November 28, 2021

The Sun (Taiyo, English: Sun, Luo: Sol) is one of the stars in the galaxy (Milky Way Galaxy). It is the physical center of the solar system including the earth where human beings live, occupies 99.8% of the total mass of the solar system, and exerts the influence of gravity on all the celestial bodies of the solar system. The Sun is one of the most common main sequence stars in the galaxy to which it belongs, and its spectral type is G2V (gold). The estimated age is about 4.6 billion years, and it is thought that about 50% of the hydrogen existing in the center is used for thermonuclear fusion, and half of the period that it can exist as a main sequence star has passed. There are many unclear points about the internal state, and the current situation is that it is required by the "standard solar model" described later. Also, since the sun is the central star of the solar system, the central star of any planetary system may be called the "sun" figuratively.

Overview and location

The radius of the sun is about 700,000 kilometers and the diameter is about 1.4 million km, which is about 109 times the diameter of the earth. The mass (solar mass) is about 1.989 × 10 ^ 30 kg, which is almost equal to about 333,000 times that of the earth, and accounts for 99.86% of the total mass of the solar system. The average density is 1.4 times that of water, which is about 1/4 that of 5.5 times that of the earth. In the galaxy to which the Sun belongs, the distance from its center to the Sun is about 25,000 light-years and is located on the Orion Arm. The average distance from the earth to the sun is about 149.6 million kilometers (about 8 light seconds 19 light seconds). This average distance can be considered as the time average of the distance between the earth's sun or the semimajor axis of the earth's orbit. The more accurate value of this average distance is 149597870700 m (error is 3 m), which is defined as one astronomical unit (au). A resolution of the International Astronomical Union (IAU) in August 2012 redefined the value of 1 au to be exactly 149597870700 m, excluding the error ± 3 m. Since the time required for light to reach this distance is 8.3 minutes, it can also be expressed as 8.3 light minutes. It is said that it takes about 225 to 250 million years for the sun to orbit in the galaxy, and if it is the orbit of the sun, the axis of rotation of the sun is tilted by about 60 degrees. There is. In the revolution movement, it moves toward Lyra and Hercules. When the numerical value of the sun is used as a unit, they are represented by the symbol ☉ representing the sun. For example, the mass of the sun is displayed as M☉, and the solar luminosity is displayed as L☉. Currently, the standard of time is also based on 1 second determined by the atomic clock, but in the past, the rotation and revolution of the earth, from the human point of view, the "day" and "year" based on the sunrise, sunset, and the cycle of the season. The solar calendar and the lunisolar calendar that determine the date were used.


The sun is an almost perfect sphere with a flattening of less than 0.01%. Unlike terrestrial planets and satellites, the sun does not have a clear surface. The sun consists of a central core (solar core), a radiative zone, a convection zone, a photosphere, a chromosphere, a transition layer, and a corona. Since the viewing angle is almost the same as when observing the sun from around the earth with visible light, the photosphere is used as the surface of the sun for convenience. Moreover, there is no means for optically observing the inside of the inside. The solar radius is defined as the distance from the center of the sun to the photosphere. The photosphere often has sunspots, which are cooler than their surroundings, and areas called plage, which are bright areas around them. The part above the photosphere that has high light transmission is called the solar atmosphere. The upper part of the plasma-ized solar atmosphere is less susceptible to the constraints of solar gravity. For this reason, it leaks into the interplanetary space and extends to the orbit of Neptune. This is the sun.

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