Sun/Sol

For other uses, see Sun (disambiguation) and The Sun (disambiguation).
The Sun is the star at the center of the Solar System. It is a massive, hot ball of plasma, inflated and heated by energy produced by nuclear fusion reactions at its core. Part of this internal energy is emitted from its surface as light, ultraviolet, and infrared radiation, providing most of the energy for life on Earth.
The Sun moves around the Galactic Center of the Milky Way, at a distance of 26,660 light-years. From Earth, it is on average 1 AU (1.496×108 km) or about 8 light-minutes away. Its diameter is about 1,391,400 km (864,600 mi; 4.64 ls), 109 times that of Earth or 4 lunar distances. Its mass is about 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.
The Sun is a G-type main-sequence star (G2V), informally called a yellow dwarf, though its light is actually white. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.
Every second, the Sun's core fuses about 600 million tons of hydrogen into helium, and in the process converts 4 million tons of matter into energy. This energy, which can take between 10,000 and 170,000 years to escape the core, is the source of the Sun's light and heat. Far in the future, when hydrogen fusion in the Sun's core diminishes to the point where the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature which will push its outer layers to expand, eventually transforming the Sun into a red giant. This process will make the Sun large enough to render Earth uninhabitable approximately five billion years from the present. After this, the Sun will shed its outer layers and become a dense type of cooling star (a white dwarf), and no longer produce energy by fusion, but still glow and give off heat from its previous fusion for trillions of years. After that it might become a super dense hypothetical black dwarf, giving off no more energy.
The enormous effect of the Sun on Earth has been recognized since prehistoric times; the Sun was thought of by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of some solar calendars. The predominant calendar in use today is the Gregorian calendar, which is based upon the standard 16th-century interpretation of the Sun's observed movement as actual movement.
== Etymology ==
The English word sun developed from Old English sunne. Cognates appear in other Germanic languages, including West Frisian sinne, Dutch zon, Low German Sünn, Standard German Sonne, Bavarian Sunna, Old Norse sunna, and Gothic sunnō. All these words stem from Proto-Germanic *sunnōn. This is ultimately related to the word for sun in other branches of the Indo-European language family, though in most cases a nominative stem with an l is found, rather than the genitive stem in n, as for example in Latin sōl, ancient Greek ἥλιος (hēlios), Welsh haul and Czech slunce, as well as (with *l > r) Sanskrit स्वर (svár) and Persian خور (xvar). Indeed, the l-stem survived in Proto-Germanic as well, as *sōwelan, which gave rise to Gothic sauil (alongside sunnō) and Old Norse prosaic sól (alongside poetic sunna), and through it the words for sun in the modern Scandinavian languages: Swedish and Danish sol, Icelandic sól, etc.
The principal adjectives for the Sun in English are sunny for sunlight and, in technical contexts, solar (/ˈsoʊlər/), from Latin sol – the latter found in terms such as solar day, solar eclipse and Solar System. From the Greek helios comes the rare adjective heliac (/ˈhiːliæk/). In English, the Greek and Latin words occur in poetry as personifications of the Sun, Helios (/ˈhiːliəs/) and Sol (/ˈsɒl/), while in science fiction Sol may be used to distinguish the Sun from other stars. The term sol with a lower-case s is used by planetary astronomers for the duration of a solar day on another planet such as Mars.
The English weekday name Sunday stems from Old English Sunnandæg "sun's day", a Germanic interpretation of the Latin phrase diēs sōlis, itself a translation of the ancient Greek ἡμέρα ἡλίου (hēmera hēliou) 'day of the sun'. The astronomical symbol for the Sun is a circle with a center dot, 
☉
It is used for such units as M☉ (Solar mass), R☉ (Solar radius) and L☉ (Solar luminosity).
== General characteristics ==
The Sun is a G-type main-sequence star that makes up about 99.86% of the mass of the Solar System. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is a Population I, or heavy-element-rich, star. Its formation may have been triggered by shockwaves from one or more nearby supernovae. This 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 heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star.
The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46.
One astronomical unit (about 150,000,000 km; 93,000,000 mi) is defined as the mean distance of the Sun's center to Earth's center, though the distance varies (by about +/- 2.5 million km or 1.55 million miles) as Earth moves from perihelion on about 3 January to aphelion on about 4 July. The distances can vary between 147,098,074 km (perihelion) and 152,097,701 km (aphelion), and extreme values can range from 147,083,346 km to 152,112,126 km. At its average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. 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 center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometers (6.2 mi). The tidal effect of the planets is weak and does not significantly affect the shape of the Sun. The Sun rotates faster at its equator than at its poles. This differential rotation is caused by convective motion due to heat transport and the Coriolis force due to the Sun's rotation. In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at the poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days. Viewed from a vantage point above its north pole, the Sun rotates counterclockwise around its axis of spin.
From NASA's fact sheet:
Comparison between Sun and Earth: Bulk parameters
 	Sun 	Earth 	Ratio(Sun/Earth)
Mass 	1,988,500.	5.9724 	333,000.
GM 	132,712. 	0.39860	333,000.
Volume 	1,412,000.	1.083 	1,304,000.
Volumetric mean radius 	695,700. 	6371. 	109.2
Mean density 	1408. 	5514. 	0.255
Surface gravity (eq.) 	274.0 	9.78 	28.0
Escape velocity 	617.6 	11.19 	55.2
Ellipticity 	0.00005 	0.0034 	0.015
Moment of inertia 	0.070 	0.3308 	0.212
Visual magnitude V(1,0)	-26.74 	-3.86 	-
Absolute magnitude 	+4.83 	 	
Luminosity 	382.8 	 	
Mass conversion rate 	4260. 	 	
Mean energy production 	0.1925 	 	
Surface emission 	62.94 	 	
Spectral type 	G2 V 	 	
Category:Star