Stars are not empty, but full of matter, which can be gas, dust or a mixture of the two. A low-temperature, non-luminous interstellar dust cloud is considered to be the basic material for star formation. The temperature of these dark interstellar dust clouds is very low, ranging from -260 to-160 degrees Celsius. Astronomers have found that if there is no external force, these interstellar dust clouds are like clouds in the sky. It floats in space forever. However, if something happens, such as a supernova explosion nearby, the shock wave will compress the interstellar dust cloud when it passes through, thus increasing the density of the interstellar dust cloud and shrinking by its own gravity. This process of shrinking the volume by its own gravity is called "gravity collapse". There are other external forces, such as the magnetism between galaxies or the collision between dust clouds. It may also cause interstellar clouds to collapse under the action of gravity. About 5 billion years ago, an interstellar dust cloud named "Primitive Solar Nebula" began to collapse under the action of gravity. The smaller the volume, the higher the temperature and density of the core. When the volume is reduced by 1 million times, it becomes a primitive star, and the temperature in the core area also rises to about 10 million degrees Celsius. When the temperature in the core region of this primitive star or fetal star is high and the hydrogen fusion reaction is triggered, it is also the reaction of hydrogen bomb explosion. At this time, a star named Sun was born. After a series of nuclear reactions, four hydrogen nuclei will be consumed to form a helium nucleus, and a little mass will be lost. According to Einstein's mass-energy exchange equation, e = MC 2, the lost mass will be converted into light and thermal radiation, which will eventually arrive after collision, absorption and re-emission. When the energy produced by hydrogen fusion reaction in the central region of the sun reaches the surface, most of it is radiated into space in the form of visible light. The sun, which was just formed 50 years ago, is unstable and its volume shrinks and expands. The gravitational force of contraction is blocked by the pressure of thermal expansion, and sometimes the thermal expansion force rises and exceeds the gravitational force, so the star atmosphere expands. However, as soon as it expands, the temperature drops. It's inflated too much. As a result, the temperature is so low that the thermal expansion pressure can't stop gravity, and then the stellar atmosphere begins to shrink. Similarly, as soon as it contracts, the temperature rises and contracts too much, resulting in too high a temperature, which makes the thermal expansion pressure exceed gravity, and the star atmosphere begins to expand again. This process of expansion and contraction occurs repeatedly, and the surrounding is still shrouded in clouds and gas, so the brightness changes irregularly. However, the degree of expansion and contraction gradually narrowed, and finally the thermal expansion force and contraction force reached a balance. Enter a stable period. At this time, the sun is a yellow star, almost the same as what we see now. After the sun enters a stable period, it emits light and heat quite stably, which can last for10 billion years. This period accounts for 90% of the life of the sun, which astronomers call the "main sequence star" period. It has been 5 billion years since the sun became the yellow main sequence star. In another 5 billion years, the sun has passed its golden age. It will enter old age. It is very important for life on the planet to have a long enough stable period. According to the experience of the earth, the earth and the sun formed at the same time, and life did not appear until nearly a billion years later. More than four billion years later, creatures with higher intelligence were discovered. So astronomers are only interested in finding extraterrestrial life, and the sun will become a red giant in old age. In its old age, the sun will run out of hydrogen in its core area. At this time, the core area of the sun is low-temperature helium, and the surrounding layers are undergoing hydrogen fusion reaction, and then the general matter of the sun is at the periphery. The light and heat produced by hydrogen fusion reaction are the same as the gravitational force of contraction. Because of the low temperature of helium in the core, the density of helium is higher than that of hydrogen, and the gravity is greater than the thermal expansion force, the heat generated by the contraction of the core diffuses to the outer layer, plus the heat generated by the fusion reaction of hydrogen in the outer layer. It makes the outside of the sun slowly expand, and the radius increases to the extent of engulfing Mercury. With the expansion of the sun, its luminous and heat dissipation surface area is also increasing. After the surface area is enlarged, the heat emitted per unit area is relatively reduced, so with the expansion of the sun, the surface temperature drops to 3000 degrees Celsius. In the electromagnetic radiation, red light is the strongest, so it will present a fiery red sun, which is called "red giant star". During the period of red giant, the sun was unstable. The disturbance of the external atmosphere will cause the pulsation effect of expansion and contraction, and the period of pulsation is related to the size. Think about the situation of jelly. If you pat it gently, it will shake, and the bigger the jelly, the less it will shake. Similarly, the larger the red giant, the longer the period of expansion and contraction. Simply put, after five billion years, the heat shrinking in the sun's core region will lead to external expansion. It became a red giant. The core area filled with helium continues to shrink and the temperature increases. When the temperature in the core region rises to 1 100 million degrees, the helium fusion reaction begins, and three helium atoms fuse into a carbon through a series of nuclear reactions, releasing more light and heat than the hydrogen fusion reaction, making the outer layer of the sun expand rapidly and engulf the earth. Become a super red Supergiant star. The end of the sun: a similar process of white dwarfs occurs again in the core