Have knowledge about the galaxy.

If we scan the sky roughly with the naked eye, it seems that we have seen all the stars in the sky. There is no place where the stars look particularly dense and there is no place where the stars look particularly thin. From this, we can draw a conclusion that for us, the stars are evenly distributed in all directions, and if the stars as a whole can form an aggregate with a certain shape, then the shape must be spherical. Obviously, all large celestial bodies are approximate to spheres. Why can't the whole galaxy be regarded as a sphere?

Of course, there are only 6, stars we can see with the naked eye, and most of them are quite close to us. What will we find if we use a telescope? The answer is that we see more stars, and they seem to be evenly distributed in the sky-except the Milky Way.

With naked eyes, the Milky Way is a weak band of light (nowadays, if we live in a city, it is difficult to see the Milky Way, because the sky is illuminated by artificial lighting). It looks pale milky white. In fact, there is a fairy story about it: once upon a time, when Zeus' wife Hera was breastfeeding her baby, her milk flowed into the sky and formed this weak light band. The Greeks called it galaxias kyklos (Silver Ring), and the Romans called it via lactea (Galaxy), from which we got its English name.

But what is the real Milky Way? If we don't consider fairy tales, then we can first think of the ancient Greek philosopher democritus. Around 44 BC, he suggested that the Milky Way actually consists of a large number of stars, which cannot be distinguished individually. But they gather to give off a soft light. Although this view has not attracted people's attention, it is exactly correct. In 169, when Galileo pointed the first telescope at the sky and found that the Milky Way contained a great number of stars, this theory was confirmed.

what does "extremely large quantity" mean? When people look at the night sky, their first impression is that there are countless stars, and there are too many to count. But I have mentioned several times that the total number of stars visible to the naked eye is only about 6, and the number of stars seen through telescopes is much larger. Does that mean they are countless?

The stars in the direction of the Milky Way are very dense, but they are relatively rare in other directions, which means that we must abandon the whole concept of stars that form spherical structures. In that case, there should be as many stars in all directions as there are in the direction of the Milky Way, and the whole sky will be lit up as the nearer stars twinkle (not as spectacular as they are now) against the background of weak light.

then, we must assume that the stars exist in large non-spherical clusters and extend farther in the direction of the milky way than in other directions. In this case, the Milky Way shows that the stars are all gathered in the shape of a lens or a hamburger. This lens-shaped cluster is called the Milky Way (Greek interpretation from the Milky Way), and the name of the Milky Way has been preserved because of the dark light band around the sky we see.

The first person who suggested that stars exist in shaded galaxies was thomas wright, a shaded astronomer. He put forward the proposal in 175, but his ideas seemed so confusing and incomprehensible that few people paid attention to him at first.

Of course, even if the Milky Way is lens-shaped, it can always extend in the long-diameter direction. Although only a few stars are seen outside the Milky Way, there are countless stars inside the Milky Way.

To illustrate the problem, William Herschel counted the number of stars. Naturally, it is impossible to count all the stars in a certain period of time.

Herschel selected 683 small areas, which were evenly distributed in the sky, and then counted the stars seen through the telescope in each area. In this way, he got the number of stars in what we now call an "imaginary poll" in the sky. This is the first example of applying statistics to astronomy.

Herschel thinks that the number of stars in each region is related to its proximity to the Milky Way. In all directions, the number of stars increases steadily with the increase of approaching the Milky Way. Judging from the number of stars he counted, we can estimate the number of stars in the Milky Way and how big the Milky Way may be. In 1785, he announced the results and proposed that the long diameter of the Milky Way is about 8 times the distance from the sun to Sirius, and the short diameter is 15 times this distance.

after half a century, the actual distance of Sirius was calculated, and it can be concluded that Herschel thought that the long diameter of the Milky Way was 8, light years and the short diameter was 1,5 light years. At the same time, he calculated that there are 8 billion stars in the Milky Way. Although this is a huge number, it is not uncountable.

