Sun, star at the center of our Solar System, a luminous sphere of gas about 865,000 mi (1.4 million km) in diameter and 93 million mi (150 million km) from earth. The sun in 332,000 times as massive as earth, and its volume is 1.3 million times greater. The temperature in its center is about 15,000,000°K. The source of its energy is a series of nuclear fusion reactions in which hydrogen is converted into helium—light, heat, and gamma rays being produced in the process. Light from the sun takes about 8 minutes to reach earth. Although the sun is entirely gaseous, its bright surface looks like a kind of skin. This outer layer is called the photosphere, whose temperature is about 6,000°K. Above the photosphere lies the chromosphere, an irregular layer of gases, sunspots, flares, and prominences visible to the naked eye only during eclipses. The corona is the sparse outer atmosphere.
See also: Solar System.