Sun

The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that 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. Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others.

The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light. Its stellar classification, based on spectral class, is G2V, and is informally designated a yellow star, because its visible radiation is most intense in the yellow-green portion of the spectrum. In this spectral class label, G2 indicates its surface temperature of approximately 5778 K (5505 °C), and V 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 430–600 million tons of hydrogen each second. Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs.[14][15] The absolute magnitude of the Sun is +4.83; however, 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 core of the Sun is considered to extend from the center to about 0.2 to 0.25 of the solar radius. It has a density of up to 150 g/cm3 (about 150 times the density of water) and a temperature of close to 13,600,000 K (by contrast, the Sun's surface is around 5,800 K). Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone. Through most of the Sun's life, energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium. Less than 2% of the helium generated in the Sun comes from the CNO cycle.

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