Galactic structure, an astronomical term.The overall structure is: the main parts of the Milky Way Galaxy matter form a thin disk, called“Silver plate”The approximately spherical part of the bulge in the center of the silver disk is called the nuclear sphere.In the nuclear sphere areafixed starIt is highly dense, with a small dense area in the center, calledSilver nucleus。Outside the silver disk is a system with a larger range and nearly spherical distribution, in which the material density is much lower than that in the silver disk, calledSilver halo。There are also silver halosSilver crownIts material distribution is also roughly spherical.
Galaxy and Milky Way Galaxy are two concepts at different levels:the galaxyIt is an irregular silver white light band that can be seen in the clear night sky, while the Milky Way is a huge celestial system composed of many stars.Ancient philosophers' understanding of the Milky Way, in essence, was only based on some subjective imagination, and lacked scientific basis.For example, Aristotle believes that the Milky Way is just an atmospheric phenomenon, which is the water vapor emitted by the earth, but does not recognize it as a heavenly thing.The first person who correctly understood the essence of the Milky Way was Democritus, the ancient Greek philosopher. He believed that the Milky Way was composed of countless stars, but because these stars were too dark and dense to be distinguished, they appeared as a fuzzy light band.
In 1608, the Dutchman Liposser invented the telescope by chance.The next year, Galileo made telescopes and used them for astronomical observation, creating a new era of astronomical observation.That winter, Galileo observed the Milky Way with a telescope. He found that the Milky Way was not a thin cloud, but a dense cluster of stars, which could not be distinguished by the naked eye. It was shown as a dim light band in the sky, which confirmed Democritus' opinion from the observation.The observational study of the structure of the Milky Way Galaxy was initiated by the British astronomer William Herschel.
In 1785, Herschel obtained from star counting that the main part of the distribution of stars in the Milky Way is a flat disk structure.He counted 117600 stars in 1083 observations with telescope.On the basis of star counting work, Herschel added some assumptions, and thus obtained the first model of the Milky Way in the history of astronomy (see figure).In Herschel's model, however, the sun is at the center of the Milky Way.In 1918, American astronomer Shapley usedGlobular clusterIt is correctly inferred that the sun is not located at the center of the Milky Way, but near the edge of the Milky Way.At that time, more than 130 years had passed since Herschel proposed the first model of the Milky Way.
According to the observation and research of modern astronomy, the Milky Way is aSpiral galaxyIts overall structure can be divided into four parts, namely, silver disk, nuclear sphere, silver halo and dark halo.Except for the dark halo, the total mass of the Milky Way is about 1.4 × 10elevenThe mass of the sun is about 90% in the form of stars and about 10% in the form of interstellar matter composed of gas and dust.The Milky Way is estimated to be 10 billion years old or older.The silver disk is the main part of the distribution of stars in the Milky Way. It is an axial and plane symmetric flat disk with a diameter of about 82000 light-years.The distance from the sun to the center of the Milky Way Galaxy (galactic distance) is about 26000 light years, not far from the symmetry plane (galactic plane) of the silver disk (20-30 light years).The thickness of the silver disk is uneven. The thickness of the silver disk near the sun is about 3300 light-years.In addition to dark halos, 85%~90% of the mass of the Milky Way Galaxy is concentrated in the silver disk.
NucleosphereIt is a star concentrated area located in the center of the Milky Way Galaxy, roughly in the shape of a flat rotating ellipsoid, with a long axis of 13000-16000 light-years and a thickness of 13000 light-years.The mass of the nucleosphere is estimated to account for about 5% of the mass of the Milky Way except for the dark halo.The nuclear sphere is mainly composed of some old objects. The closer to the center, the higher the concentration of nuclear sphere stars.In the direction of the center of the Milky Way Galaxy, it is observed that there is a complex structure of strong radio source Sagittarius A, which contains at least five sub sources. In a sub source with a diameter of 33 light-years, there is a bright core with a diameter of nearly 5 light-years, which is the silver core.The mass of the silver core is about several million solar masses, and now it is generally believed that there is aSupermassive black holeHowever, it is not in the period of intense activity.Surrounding the silver disk is an area composed of sparsely distributed stars and interstellar matter, called the silver halo.The silver halo is generally spherical, with a diameter of about 100000 light-years, and its range is much larger than that of the silver disk. However, because the material distribution is very sparse, its mass is only about 10% of that of the silver disk.