In the past two centuries, astronomers have explored the Milky Way with much better instruments and techniques than those available to Herschel, and now they know that the Milky Way is much bigger than Herschel expected. It extends at least 1, light-years in the long direction and may have 2 billion stars. However, it can be said that we have confirmed that the Milky Way and the stars are not numerous but computable, which is due to Herschel.

the milky way galaxy

a huge, disk-shaped system composed of stars and galactic matter, in which the sun is a member. The light of many stars in the Milky Way formed the Milky Way, which became an irregular luminous belt around the night sky. This starlight band 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 among galaxy types. Its core is surrounded by a huge central nuclear ball with spiral arms wrapped around it. These curved arms make the shape of the Milky Way look like a huge wheel. The radial arm sinks evenly in the silver plate. The silver disk is the main part of the Milky Way, with a diameter of about 7, light years. Silver cores are shielded by interstellar dust particles, which absorb visible light and ultraviolet light in silver nuclear radiation. However, scientists can record and study the radiation emitted by the silver nucleus in the bands of radio, infrared, X-ray and γ-ray. Especially the strong emission in infrared radiation and X-rays, indicates the existence of ionized gas clouds moving at high speed. It is now widely believed that this gas cloud is orbiting a massive celestial body, which is probably 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 old stars and old clusters. The composition of the spiral arm is a completely different kind of 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 highest concentration area 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, about two-thirds of the radius from the center of the Milky Way. The silver core is located in the direction of Sagittarius, and the distance from the sun is about 23, light years. Above and below the silver plate is a spherical area (called spherical component), which is filled with spherical clusters and other very old celestial bodies. Such as dwarfs that are poor in heavy elements. The periphery of the Milky Way galaxy reaches the visible edge, which is a huge massive silver halo. Its composition, shape and extension are not very clear. The whole galaxy system rotates around the galactic center, but the celestial bodies with different components do not revolve 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, moves around the galactic center in an approximate circular orbit, and the speed is estimated to be 225 km/s. Because of the slow revolution speed of the sun, it takes about 2 million years for it to revolve around the silver heart.

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 Yinhan in ancient times. In the northern half-day, the Milky Way starts from Eagle to the northwest, passes through Sagittarius, Foxes, Cygnus, Cepheus and Cassiopeia, then turns to the southeast, passes through Perseus, Auriga, Taurus, Gemini and Orion, runs through the unicorn on the equator of the celestial sphere, enters Canis canis, Pisces and Vela in the southern half-day, and turns to the northwest, crossing the carina and the carina. After 23 constellations, the Milky Way returned to Aquila on Sunday. Looking through a telescope, we can see that the Milky Way is made up of numerous stars and nebulae. Nebulae are bright and dark. Dense bright nebulae brighten the Milky Way, for example, the bright areas around Shield and Sagittarius. Dark nebulae appear as dark areas on the Milky Way, such as the "Big Bifurcation" to the south of Aquila and the "Coal Bag" near the Southern Cross. The milky way draws a band with irregular outline and inconsistent width in the starry sky, which is called the galactic belt. The widest part of the galactic belt is 3, and the narrowest part is more than 1.

Astronomical Milky Way

At the beginning of the 2th century, Kapteyn established a model of the Milky Way with the solar system as the center and a diameter of 4, light years through the statistical study of the star count and luminosity function. In 1918, shapley challenged the traditional idea that the solar system was the center of the Milky Way. He analyzed the apparent distribution of globular clusters known at that time, and estimated their distances according to period-luminosity relation, the Cepheid variable star, and concluded that the Milky Way is a lens-shaped star and nebula system with a diameter of 3, light years and a thickness of 3, light years. The center of the Milky Way is in the direction of Sagittarius, and the sun is 5, light years away from the galactic center. This is a feat of announcing that the solar system is not at the center of the universe since Copernicus Heliocentrism. In the past half century, the shape of shapley model has stood the test of new observation facts and has been recognized by the world. However, due to the incorrect assumption that there is no light-absorbing material between stars, the distance scale is overestimated. It was not until 193, when trumpler confirmed the existence of interstellar light absorption by studying the galactic cluster, that he revised the size of the galactic model again. Today's recognized values are about 81,5 light-years in diameter, about 3,3 to 6,6 light-years in thickness, and about 32,6 light-years from the galactic center.

In p>1926, Lindblad pointed out that the asymmetric effect of star motion is a reflection of the rotation of the Milky Way. Subsequently, the poor rotation of the Milky Way was confirmed by Oort, and it was found that the sun moves around the galactic center in a circular orbit at a speed of 25 kilometers per second, and it is estimated that it will make a revolution in 25 million years. He also estimated that the mass of the Milky Way is 1.4×1□ the mass of the sun. According to the revelation of extragalactic galaxies, it is speculated that the Milky Way also has a vortex structure. In the early 195s, Morgan's research on the spatial distribution of high luminosity stars and celestial 21 cm-hydrogen line radio analysis by Ault et al. both accurately described the spiral structure and spiral arms of the Milky Way. In 196s, Lin Jiaqiao successfully explained the vortex structure and its maintenance mechanism with density wave theory.

