The evolution of stars(恒星的演化)

The evolution of starsStars are the most noticeable objects in the vast universe except the sun, moon and a few planets. In ancient times, people were full of curiosity and fantasy about stars, and very moving myths and legends were popular in China and abroad. However, it was not until the telescope appeared that people had the most basic understanding of stars and realized that stars were not constant in the sky.At the beginning of the20th century, Einstein published the famous mass-energy relationship, people gradually realized the huge energy produced by the nuclear reaction and knew the source of the star's energy before they gradually realized that the star itself also had a life cycle, they would be born, grow, and die just like people. However, the birth of stars was still a mystery for a long time. It was not until the 1960s that astronomers discovered molecular gas in interstellar space that they had the most preliminary understanding of the evolution process of stars. Next, I’d like to share it. Gravitational contraction stageThe star was originally born from interstellar dust in space. Scientists call it "nebula" or " interstellar cloud" vividly. Its main component is hydrogen, which is extremely small indensity but huge in volume and mass. The nebula with enough density continuously shrinks and its temperature rises under the action of its own gravity.When the temperature reaches 10 million degrees, a thermonuclear fusion reaction takes place inside it, releasing a large amount of atomic energy and forming radiation pressure, when the pressure increases enough to compete with the gravitational pull of its contraction, a star is born.Main sequence starThe stage in which a star uses internal hydrogen and helium fusion as its main energy source is the main sequence stage of the star, which is the ‘youth” of the star. It is the longest golden stage in the life of the star and occupies 90 % of its entire life span.During this period of time, the star is relatively stable, the two forces of outward expansion and inward contraction are roughly balanced. The star does not shrink or expand basically, and emits heat with almost constant luminosity, illuminating the surrounding space.The time that different stars stay in the main sequence stage varies greatly with different masses.The greater the mass, the greater the luminosity, the faster the energy consumption, and the shorter the time spent in the mainsequence stage.Stars whose mass is equal to 15, 5, 1, and 0.2 times the mass of the sun are in the main sequence stage for 10 million years, 70 million years, 10 billion years, and 1 trillion years respectively.Red giant star stageWhen a star passes through its long main sequence stage, it will first become a red giant star.Because the burning consumption of hydrogen in thermonuclear reactions increases, the rate of helium formation will continue to increase.Soon, the hydrogen around the helium core will become very small, and the energy generated by the central core will no longer be sufficient to maintain its radiation pressure, so the balance will be broken and the gravitational force will be greater than the radiation pressure. After that, the process of star evolution is that the inner core shrinks and the outer shell expands. At the same time, the outer shell of the star expands outward and keeps getting cold, and the surface temperature is greatly reduced. The process only lasts hundreds of thousands of years, and the star becomes a red giant in rapid expansion.Because the volume will expand to 1 billion times and is very huge, it is called "superstar".While the star isexpanding rapidly, its outer surface is getting farther and farther away from the center, its mass is easy to lose and its temperature is getting lower and lower, and the emitted light is getting redder and redder, so it is called a "red" giant star. Because of its huge volume, its luminosity has also become very large, extremely bright. Many of the brightest stars visible to the naked eye are red giant stars.Outbreak stageThe stars in their later years are very unstable. One day they will explode violently. By then, the whole star will have formed its own life with an extremely spectacular explosion, throwing most of its material into the sky, turning it into a nebula again, and releasing huge energy at the same time. In this way, in just a few days, its luminosity is likely to increase by several hundred thousand times. This kind of star is called "nova". If the stellar explosion is more violent, its luminosity can even increase by more than 1,000,000 times or even 10,000 times. This kind of star is called "supernova." The explosion of new stars or supernovae is an important link in the evolution of celestial bodies. It is the brilliant funeral of old stars and also the promoter of the birth of new stars. Theexplosion of supernovae may lead to the birth of countless stars in nearby nebulae.High density stageAfter the explosion, the supernova left only ahigh-density debris, instead of a star.Stars with a mass below 1 - 3 times the mass of the sun lose enough mass without drastic changes after leaving the main star sequence belt, and end their lives relatively peacefully and become a white dwarf.Stars with 3 times the mass of the sun will end up in supernova explosions. Within seconds of the explosion, the core will begin to collapse and eventually collapse into dense neutron stars.The mass of neutron stars is not infinite.If the core residual mass still exceeds about three times the mass of the sun after the supernova explosion, the central portion will continue to shrink.Finally, when the mass shrinks to a very small extent, the gravitational force near it is large enough to make the fastest moving light unable to get rid of its confinement.This celestial body cannot send any information to the outside world, which is called a black hole.In this way, stars come to the nebula and return to the nebula, completing its glorious life.。