of red giant. The more carbon accumulates, the denser carbon is than helium, and the gravity of relative contraction is also greater. The historical carbon core area has shrunk. But when this region shrinks to a very tight and solid degree, that is, all the electrons around the carbon atom nucleus are squeezed together and can no longer be squeezed, this tight pressure prevents gravity contraction. Although the temperature at this time is much higher than 1 100 million degrees Celsius, it is not enough to produce carbon fusion reaction. Therefore, the core area of the sun no longer shrinks, but there is no excess heat to expand the outer layer, thus forming a white dwarf. Because the core of the white dwarf has no nuclear fusion reaction to supply light and heat, the whole planet becomes darker and darker. Finally become a dead planet without light-black dwarf. According to theoretical calculation, the process of white dwarfs slowly cooling into black dwarf is very long, exceeding/kloc-0.00 billion years, while the formation of the Milky Way is only/kloc-0.00 billion years, so astronomers think that the Milky Way is not old enough to form black dwarf. After calculation, the volume of the sun is reduced by one million times, which is about as big as the earth, and the degree of crowding between materials is enough for defying gravity to shrink. The mass is equivalent to the sun, but the volume is only the size of the earth. It is easy to calculate that the density of white dwarfs is one million times heavier than water. In other words, a centimeter of matter weighs about one metric ton, which is a very special state of matter, which physicists call degeneracy. Atoms consist of nuclei and electrons. Most people have seen pictures or animations of electrons around the nucleus, although it is a simplified schematic diagram. However, it also reflects the tiny state of matter. Usually, electrons circle away from the nucleus. If the temperature decreases gradually or the external force increases gradually, the activity range of electrons will be squeezed smaller and smaller, and gradually close to the nucleus. But the distance between the electron and the nucleus has its minimum range, and the electron cannot cross this boundary. Like sand grains around glass beads, sand grains are attached to the surface of glass beads at most. Similarly, when all electrons are forced to squeeze the surface layer of heavy atoms, the state of matter reaches a critical point, and even if the pressure is increased, electrons cannot squeeze inward. This back pressure caused by the innermost electrons is called electron degeneracy pressure. According to theoretical calculation, the gravitational force of planets with mass less than 1.4 solar masses is not enough to crush electron degeneracy pressure. Therefore, the mass of a white dwarf cannot be greater than 1.4 suns. So far, the number of white dwarfs has exceeded hundreds. This upper limit was first calculated by Indian astronomer subrahmanyan chandrasekhar19101995 in 193 1 year with quantum mechanics. Therefore, it is called the Chandraseka limit. When Chandraseka proposed this planet blocked by electron degenerate pressure in 1935, it was not praised. At the seminar held by the Royal Astronomical Society in 1935, it was even suppressed by Sir Authoreddington, a contemporary master, who thought that such celestial bodies did not exist in the universe. After this attack, Draca Ha was unable to publish a paper in the near future, so he wrote. & lt Structure and Evolution of Stars >:> Later became a classic in this field. Why is it called a white dwarf? This is because the white dwarf identified by Brother Yi is the companion star of Sirius, which is bluish white at high temperature, but so small in size, so it is called a white dwarf. But later, many similar stars were discovered one after another. Their starlight colors were low-temperature yellow and orange, but they were still called white dwarfs. So white dwarfs became a proper term, specifically referring to this kind of planet whose gravitational contraction was blocked by electronic degeneracy pressure. There are records and legends about the history of stars at home and abroad. The Pleiades cluster has been recorded for more than 3000 years, and it is one of the brightest clusters in the northern sky. This seemingly vague celestial body, known as the Pleiades in China, is one of the 28 stars. In the Book of Songs, I have already mentioned accommodation in A Journey to Zhao Nan, and Shi Tianzhong has also mentioned Xi Lu Ang, which means that it is called Ang. This is the meaning of arrogance in Historical Records-Official Book. The Pleiades cluster has many different names and stories in Japanese fairy tales, but most of them are related to agriculture and fisheries. For example, in some agricultural areas of Japan, when the Pleiades star cluster rises with the sun, it means that spring is the season of sowing. In some coastal areas, I watched the Pleiades rise and fall and decided whether to cast a net. In Greek mythology, Seven Sisters was the daughter of Atlas, the giant god. They are the maids of themis, the goddess of the moon. One day, Olivon, an Orion, suddenly broke in while playing on the grassland. Seven Sisters fled to the sky in fear and hid in the sleeve of the goddess. Afterwards, the goddess opened her sleeves and saw seven pigeons curled up into a ball. Although Leon couldn't catch them, he pursued them until Zeus sympathized with them and placed them in the sky, becoming the Seven Sisters group. From an astronomical point of view, the seventh star is most likely a variable star, which was originally bright and then dimmed. According to the research of astronomers, the Pleiades cluster is a young cluster with a journey of about 1 100 million years, which contains many variable stars with irregular brightness changes. Because the Pleiades cluster is a young cluster, some short-lived stars have just entered the final stage of evolution, and the brightness of these stars is mostly unstable. For example, the BU star in Taurus is a variable star, and its brightness changes irregularly. The evolution of heavy stars When these substances collide with the extremely hard core region at high speed, they will generate a strong rebound force and form a