The Sun is the star at the center of the Solar System. It is
almost perfectly spherical and consists of hot plasma interwoven with magnetic
fields. It has a diameter of about 1,392,684 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. Chemically, about three
quarters of the Sun's mass consists of hydrogen, while the rest is mostly
helium. The remainder (1.69%, which nonetheless equals 5,628 times the mass of
Earth) consists of heavier elements, including oxygen, carbon, neon and iron,
among others.
The Sun formed about 4.6 billion years ago from the
gravitational collapse of a region within a large molecular cloud. 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 initiating thermonuclear fusion in its core. It is
thought that almost all other stars form by this process. The Sun's stellar
classification, based on spectral class, is G2V, and is informally designated
as a yellow dwarf, because its visible radiation is most intense in the
yellow-green portion of the spectrum and although its color is white, from the
surface of the Earth it may appear yellow because of atmospheric scattering of
blue light.
In the 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 620
million metric tons of hydrogen each second.
