Who can introduce the Milky Way?

Our star system, where the earth and the sun are located, is an ordinary galaxy, named after its milky bright band projected on the celestial sphere-the Milky Way. The Milky Way is a lens-shaped system with a diameter of about 25,000 parsec and a thickness of about 1 ~ 2000 parsec. Its main body is called a silver plate. The spiral structure composed of high luminosity stars, galaxy clusters and galactic nebulae is superimposed on the silver disk. At the center of the Milky Way is a massive nuclear sphere with a long axis of 4-5 thousand parsec and a thickness of 4 thousand parsec. The Milky Way galaxy is shrouded in a silver halo about 30,000 parsec in diameter. The brightest member of the silver halo is a globular cluster. The mass of the Milky Way is1.4×101solar mass, of which stars account for about 90%, and interstellar matter composed of gas and dust accounts for about 10%. The galaxy as a whole rotates badly. The sun is about 65,438+00 thousand parsec from the galactic center in parsec, about 8 kilometers north of galactic plane, and it orbits the galactic center at a speed of 250 kilometers per second, once every 250 million years. The total density of matter (stars and interstellar matter) near the sun is about 0. 13 solar mass/parsec 3 or 8.8× 10-24 g/cm 3. The Milky Way is an Sb or Sc spiral galaxy with 200 billion stars, and it is the largest giant galaxy except Andromeda. Its apparent absolute magnitude is Mv=-20.5. It evolves on the time scale of 10 10 years.

In the middle of the 8th century, people realized that all the stars in the sky are distant "suns" except planets and satellites in the solar system. Wright, Kant and Lambert first thought that it was likely that all the stars were assembled into a huge system with limited space.

The first person who studied the origin of star system through observation was F.W. Herschel. He used his reflecting telescope to count the stars in several celestial bodies. 1785, according to the statistical study of star counting, he drew a horizontal organization map of the Milky Way with the sun as the center. He observed with telescopes with diameters of 50 cm and 120 cm, and found that when the penetration of the telescope increased, the number of dark stars observed also increased, but the edge of the Milky Way still could not be seen. F.W. Herschel realized that the Milky Way was much bigger than he had originally estimated. After the death of F. W. Herschel, his son J. F. Herschel inherited his father's business and expanded the scope of counting stars to the south for half a day. /kloc-in the middle of the 0/9th century, it began to measure the distance between stars and compile the all-sky star map. 1906, in order to re-study the structure of the star world, Kapteyn put forward the plan of "selecting star regions", which was later called "Kapteyn constituency". 1922, he drew a model similar to that of F.W. Herschel, which is also a flat system with the sun at the center, dense stars and sparse edges. Shapley explored the size and shape of the Milky Way on a completely different basis. He used the period-luminosity relationship of Cepheid variables in Magellanic Cloud discovered by Loewit in1908 ~12 to measure the distance between globular clusters and Cepheid variables. Under the assumption that there is no obvious interstellar extinction, 19 18 establishes a lenticular model of the Milky Way, and the sun is not in the center. By the 1920s, shapley model had been recognized by the astronomical community. Shapley overestimated the Milky Way because he didn't consider the interstellar extinction effect. It was not until 1930 that trumpler confirmed the existence of interstellar matter, and this deviation was corrected.

About 90% of the materials that make up the Milky Way are concentrated in the stars. 1905, hertzsprung discovered that stars can be divided into giant stars and dwarf stars. 19 13 years, after Herotto was published, according to the spectral type and luminosity, besides the main sequence star, there are five branches: Supergiant star, Superstar, subgiant, Sub Dwarf and White Dwarf. During the period of 1944, Budd observed the Andromeda galaxy and determined that stars can be divided into two different star groups: star group ⅰ and star group ⅱ. Xingzu I is a young and gold-rich celestial body, distributed on the spiral arms and related to interstellar matter. Xingzu Ⅱ is an ancient metal-poor celestial body, which has no tendency to gather in galactic plane. 1957, according to the metal content, age, spatial distribution and motion characteristics, these two star groups were further subdivided into intermediate star group ⅰ, spiral arm star group ⅰ, discoid star group ⅱ and halo star group ⅱ.

Stars in pairs, groups and clusters are common phenomena. Within 25 parsec near the sun, less than half of the stars are single stars. So far, 65,438+032 globular clusters, more than 65,438+0,000 galactic clusters and quite a few star complexes have been observed. According to statistics, there should be 18000 galactic clusters and 500 globular clusters. At the beginning of the twentieth century, Barnard discovered a large number of bright and dark nebulae through photographic observation. 1904, the discovery of ionized calcium in the stellar spectrum revealed the existence of interstellar matter. Subsequent spectral and polarization studies have determined the gas and dust components in the nebula. In recent years, through the detection of infrared band, it is found that there are forming stars in the dense area of dark nebula. After the birth of radio astronomy, the vortex structure of the Milky Way was outlined by using the neutral hydrogen 2 1 cm spectral line. According to the description of ionized hydrogen region, it is found that there are three spiral arms near the sun: Sagittarius, Orion and Perseus. The sun is in the arm of Orion. In addition, an arm of 3000 parsec was found in the direction of Yin Xin. The distance between the two arms is about 1.6 thousand parsec. In 1963, the interstellar molecule OH was observed in radio astronomy, which was a major breakthrough since the identification of interstellar molecules ch, CN and CH+ in the optical band during the period of 1937 ~ 194 1. By the end of 1979, more than 50 kinds of interstellar molecules had been discovered.

