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             The Sun is a regular star of intermediate size and luminosity. It is one of more than 100 billion stars in our galaxy. The Sun is by far the largest object in our solar system. The Sun is personified in many mythologies, the Greeks called it Helios and the Romans called it Sol. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun's hot, dense interior. The Sun is, at present, about 75% hydrogen and 25% helium by mass; everything else amounts to only 0.1%. This changes slowly over time as the Sun converts hydrogen to helium in its core. The Sun's outer layers exhibit different rotation, at the equator the surface rotates once every 25.4 days: near the poles it's as much as 36 days. This weird behavior is caused by the fact that the Sun is not a solid body like the Earth. The different rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body. The Sun's core conditions are extreme. The pressure is 250 billion and the temperature is 15.6 million Kelvin. At the center of the core the Sun's density is more than 150 times that of water. The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. For the Sun's entire steadiness, it is an extremely active star. On its surface dark sunspots bounded by intense magnetic fields come and go in 11-year cycles. Sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth; and a continuous stream of protons, electrons and ions leave the Sun and move out through the solar system, spiraling with the Sun's rotation. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil. The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billio...