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The sun is the star in the center of the solar system. It is a nearly perfect sphere of hot plasma, which has internal convection motion and generates magnetic field through generator process. So far, it is the most important energy source for life on earth. Its diameter is about 1.39 million kilometers, 109 times of the earth's, and its mass is about 330000 times of the earth's, accounting for 99.86% of the total mass of the solar system. About three-quarters of the sun's mass is made up of hydrogen (about 73%); the rest is mostly helium (about 25%), which contains less heavy elements, including oxygen, carbon, neon and iron.
The sun is a G-type main sequence star (g2v) based on its spectral category. As a result, it is informally known as the yellow dwarf. It was formed about 4.6 billion years ago by the gravitational collapse of matter in the region of macromolecular clouds. Most of the matter is concentrated in the center, while the rest is flattened into an orbital disk and becomes the solar system. The central material became so hot and dense that it eventually led to fusion. It is believed that almost all stars are formed by this process.
The sun is about middle age. It has not changed much in more than 4 billion years, and will remain fairly stable for more than 5 billion years. When the hydrogen fusion in its core is reduced to no longer in hydrostatic equilibrium, the core of the sun will experience a significant increase in density and temperature, and its outer expansion will eventually turn into a red giant. It is calculated that the sun will become large enough to engulf the current orbit of mercury and Venus and make the earth uninhabitable.
Since prehistory, people have realized the great influence of the sun on the earth, and in some cultures, the sun has been regarded as a God. The conjugate rotation of the earth and its orbit around the sun is the basis of the solar calendar, one of which is the main calendar used today.
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