The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years. Its mean distance from Earth is about 1.496×108 kilometres or about 8 light-minutes.
The distance between the Sun and the Earth was used to define a unit of length called the astronomical unit (au), now defined to be 149.5978707×106 kilometres.
It is the largest and most massive object in the Solar System; its diameter is about 1,391,400 km (864,600 mi), around 109 times that of Earth. The Sun's mass is around 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System.
The mass of the Sun's surface layer, its photosphere, consists mostly of hydrogen (~73%) and helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.
The Sun 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 centre; 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 now classified as a G-type main-sequence star (G2V). Every second, the Sun's core fuses about 600 billion kilograms (kg) of hydrogen into helium and converts four billion kilograms of matter into energy.
About four to seven billion years from now, 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 cause its outer layers to expand, eventually transforming the Sun into a red giant.
After the red giant phase, models suggest 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 will still glow and give off heat from its previous fusion for perhaps trillions of years. After that, it is theorised to become an extremely dense black dwarf, giving off negligible energy.
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 स्वर् (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.
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 lowercase s is used by planetary astronomers for the duration of a solar day on another planet such as Mars.
The astronomical symbol for the Sun is a circle with a central dot: ☉. It is used for such units as M☉ (Solar mass), R☉ (Solar radius) and L☉ (Solar luminosity). The scientific study of the Sun is called heliology.
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 is classed as a G2 star, meaning it is a G-type star, with 2 indicating its surface temperature is in the second range of the G class. It 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.
It is more massive than 95% of the stars within 7 pc (23 ly). The Sun is a Population I, or heavy-element-rich, star. Its formation approximately 4.6 billion years ago 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.
Shape
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 centre to the edge of the photosphere, the apparent visible surface of the Sun. The roundness of the Sun is the relative difference between its radius at its equator, , and at its pole, , called the oblateness,
The value is difficult to measure. Atmospheric distortion means the measurement must be done on satellites; the value is very small meaning very precise technique is needed.
The oblateness was once proposed to be sufficient to explain the perihelion precession of Mercury but Einstein proposed that general relativity could explain the precession using a spherical Sun. When high precision measurements of the oblateness became available via the Solar Dynamics Observatory and the Picard satellite the measured value was even smaller than expected, 8.2×10−6, or 8 parts per million.
These measurements determined the Sun to be the natural object closest to a perfect sphere ever observed. The oblateness value remains constant independent of solar irradiation changes. The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.
Rotation
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.
A survey of solar analogues suggests the early Sun was rotating up to ten times faster than it does today. This would have made the surface much more active, with greater X-ray and UV emission. Sunspots would have covered 5%–30% of the surface.
The rotation rate was gradually slowed by magnetic braking, as the Sun's magnetic field interacted with the outflowing solar wind. A vestige of this rapid primordial rotation still survives at the Sun's core, which rotates at a rate of once per week; four times the mean surface rotation rate.
Composition
The Sun's photosphere consists mainly of the elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.
The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. Typically, the solar heavy-element abundances described above are measured both by using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures.
These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by settling of heavy elements within the Sun. The two methods generally agree well.
The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.
The interior composition of the Sun is not known. Estimates based on models of stellar nuclear reactions give different values than estimates based on observations of solar seismology.
Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over the past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the centre of the Sun because of gravity.
The proportions of heavier elements are unchanged. Heat is transferred outward from the Sun's core by radiation rather than by convection (see Radiative zone below), so the fusion products are not lifted outward by heat; they remain in the core, and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium.
In the current photosphere, the helium fraction is reduced, and the metallicity is only 84% of what it was in the protostellar phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the main sequence and become a red giant.
Structure
Core
The core of the Sun extends from the centre to about 20–25% of the solar radius. It has a density of up to 150 g/cm3 (about 150 times the density of liquid water) and a temperature of close to 15.7 million kelvins (K).
By contrast, the Sun's surface temperature is about 5800 K. Recent analysis of SOHO mission data favours the idea that the core is rotating faster than the radiative zone outside it. Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through the proton–proton chain; this process converts hydrogen into helium.
Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous.
The core is the only region of the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).
The proton–proton chain occurs around 9.2×1037 times each second in the core, converting about 3.7×1038 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9×1056 free protons in the Sun), or about 6.2×1011 kg/s.
However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen), for 384.6 yottawatts (3.846×1026 W), or 9.192×1010 megatons of TNT per second.
The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per cubic metre. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the centre of the core, which, according to Karl Kruszelnicki, is about the same power density inside a compost pile.
The fusion rate in the core is in a stable equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.
Radiative zone
The radiative zone is the thickest layer of the Sun. It starts above the core at about 0.25 solar radii and out to about 0.7 solar radii. The zone is so named because thermal radiation is the primary means of energy transfer: photons scatter from dense gas so often that they take a million years to cross this zone. The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.
This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why the transfer of energy through this zone is by radiation instead of thermal convection. The density drops a hundredfold (from 20,000 kg/m3 to 200 kg/m3) between 0.25 solar radii and 0.7 radii, the top of the radiative zone.
Tachocline
The radiative zone and the convective zone are separated by a transition layer, the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear between the two—a condition where successive horizontal layers slide past one another.
Presently, it is hypothesised that a magnetic dynamo, or solar dynamo, within this layer generates the Sun's magnetic field.
Convective zone
The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface. In this layer, the solar plasma is not dense or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface.
Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry most of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues.
At the photosphere, the temperature has dropped 350-fold to 5,700 K (9,800 °F) and the density to only 0.2 g/m3 (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone).
The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun. The Sun's thermal columns are Bénard cells and take the shape of roughly hexagonal prisms.
Atmosphere
The solar atmosphere is the region of the Sun that extends from the top of the convection zone to the inner boundary of the heliosphere. It is often divided into three primary layers: the photosphere, the chromosphere, and the corona. The chromosphere and corona are separated by a thin transition region that is frequently considered as an additional distinct layer. Some sources consider the heliosphere to be the outer or extended solar atmosphere.