in p>1944, based on the study of the Herro diagram of clusters, Budd put forward the concept of star family, and divided the stars into two categories: star family ⅰ and star family ⅱ. In 1957, at an international academic conference held in the Vatican, according to the parameters such as the spatial velocity of stars, the distance from galactic plane, the concentration degree to galactic center, helium content and age, the star groups were subdivided into intermediate star group ⅰ, spiral arm star group ⅰ, disk star group ⅱ and halo star group ⅱ. The member celestial bodies of these five sub-systems constitute a silver crown, a silver halo, a silver heart, a silver disk and a spiral arm.

Galaxy World In 1912, Loewit observed the Cepheid variable of the Small Magellanic Cloud and discovered the period-luminosity relation, thus inferring that the distance of the Small Magellanic Cloud may be very far away, perhaps outside the Milky Way. At the end of 1924, Hubble announced that he had calculated the distance between Andromeda Galaxy (M31) and Sagittarius Irregular Galaxy (NGC6822) by using Cepheid period-luminosity relation, and pointed out that they were star systems outside the Milky Way. Since then, galaxy astronomy has been born. The ancient concept of cosmic island has been proved to be an objective reality; The establishment of the concept of "the universe beyond the milky way" is another great achievement of astronomy in the twentieth century.

in p>1929, Hubble discovered that the redshift of the spectral lines of extragalactic galaxies is directly proportional to the distance between galaxies. If the redshift is recognized as the Doppler effect of celestial retrogression, then the redshift-distance relationship means that galaxies generally regress, and the space in which they live is expanding as a whole. The expansion of the universe is one of the expected results of relativistic cosmology. In 1956, M.L. Hammason extended the linear relationship between redshift and distance to redshift □=.2, that is, the regression speed reached 1/5 of the speed of light. In 1977, Sandage extended to □=.75, that is, the retrogression speed was half the speed of light. According to this, the distance has exceeded 5 billion light years. This is the observation basis that we live in a constantly moving and evolving universe.

in the 196s, the exciting phenomena and special celestial bodies with abnormal activities, such as extragalactic radio sources, X-ray sources and quasars, were discovered in the galaxy world. Compared with most normal galaxies whose evolution scale is 1□ years, their existence is only a short moment. Since the 197s, exploring the deep space of the universe, which is more than ten billion light years away, has become the main topic of modern astronomy.

Milky Way

Our Milky Way contains about 2 billion stars, including about 1 billion stars, and the sun is a typical one. The Milky Way is a fairly large spiral galaxy, which has three main components: a silver disk with spiral arms, a silver center protruding from the center and a halo.

The silver disk:

The silver disk is the main body of the galaxy, with a diameter of about 8, light years, a thickness of about 6, light years in the middle part, and a thickness of about 3, light years near the sun. The silver disk is mainly surrounded by four huge spiral arms, and it is composed of countless blue stars. The sun is located on the arm of Orion between Sagittarius and Perseus, 28, light years or 8.5 thousand parsec away from the silver heart. The formation of spiral arms is related to the activity of galactic nuclei during the creation of the Milky Way.

Silver Heart:

The central bulge of a galaxy is a bright sphere with a diameter of about 2, light-years and a thickness of 1, light-years. This region is composed of high-density stars, mainly red stars with an age of more than 1 billion years. Many evidences show that there is a huge black hole in the central region, and the activity of galactic nuclei is very intense.

Silver Halo:

The galactic halo is scattered in a spherical area around the silver disk. The diameter of the silver halo is about 98, light years. The density of stars here is very low, and there are some spherical clusters composed of old stars. Some people think that there is a huge spherical radio emission area outside the silver halo, called the silver corona, which extends at least 1, parsec or 32, light years away from the silver heart.

Milky Way

The star system in which the solar system is located includes 12 billion stars, a large number of star clusters and nebulae, and various types of interstellar gas and interstellar dust. Its total mass is 14 billion times that of the sun. Most of the stars in the Milky Way are concentrated in a oblate sphere, which is shaped like a discus. The protruding part in the middle of the oblate sphere is called the "nuclear sphere" with a radius of about 7, light years. The middle of the core ball is called "silver core" and the periphery is called "silver plate". There is a bigger sphere outside the silver disk, where there are fewer stars and the density is small, which is called "silver halo" with a diameter of 7 thousand light years. The Milky Way is a spiral galaxy.