The Sun is almost perfectly spherical and consists of hot plasma with magnetic fields. It has a diameter of about 1,392,684 km (865,374 mi), around 109 times that of Earth, and its mass (approximately 330,000 times the mass of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. The remainder consists of heavier elements, including oxygen, carbon, neon and iron, among others. The solar atmosphere (a thin layer of gases) is where we see features such as sunspots and solar flares on the sun.The Sun is a G-type main-sequence star (G2V) based on spectral class and it is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum, although it is actually white in colour. From the surface of the Earth it appears yellow because the Earths atmosphere scatters blue light.
Its surface temperature, of approximately 5778 K (5505 °C), indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen each second.Astronomers once thought that the Sun was a small and relatively insignificant star, but it has been found to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. As the star closest to Earth, the Sun is the brightest object in the sky with an apparent magnitude of -26.74. The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.
The distance of the Sun from the Earth changes as the Earth moves from perihelion in January to aphelion in July. The average is calculated as approximately 1 astronomical unit (150,000,000 km; 93,000,000 mi). Light travels from the Sun to Earth in about 8 minutes and 19 seconds and its energy supports almost all lifeon Earth by photosynthesis and is the means by which Earth's climate and weather is affected. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. Not very much was known about the Sun and, until the 19th century, prominent scientists did not understand the Sun's physical composition and source of energy.
Today, although much more is known about the Sun, there are still many anomalies which remain unexplained.
The Sun
The Sun formed about 4.6 billion years ago from the collapse, caused by gravity, of a region within a large cloud of dust. Most of the matter gathered in the center, while the rest flattened into an orbiting disk that would become the Solar System. The central mass became increasingly hot and dense, eventually igniting into thermonuclear fusion. It is thought that almost all stars form by this process. The Sun is the star at the center of the Solar System.
A star does not have a solid surface, but is a ball of gas held together by its own gravity. Since the Sun is not a solid body, different parts of the sun rotate at different rates. At the equator, the sun spins once about every 25 days, but at its poles the sun rotates once on its axis every 36 Earth days.
