SUN

The Sun is a G-type main-sequence star that makes up about 99.86% of the mass of the Solar System. The Sun is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. One astronomical unit (about 150 million kilometres; 93 million miles) (au) was originally defined as the mean distance between the centres of the Sun and the Earth. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather.[The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its centre to the edge of the photosphere, the apparent visible surface of the Sun.The value is difficult to measure. Atmospheric distortion means the measurement must be done on satellites; the value is very small meaning very precise technique is needed