shock wave that spreads outward. This situation is like a person banging on the table. The harder he works, the greater the rebound. The shock wave vibrates outward at supersonic speed, squeezing the outer material, causing the temperature to rise rapidly. Therefore, the stable period of inner heavy stars on the whole planet varies greatly according to their mass. The lifetime of a star with impact mass is quite short, only tens of millions of years. The life span of a star whose mass is more than twice that of the sun is about several hundred million to several billion years. The reason for the short life span of a heavy star is that its mass is large, and the gravitational force that leads to contraction is also very powerful, which makes the temperature inside and in the region of the star relatively high, thus making the nuclear reaction rate more explosive and emitting fierce light and heat, resulting in the surface temperature of the nuclear star being several times higher than that of the sun star, and the light and heat radiated into space increasing geometrically. When a star is formed, its mass is already fixed, so the star is eating its own roots. Although the cost of a heavy star is more than that of the sun, its heating speed is dozens of times or even more than that of the sun. Obviously, it will soon run out of raw materials for nuclear reactions and enter the end of evolution. The diameter range of neutron star atoms is 10 thousand times larger than that of the nucleus, so when electrons are squeezed into the nucleus, the diameter is reduced by more than 10 thousand times and the volume is reduced by more than 1 trillion times. So when all matter becomes neutrons, the volume can be said to be surprisingly small and the density scary. The pressure resistance is stronger. The pressure resistance of this neutron tightly squeezed together is called {neutron degenerate pressure}. According to theory, at the end of the evolution of heavy stars, if the mass of the core region is between two and three solar masses, strong gravity will squeeze the matter into neutrons. At this time, the diameter of the planet is about 30 kilometers, and the strong neutron degenerate pressure blocks the gravity, so the planet does not shrink into a neutron star. Speaking of this, the story of neutron star is not complete, only the central region of the star is in front, and it is complete if the changes in the peripheral region are added. Through a process that physicists still don't fully understand, when all the materials in the central region are squeezed into neutrons, the materials inside the star sink toward the center under the strong gravity, and the falling speed is very fast, and the temperature from the nuclear region to the surface layer is high enough to produce nuclear fusion reaction. Imagine if all the hydrogen bombs on the earth exploded at the same time. This is a nuclear fusion reaction that takes place all over the earth and will blow the whole planet to pieces, forming what astronomers call a "supernova explosion". Supernova explosions are like fireworks, but they are much larger and last for a long time. The whole supernova explosion is like fireworks, but it is much larger and lasts for a long time. The whole process of supernova explosion and diffusion can last for thousands to tens of thousands of years, with a width of dozens of light years. Supernova explosion is the most spectacular event in the galaxy. To sum up, a star with a mass more than three times that of the sun may produce a supernova explosion. After the explosion, a neutron star was left in the center. Neutron stars are mainly composed of neutrons, with a diameter of tens of kilometers and a density of tens to 100 million times that of water. It is really an abnormal planet. When a star with a black hole mass of more than six to eight suns explodes more energy than a supernova at the end of its evolution, even the degenerate neutron pressure cannot resist the strong gravitational contraction, so the matter has to shrink all the way. At present, only Einstein's general theory of relativity can explain this problem. According to theory, matter has shrunk to about three kilometers and entered a range where even light can't escape. Except for the total amount and the rotation of the nucleus, all the missing information is called "black hole" by physicists. Since black holes don't shine, how can we find them? For a single black hole, physicists can't think of a good way, but if the black hole is one of the binary systems, we can estimate the invisible companion mass by observing the motion of the binary stars. If the companion is more than three solar masses, it may be a black hole if it cannot be seen. In the binary system, if one of them is a black hole, when the other star expands into a Supergiant star in the later stage of evolution, the expanded matter will be attracted by the strong gravity of the black hole and fall into the black hole in a spiral way. In the process of spiraling down, an accretion disk is formed. In the process of hovering and falling in the accretion disk, the material collides and pushes all the way, the radius is getting smaller and smaller, and the temperature is also rising. The temperature of the inner layer of the accretion disk is as high as 1 million degrees Celsius, emitting X-rays. Therefore, astronomers search X-ray binary systems to calculate the mass of invisible companions. If the mass of an invisible companion star exceeds three solar masses, it is considered as a candidate for a black hole. After nearly a hundred years of research, the evolution of the star structure is outlined, so that we can understand the story of how the star evolved and come to an end. Some of these stars violently exploded involuntarily, and many elements such as calcium, silicon and iron were scattered into the interstellar medium by supernova explosion. Coincidentally, these substances have become spring mud for more protection, and after being compressed by gravity, they have become a brilliant star. Because of these elements, planets similar to the earth can be formed, and the occurrence of death depends on these elements. For example, in the human body, blood contains iron and bones contain calcium, so astronomers often say "we are supernova people".