Silver haloIt is mainly composed of two types of celestial bodies, namely, old stars and globular clusters, in addition to a small amount of gas.Beyond the silver halo, there is a larger area of material distribution, which is the dark halo, also known as the silver crown.The composition of the dark halo is dark matter that cannot be observed at present. Its diameter may be 10 times the diameter of the silver halo, and its mass may be 10 times the total mass of other parts of the Milky Way.The dark halo is mainly calculated according to the kinematic condition of the galactic disk objects: if the material distribution of the Milky Way Galaxy is concentrated in the center, then the stars farther away from the center will rotate around the silver center more slowly, but the measured results are not like this.Near the sun and farther away, the speed of stars' movement remains roughly the same, or even slightly increases, which leads to the conclusion that there must be a lot of dark matter in the periphery of the Milky Way, that is, there is a dark halo.
In 1944, the German astronomer Bud put forward the concept of star family. According to the physical properties, spatial distribution and motion characteristics of stars, he divided the stars in the Milky Way into two categories: star family I and star family II.Star family I objects are distributed in an oblate circle with the silver center as the center. They are younger and move faster around the silver center, butVelocity dispersionSmall;Star family II objects are distributed in a slightly flat sphere centered on the silver center. They are relatively old and move slowly around the silver center, but their speed dispersion is large.Therefore, the silver disk is mainly composed of star group I objects, while the nuclear sphere and silver halo are mainly composed of star group II objects.The motion state of stars in the Milky Way depends on thegravitational fieldThe gravitational field depends on the distribution of matter in the Milky Way.The motion of the Milky Way objects is neither like that of the planets in the solar systemKepler movementIt is not the rigid body rotation, but the so-called poor rotation, that is, the silver centers have different rotational angular velocities from different stars.
In 1925, Swedish astronomer Lindbergh put forward the correct concept of the rotation of the Milky Way.In 1927, Holt, a Dutch astronomer, derived a formula for calculating the influence of the differential rotation of the Milky Way Galaxy on the observation movement of stars.Olt Lindbergh theory is one of the most successful theories about the rotation of the Milky Way, and has been confirmed by a large number of observations.In the galactic disk, star group I objects rotate around the galactic center in the same direction in a near circular orbit. The intersection angle between the orbital plane and the galactic plane is not large. The curve of the linear velocity of motion varying with the distance between the galactic centers is called the rotation curve of the galaxy.The rotation speed of the stars near the sun is about 220km/s, and it takes more than 200 million years to circle the silver center, so the sun has revolved more than 20 times in the Milky Way since its formation[1]。
The nuclear sphere isGalactic centerThe area with dense stars is nearly spherical, with a diameter of about 4000 seconds, and its structure is complex.The nuclear sphere is mainly composed ofStar Family IICelestial composition, there are also a few star family I objects.The central part of the nuclear sphere isSilver nucleus。It emits strong radio, infrared, X-ray and gamma rays.Its nature is unclear, and may includeblack hole。The silver halo is mainly composed of halo family objects, such asSub dwarf、Metal poor star, globular clusters, etc. There is no young O and B stars, but a small amount of gas.The material density in the silver halo is much lower than that in the silver disk.
Silver halo
The system with a larger scope outside the silver plate and a near spherical distribution is calledSilver haloThe density of the material is much lower than that of the silver plate.The diameter of the silver halo is about 98000 light-years. The density of stars here is very low, and there are some globular clusters composed of old stars.
