O Ⅱ)Forbidden line。Due to the strong penetrability of red light, the H α line of hydrogen atom in the spectrum of diffuse gas nebula is very strong.Therefore, the photographic observation of faint nebulae is generally carried out by only penetrating strong radiation (such as H α)Narrowband filterAnd red sensitive negative for a long time exposure.Many weak ionized hydrogen regions are found by this method.The brightest gas nebula isOrion nebula (M42); the largest gas nebula is the Gum nebula in the southern sky, with an angular diameter of 30 °; the most beautiful gas nebula is the reticular nebula in the constellation Cygnus (NGC6960).In manyExtragalactic galaxyIn, especially inSpiral galaxyA large number of gas nebulae have also been observed in the spiral arms of.
formation
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Many nebulae or stars are formed in the gravitational collapse of interstellar medium gas or ISM.When matter collapses due to its own weight, massive stars may form in the center, and their ultraviolet radiation will ionize the surrounding gas, making them emit the wavelength of visible light.The Rosette NebulaAnd the Pelican Nebula are examples of such nebulae.The size of these nebulae, the so-called ionized hydrogen region, will vary according to the size of the original molecular cloud.They are located in the star birth area, and the stars formed are often young, loose groups known.
Some nebulae are formed as a result of supernova explosions of massive, short-lived stars.The matter ejected from the supernova explosion is ionized with high energy, and it will also produce dense matter.In this, TaurusCrab NebulaIs the most famous example.This supernova event occurred in 1054 AD, so it is marked SN 1054. The dense material created is the neutron star at the center of the Crab Nebula.
Another possible nebula isPlanetary nebulaThis is the last stage of the life of low-quality stars, like the Earth's sun.A star whose upper mass limit is about 8-10 times the mass of the sun will evolve into a red giant star, and the outer atmosphere will lose mass at a slow rate when it expands and contracts.When a star loses enough material, the temperature of its surface will rise, and the ultraviolet rays it emits will ionize the gas that was thrown out earlier and surrounding it.97% of this nebula is hydrogen, 3% is helium, and the rest is trace but detectable.The main function of this stage is to complete balance.
Planetary nebula
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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 a slightly green round surface with clear edges like Uranus and Neptune.In 1777, F. W. Herschel discovered such objects and called them 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 near the galactic plane, and are affected by interstellar extinction. A large number of planetary nebulae are obscured by dark nebulae, which are difficult to observe. Their 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. Its mass is generally between 0.1 and 1 solar mass, its density is between 100 and 10000 atoms [ions] per cubic centimeter, its temperature is between 6000K and 10000K, and the temperature of the central star is more than 30000K.
Planetary nebulae are nebulae formed by the gas ejected from the outer shell when the low mass asymptotically giant star branches into a white dwarf star.[2]These nebulae emit spectra similar to those found in star forming regions.Physically, most hydrogen is ionized, so they are HII regions.However, the density of planetary nebulae is higher and more compact than that of nebulae in star forming regions.They are called planetary nebulae because when astronomers first saw these objects, they thought they were like the disk of planets, although they had no relationship with planets.It is believed that our sun will become one of them 12 billion years after its birth.[3]
average life span
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The average lifetime of planetary nebulae is about 30000.The appearance of such nebulae indicates that the stars have reached their old age.During the existence of the Milky Way [about 1-10 billion years], there were nearly 1 billion to 10 billion stars that 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 thousandCubic second gapIt is estimated that there should be 40000 to 50000 in the whole Milky Way, and only a small part of them are observed now.
Planetary nebulae are round, oblate or annular, and some are similar to major planets, hence the name.These nebulae are associated withDiffuse nebulaThey are completely different in nature. They are the products of stars of the same mass as the sun when they are dying after the nuclear reaction stops in their late evolution.The volume of these nebulae is expanding, and eventually tends to dissipate.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.
quality
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The mass of planetary nebulae ranges from one tenth to one solar mass, and the density of nebulae ranges from 100 to 10000 atoms (ions) per cubic centimeter.The central stars of the planetary nebula are very hot (≥ 30000K), and the nebula absorbs the strong ultraviolet radiation it emits and converts it into visible light through a cascade transition process.Planetary nebulae represent a star in its later years. It is estimated that the average lifetime of planetary nebulae is about 30000 years. The nebular gas gradually diffuses and disappears into interstellar space, leaving only a central white dwarf star.
Planetary nebulae are essentially shells of dust and gas ejected by dying stars.At the end of its evolution, a star whose mass is less than ten times that of the sun will throw its material shell into space due to the depletion of helium fuel in its core.The diameter of planetary nebulae is generally about one light year.
