The Sun (occasionally referred to as Sol) is the star at the centre of our solar system. Planet Earth orbits the Sun, as do innumerable other bodies including other planets, asteroids, meteoroids, comets and dust.
The primary stellar body around which an object orbits is called its "sun", and stars in a multiple star system are referred to as the "suns" of bodies in that system.
The Sun is a main sequence star, with a spectral class of G2, meaning that it is somewhat more massive and hotter than the average star but far smaller than a blue giant star. A G2 star is on the main sequence, and has a lifetime of about 10 billion years (10 Ga), and the Sun formed about 5 Ga (5 billion years) ago, as determined by nucleocosmochronology. The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 Ma (226 million years). The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days.
The astronomical symbol for the Sun is a circle with a point at its centre (☉).
Caution: looking directly at the Sun can damage the retina and one's eyesight.
Structure of the sun
The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths (mostly due to the gravitation of Jupiter), which means the polar diameter differs from the equatorial by 10 km at most. This is because the centrifugal effect of the Sun's rather sedate rotation is 18 million times weaker than its surface gravity (at the equator).
The Sun does not have definite boundaries as rocky planets do. Instead, the density of gases comprising the Sun drops off following an exponential relationship with distance from the centre of the Sun. The Sun's radius is measured from centre to the edges of the photosphere.
At the centre of the Sun, where its density is 150 g/cm3, thermonuclear reactions (nuclear fusion) convert hydrogen into helium. About 8.9×1037 protons (hydrogen nuclei) are converted to helium nuclei every second. This releases energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 yottawatts (9.15×1016 tons of TNT per second) which escapes from the surface of the Sun in the form of electromagnetic radiation and neutrinos (and to a smaller extent as the kinetic and thermal energy of solar wind plasma and as the energy in the Sun's magnetic field). A fusion reactor, which some physicists believe may one day provide power for human use, would use a similar process to extract atomic energy.
The corona has a particle density of 1011/m3, and the photosphere a particle density of 1023/m3.
For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only one third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were created including the Sudbury Neutrino Observatory to try and measure the amount of neutrinos given off by the Sun. From these observatories and experiments it was recently found that neutrinos had rest mass, and could therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth; thus measurement and theory were reconciled.
All matter in the Sun is in the form of plasma due to its extreme temperature. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (28 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences. (See magnetic reconnection) The solar activity cycle includes old magnetic fields being stripped off the Sun's surface starting from one pole and ending at the other. The magnetic field of the sun reverses once for each 11-year sunspot cycle.
Calculating the position of the Sun
Since the path of the sun across the sky varies throughout the year, a completely automatic heliostat, or sun tracker, must be guided by continuous calculations. The National Renewable Energy Laboratory has released its Solar Position Algorithm (SPA) with complete documentation. Another resource is the libnova Celestial Mechanics and Astronomical Calculation Library, which also calculates variables such as apparent position and rise, set and transit times among many others of astronomical objects.
Solar space missions
To obtain an uninterrupted view of the Sun, the European Space Agency and NASA cooperatively launched the Solar and Heliospheric Observatory (SOHO) on December 2, 1995.
Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is much less well known. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. It returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on reentry to Earth's atmosphere.
The history and future of the Sun
Our Sun does not have enough mass to explode as a supernova. Instead, in 4-5 billion years it will enter its red giant phase, expanding as the hydrogen fuel in the core is consumed. Then it will start to fuse helium and the core temperature will rise to 3×108 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. Following the red giant phase, giant thermal pulsations will cause the Sun to throw off its outer layers forming a planetary nebula. The Sun will then become a white dwarf, slowly cooling over eons.
Human understanding of the Sun
In many prehistoric and ancient cultures, the Sun was thought to be a deity or other supernatural phenomenon. One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of rock or metal, and not the chariot of Apollo. For teaching this heresy he was imprisoned by the authorities and sentenced to death.