Silver crown
The silver halo is roughly spherical.The silver disk has a diameter difference of about 25 thousand seconds and a thickness difference of 1~2 seconds. It gradually thins from the center to the edge. The sun is located in the silver diskSilver coreAbout 8.5 kilosecond differenceGalactic surfaceAt a distance of 8 seconds to the north.There are spiral arms in the silver disk, where gas, dust and young stars are concentrated.The nucleosphere is a region of dense stars in the center of the Milky Way Galaxy, which is approximately spherical, with a diameter of about 4000 seconds, and a complex structure.The nuclear sphere is mainly composed of star family II objects, and there are a few star family I objects.The central part of the nuclear sphere isSilver nucleus。It emits strong radio, infrared, X-ray and gamma rays.Its nature is not yet clear and may contain a black hole.
The silver halo is mainly composed of halo family objects, such as sub dwarf stars, metal poor stars, globular clusters, etc. There are no young O and B stars, but a small amount of gas.The material density in the silver halo is much lower than that in the silver disk.The diameter of the long axis of the silver halo is about 30 thousand seconds, and the age is about 10 billion years[3]The quality is not very clear.The silver corona outside the star distribution area of the silver halo is a roughly spherical radio radiation area, and its properties are poorly understood.
Research history
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A preliminary description of the structure of the Milky Way
On a sunny summer night, if there is no moon, you will see a silver white light band stretching from the northeast to the other end of the sky bend, which is like a gauze light band that is the Milky Way.In ancient China, people regarded the Milky Way as a river in the sky.Our ancestors gave it many beautiful names: Milky Way, Changhe, Hehan, Minghe, Qiuhe, Yinhan, Tianhan, Xinghan, etc.In the West, the Milky Way is called "Milk Way", which means "Milky Way". According to ancient Greek mythology, this milky white Milky Way runs across the sky.
In 1601, when Galileo, an Italian scientist, pointed his telescope at the Milky Way for the first time, he found that this vast white band of light was actually composed of countless stars.With the improvement of the penetration ability of astronomical telescopes, more and more stars are observed in all directions.But for more than a hundred years since then, human beings have been studying the positions and changes of stars in the Milky Way.
It was not until the middle of the 18th century that western scientific and philosophical circles began to discuss the composition of the stellar universe.In 1705, British astronomer Ding Wright (1711-1786) proposed that the Milky Way around the bend of the sky would not extend endlessly in all directions, but only distributed within a limited range. He believed that the star world was actually a flat group like a lens, and the sun was a member of the stars.In 1755, the German philosopher Kant also said that the universe is like a vast ocean, and the stars of the Milky Way form an isolated group of stars, like an island - "Cosmic Island";There are many such islands in the universe.In 1761, German scientist Lambert (1728-1777) also expounded the assertion of star systems.Their common view is that the stellar universe is a huge system with a limited range of stars.
William Herschel (1738-1822) was the first person to use observational facts to derive the structural model of the Milky Way Galaxy.Herschel studied astronomy through amateur self-study. After decades of persistent efforts, he finally became a famous astronomer, known as the "father of stellar astronomy".In addition, three members of their family had made outstanding contributions to astronomy, and Herschel's surname became the object of admiration in the international astronomical community.Herschel was born in a music family. His six brothers and sisters, who had received music education since childhood, showed their musical talents.While studying music, his father also taught William Herschel some astronomy knowledge.In 1753, 16-year-old William joined the military band as a violin and oboe player in order to reduce the burden on his family.In 1757, the second year after the French army invaded English and Chinese Norway, Herschel, as a musician, could not bear the pain of the war. He crossed the sea alone and fled to London, England, to make a living as a musician.But he made a living by music during the day, and still insisted on self-study astronomy and astronomical observation at night.
In 1772, his sister Caroline Herschel (1750-1848) came to England. Since then, she has been assisting William in astronomical observation, grinding telescopes for him, and managing his life. She never left, even never married.In 1781, Herschel was 43 years old. During his observation, he accidentally discovered Uranus. From then on, he became famous and won the Copley Medal of the Royal Society of England, and was canonized by King George III of England. Herschel became a member of the Royal Society of England, and was given a good salary and residence. From then on, he began to work full-time in astronomy.Herschel's telescopes for observation are all made by himself, so his family has become a workshop. In order to make a good telescope, he works day and night, regardless of the cold or the heat. He is often hungry and still keeps working in order to make the telescope. Caroline loves her brother, so she often feeds him one mouthful. When she is tired, she lies in the workshop for a rest,Caroline read him stories to relieve fatigue.In this way, Herschel honed more than 400 telescope lenses in his lifeReflecting telescopeThey were the best at that time.