Structure The overall structure of the Milky Way is: the main part of the Milky Way material consists of a thin disk called a silver disk, and the part near the spherical center of the silver disk is called a nuclear ball. Stars are very dense in the nuclear sphere, and there is a small dense area in its center, which is called the silver core. Outside the silver plate is a larger, nearly spherical distribution system, in which the density of substances is much lower than that inside the silver plate, which is called silver halo. There is also a silver corona outside the silver halo, and its material distribution is also roughly spherical. For details about the Milky Way, please see The Structure of the Milky Way.

Little is known about the origin and evolution of the origin of the Milky Way. This is not only to study the origin and evolution of general galaxies, but also to study cosmology. According to the Big Bang cosmology hypothesis, all the galaxies we observed were protogalaxies and evolved into galaxy clusters including the Milky Way due to the fluctuation of high-density primitive materials caused by density, unstable gravity and continuous expansion/10 10 years ago. The hypothesis of the steady-state universe model is that galaxies are constantly formed in the core region of high-density protogalaxies.

Only in recent years have some achievements been made in the study of the evolution of the Milky Way. The data about the spatial movement of old stars near the sun show that the halo star family was first born in the process of the collapse of the original galaxy nebula. Their age is over 654.38 billion years, and their chemical composition is about 73% hydrogen and 27% helium. Most of the gaseous substances are concentrated into silver disks, which in turn form disk clusters. In recent years, people have discussed the overall evolution of the Milky Way from the perspectives of the formation and evolution of stars, the changes of element abundance, the activities of silver nuclei and their position in evolution. The density wave theory, developed in the 1960s, well explains the overall structure of the spiral structure of the Milky Way and its long-term maintenance mechanism.

Galactic System

The sun is a member of a huge disk system composed of stars and galaxies. The light from many stars in the Milky Way formed the Milky Way, which became an irregular luminous band around the night sky. This starlight belt is generally located on the plane of the silver plate. The Milky Way is one of the billions of galaxies that make up the universe. It has tens of billions of stars and a considerable amount of interstellar gas and dust. The Milky Way is a typical spiral galaxy in the galaxy type. Its core is surrounded by a huge central nuclear ball, surrounded by spiral arms. These curved arms make the shape of the Milky Way look like a giant wheel. The spiral arms sink evenly into the silver plate. The silver disk is the main part of the Milky Way, with a diameter of about 70,000 light years. The silver core is shielded by interstellar dust particles and absorbs visible light and ultraviolet light in silver nuclear radiation. However, scientists can record and study the radiation emitted by silver nuclei in radio, infrared, X-ray and γ-ray bands. In particular, the intense emission of infrared radiation and X-rays indicates the existence of ionized gas clouds moving at high speed. It is generally believed that this gas cloud is orbiting a massive celestial body, which is likely to be a black hole with a mass of about 4 million suns. Scientists have confirmed that the main components of the central nuclear sphere are some ancient stars and ancient clusters. The composition of the spiral arm is a completely different celestial body. The celestial bodies in the spiral arms belong to very young bright stars and open clusters. In addition, the spiral arm region is the region with the highest concentration of interstellar gas and dust particles, so it is also the most suitable place for the formation of new stars. The sun is located near the inner edge of one of these spiral arms, the Orion arm, with a radius of about two-thirds from the center of the Milky Way. The silver core is located in the direction of Sagittarius, about 23,000 light years away from the sun. Above and below the silver plate is a spherical area (called spherical component), which is full of globular clusters and other very old celestial bodies. Such as dwarfs that are poor in heavy elements. The periphery of the Milky Way reaches the visible edge, which is a huge silver halo with huge mass. Its composition, shape and extension are not very clear. The whole galaxy revolves around the center of the galaxy, but celestial bodies with different compositions do not rotate at the same speed. Objects far from the galactic center are slower than those near the galactic center. The sun, which is quite far away from the galactic center, orbits the galactic center in an approximate circular orbit with an estimated speed of 225 km/s. Because of the slow revolution speed of the sun, it takes about 200 million years for it to orbit the galactic center once.

The solar system where the earth is located is in the Milky Way. Looking at the Milky Way on the earth, you will find a milky white bright belt across the starry sky, which is the projection of the main body of the Milky Way on the celestial sphere. China was also called Han Yin in ancient times. In the northern half-day, the Milky Way starts from the eagle to the northwest, passes through Sagittarius, Fox, Cygnus, Cepheus and Cassiopeia, then turns to the southeast, passes through Perseus, Auriga, Taurus, Gemini and Orion, passes through the unicorn on the equator of the celestial sphere, enters Canis Canis, Pisces and Vela in the southern half-day, turns to the northwest, and passes through the carina and carina. After 23 constellations, the Milky Way returned to Aquila on Sunday. Through the telescope, we can see that the Milky Way is made up of countless stars and nebulae. Nebulae are bright and dark. Dense and bright star clouds illuminate the Milky Way, for example, the bright areas around Shield and Sagittarius. Dark nebulae appear in the form of dark areas in the Milky Way, such as the "Big Bifurcation" south of Aquila and the "Coal Bag" near the Southern Cross. The Milky Way draws a belt with irregular outline and inconsistent width in the starry sky, which is called the Galaxy Belt. The widest part of the Silver Island Belt is 30, and the narrowest part is above 10.