Asimov, a famous American popular science writer, commented on Herschel like this: "In a whole generation, Herschel and his greatReflecting telescopeIt rules astronomy. "In order to explore the structure of the stellar system, the Herschel brothers and sisters worked together to conduct a systematic sky survey with their self-made telescopes with unprecedented penetration skills. They divided the sky area into 683 sampling areas, and observed the stars in each sample area. The maximum star size of the star count reached 12 dark stars.Year after year, day after day, they made a hard journey, never missing a sunny night, or even working all night. After more than ten years of work, they made 1083 observations and counted 117600 stars in total.The huge amount of data recorded by them would require nearly 6000 pages if they were copied with 400 words per page today, which shows the huge workload.In 1785, they obtained the structure map of the Milky Way (see the figure) through the analysis of the observation data: the shape of the Milky Way is flat, uneven, and the sun is in its center.This is the first time that human beings have outlined the image of the Milky Way with observation facts, and it is the first time to prove that the milky white Milky Way seen by the naked eye and stars scattered throughout the sky constitute a huge celestial system - the Milky Way.
Since the distance of any star was unknown at that time, Herschel estimated the approximate proportion of the diameter and thickness of the Milky Way Galaxy.This model is called "Herschel 1785 Galaxy Model".After 35 years of observation and exploration, Herschel revised the first model of the Milky Way. He confirmed that the Milky Way is a flat, limited space star system in which the sun resides. However, due to the penetration ability of telescopes, the diameter of the Milky Way is much larger than before, but its size cannot be measured. Later generations called it "Herschel's model of the Milky Way in 1817"。Over the next century, Herschel's research methods and achievements were regarded as a model by the astronomical community, and a new discipline - stellar astronomy was created. Herschel is also known as the "father of stellar astronomy".
Move the sun out of the center of the Milky Way
In 1906, the Dutch astronomer Karptan (1851-1922) proposed to use modern astronomical methods to carry out astronomical counting again and divide the sky into 206 '' selection regions ".By this time, the distance of some stars was known, and the astronomical photography technology that was not available in Herschel's time was used, which greatly improved the technical results.In 1922, Kaptan put forward the model of the Milky Way, which is similar to Herschel's model in outline, but 4 times larger in diameter, about 4 light years. The sun is in the center of the Milky Way, and the farther away from the sun, the more sparse the stars are.Kaptan model is actually a quantitative development of Herschel model.Shapley, an American astronomer, made a breakthrough in understanding the structure of the Milky Way.Shapley's choice of astronomy career is very dramatic.He was born in a farmer's family. His father was a farmer and a primary school teacher.He studied in a rural primary school for five years, and then learned a business course. At the age of 16, he became a reporter. He completed the middle school course by self-study in his spare time.
Before he was admitted to the University of Missouri in 1907, he was keen on journalism and was responsible for reporting disputes and shootings.However, he was dissatisfied with the way of life in the press, especially the exaggeration of the facts, which made him tired, so he considered developing in other fields.When looking at the university subject catalogue, because the catalogue is arranged in alphabetical order, because he can't pronounce the word archaeology, he put it aside and chose the next astronomy, so he simply entered the field of astronomy and became a famous astronomer.As his good fortune came one after another, he was called the "nouveau riche" in astronomy by some older astronomers.In his life, he was lucky to get the guidance and help of several first-class astronomers. First, Sears, the director of the University of Missouri Observatory, asked Shapley to act as an assistant and often taught and trained him. After Shapley obtained his master's degree in 1911, Sears recommended him to work at the Princeton University Observatory.Here, he met Russell, a famous astrophysicist, who completed his famous dissertation on eclipsing binaries under his guidance.
In the spring of 1914, Shapley accepted the invitation of Haier (1868-1938) and Sears to go to Southern CaliforniaMount Wilson Observatory。This observatory was founded by Haier in the early 20th century. Soon, it became one of the most important astronomical institutions in the world and the leading edge of astronomical research.There, the world's largest 2.54-meter Hooke reflection telescope will be completed soon, and it also has a powerful 60 inch (1.52-meter) reflection telescope at that time.Before going to Wilson Observatory, Shapley paid a special visit to his mentor Solon Bailey, who had a long historyHarvard College Observatory He specializes in variable stars in globular clusters and has made a lot of variable star observations.Globular star cluster is a special kind of star cluster, which is formed by thousands or even hundreds of thousands of stars. Its star distribution density is very large, 50 times that of stars near the sun, while the density of the cluster center is more than 1000 times.There are Cepheid variables in globular clusters. Bailey has found some. Bailey suggested Shapley make full use of the excellent equipment of Wilson Observatory to find more Cepheid variables in globular clusters.
Shapley seriously accepted Bailey's suggestion. As soon as he arrived at Wilson Observatory with his new wife, he devoted himself to the observation and research of Cepheid variables in multiple globular clusters.For this reason, Shapley once said: 'Our goal is to do things we have never done before.'"To become a leader in scientific research and realize Haier's dream of making Mount Wilson Observatory a famous research institution".He spent four years working day and night, taking hundreds of star cluster photos, inspecting them one by one, measuring the brightness of Cepheid variable stars, and determining their brightness change cycle and average brightness. Then he used“Cepheid distance ”, determine its luminosity and calculate its distance.Then, he studied the distribution of globular clusters on the celestial sphere. He found that the distribution of globular clusters relative to the sun was uneven, and more than 90% of globular clusters were located on the half celestial sphere with Sagittarius as the center, and their distance was large;10% of globular clusters are located on the other half of the globe, and they are closer together.Scientists and philosophers have always had the idea that nature is symmetrical, so Shapley believes that globular clusters are evenly distributed on the celestial sphere, but their center is not the sun, but near Sagittarius, which is the center of the Milky Way.
In 1919, Shapley put forward his model of the Milky Way. The shape of the Milky Way is like a lens. Its diameter is about 300000 light years. The distance from the sun to the center of the Milky Way is about 60000 light years.People have thought that the sun is the center of the universe for hundreds of years. Shapley, with his bold courage, amazing imagination and creativity, has described a new picture of the Milky Way, invited the sun out of the center of the Milky Way, and opened a new chapter in the history of astronomy.Bard, a German astronomer, once praised that "I always admire Shapley's approach. He solved this problem in a short time, and finally got the image of the Milky Way, which broke down all the old ideas about the scale of the Milky Way." Shapley placed the sun on the edge of the Milky Way, just like Copernicus moved the earth out of the center of the solar system,It is a crucial step to establish a correct image of the Milky Way.
In 1930, Swiss astronomer Trumpler (1886-1956) corrected the distance of globular clusters after studying the interstellar extinction effect, and calculated that the diameter of the Milky Way is less than 100000 light-years, while the distance from the sun to the center of the Milky Way is about 30000 light-years.
Looking back at the Milky Way in radio waves
When Einstein and Friedman used the general relativity equation to study the whole universe, astronomers only observed objects in the visible light band directly acceptable to the human eye.Due to the existence of a large amount of interstellar dust near the galactic plane in the Milky Way, the interstellar extinction caused by this hinders the observation of visible light, so that humans can see the distantExtragalactic galaxyAt that time, we were still unable to recognize the true nature of the galaxy in which human beings live - the Milky Way.Just as Su Dongpo, a famous poet in the Song Dynasty, said, "When you look across the mountain, you can see the peaks on its side. The distance is different from the height, and you don't know the true face of Mount Lushan. It is only because you are in this mountain." Since the American astronomer Hubble created the first classification system for extragalactic galaxies in 1925, he divided galaxies into four categories: vortex, rod spiral, ellipse and irregular.But what kind of galaxy does our own Milky Way belong to.In 1927, Holt (1900-1992), a Dutch astronomer, and B.Lindbergh collaborated to discover the rotation of the Milky Way.From then on, Ault began to explore the structure of the Milky Way for a long time.
In 1938, with the method of optical observation, Oldt found that there was a vortex structure in the Milky Way, but the intense interstellar extinction near the galactic plane hindered the in-depth research.In 1931, Bell Telephone Laboratory in New Jersey, the United States, built a 30, 5 meter long and 3.66 meter high rotating antenna array for the radio telephone across the ocean.When studying noise interference, K. Gdansky (1905-1950), a radio engineer working here, found a radio noise of unknown origin at the wavelength band of 1.46m.At first, he thought that the radio noise was from the sun, but with continuous tracking and observation, he found that the noise source was not completely synchronized with the sun's operation, but every day it was 4 minutes earlier, and the period of the star time was just 4 minutes shorter than the sun.So Jansky realized that the noise source came from a fixed position in the outer space of the solar system.And this position just points to the direction of Sagittarius. After a year of monitoring, he found that when his radio array points to the direction of the center of the Milky Way predicted by Shapley Way, this radio noise is the strongest.So in 1935, Jansky published a paper that clearly concluded that he measured the radio radiation from the center of the Milky Way Galaxy, which was the first time that humans detected cosmic radio, leading toRadio astronomyThe birth of.Since then, it has broken through the history that human beings can only detect the universe in the visible light band, opened a new window for human beings to understand the universe, and created a new era of using radio waves to study celestial bodies.
What is light? Modern physics tells us that light is electromagnetic radiation, and the visible light band that human eyes can distinguish only accounts for a very narrow part of the electromagnetic spectrum.Astronomers understand the universe by detecting the light radiation generated by cosmic objects. The light radiation of cosmic objects is not limited to the visible light band, but is widely distributed in the radio band, infrared, ultraviolet, x-ray, ray and other electromagnetic bands.Therefore, with the development of observation technology, human began to make full band astronomical observations.When the radio window was opened, Oldt, who was the director of Leiden Observatory in the Netherlands, immediately realized that radio radiation might provide a new image for the study of the structure of the Milky Way Galaxy, because radio waves are different from visible light and can pass through the obstacles of interstellar gas and dust.So Olt asked his student Van der Hoost (1918-1) to theoretically find the radio spectrum lines that can be observed.
Van der Hoost made serious research. In 1944, he predicted that the 21 cm spectral line of neutral hydrogen atoms rich in interstellar space could be detected.In 1945,the Second World WarJust after the end, Ault proposed to the Netherlands Academy of Sciences to build a 25m radio telescope to detect this spectral line, but it failed to achieve.Finally, in May 1951, a 7.5m radar antenna captured from the German army was converted into aradio telescopeWith it, the signal of 21 cm spectral line from the Milky Way was observed.It was about 3 months later than the same discovery of Eun and Paissel in the United States.In June of the same year, Australian astronomers Christensen and Heitman also measured this spectral line.These three measurement results were published at the same time, which has aroused great attention in the world scientific community to the study of the structure of the Milky Way Galaxy.
In 1956, the 25m radio telescope proposed by Ault began to be put into operation at Tevingoro Observatory, continued to carry out systematic survey of the 21cm spectral line of the Milky Way, and used the density distribution of neutral hydrogen to study the structure of the Milky Way.In 1958, the radio astronomy team led by Ault and the radio astronomy team of the Commonwealth Science and Industry Organization of Australia synthesized the observation results of the northern and southern hemispheres and drew the first picture of the Milky Way, 21 cmNeutral hydrogenWe can clearly see the vortex structure of the Milky Way.
This picture was jointly published in the paper entitled "The Milky Way as a Vortex Nebula" by Oort, Kerr and Westout.Through the observation of radio telescopes and avoiding the obstruction of interstellar extinction materials, mankind finally explored the true image of the Milky Way galaxy by using the distribution of neutral hydrogen at 21cm.As an important branch of radio astronomy, radio spectroscopy was also born. This method also played a huge role in the detection of extragalactic galaxies[2]。
