Editing Sol (section)

==Structure==
1. Core <br />
2. Radiative zone<br />
3. Convective zone<br />
4. Photosphere<br />
5. Chromosphere<br />
6. Corona<br />
7. Sunspot<br />
8. Granules<br />
9. Prominence

The Sun is a yellow main sequence star comprising about 99% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a Plasma (physics) plasmatic state and is not solid, it rotates faster at its equator than at its poles.  This behavior is known as differential Solar rotation. The period of this ''actual rotation'' is approximately 25 days at the equator and 35 days at the poles.  However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the ''apparent rotation'' of the star at its equator is about 28 days.  The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun.

The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops approximately exponentially with increasing distance from its center. Nevertheless, it has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye. The solar core comprises 10 percent of its total volume, but 40 percent of its total mass.

The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the star's inner structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.

===Core===

The Solar core core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150,000 kg/m³ (150 times the density of water on Earth) and a temperature of close to 13,600,000 kelvin (by contrast, the surface of the Sun is around 5,800 kelvin). Recent analysis of Solar and Heliospheric Observatory (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 Proton-proton chain reaction–p (proton–proton) chain; this process converts hydrogen into helium. The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

About 3.4e38 protons (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9e56 total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million tonnes per second, 383 Yotta-yottawatts (3.83e26 W) or 9.15e10 megatons of Trinitrotoluene (TNT) per second. This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 W/m³ (watts per cubic meter).  This is less power than generated by a candle.  Power density is about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, roughly a million times greater per unit mass. The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW fusion power plant would require about 170 billion tonnes of plasma occupying almost one cubic mile. Hence, terrestrial fusion reactors utilize far higher plasma temperatures than those in Sun's interior.

The rate of nuclear fusion depends strongly on density and temperature, so the fusion rate in the core is in a self-correcting 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 fusion rate and correcting the Perturbation (astronomy)perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.

The high-energy photons (gamma rays) released in nuclear fusion reactions are absorbed in only few millimeters of solar plasma and then re-emitted again in random direction (and at slightly lower energy)—so it takes a long time for radiation to reach the Sun's surface. Estimates of the "photon travel time" range between 10,000 and 170,000 years. 

After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were Solar neutrino problem lower than theories predicted by a factor of 3. This discrepancy was recently resolved through the discovery of the effects of neutrino oscillation: the Sun in fact emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 2/3 of them because the neutrinos had changed flavor (particle physics)flavor.

===Radiative zone===
From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone there is no thermal convection; while the material grows cooler as altitude increases, this temperature gradient is less than the value of adiabatic lapse rate and hence cannot drive convection. Heat is transferred by radiation- ions of hydrogen and helium emit photons, which travel a brief distance before being reabsorbed by other ions.  In this way energy makes its way very slowly (see above) outward.

Between the radiative zone and the convection zone is a transition layer called 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- a condition where successive vertical layers slide past one another.

===Convection zone===

In the Sun's outer layer (down to approximately 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone.

The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the granule (solar physics)solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.

The Sun's thermal columns are Bénard cells and therefore tend to be hexagonal prisms.

===Photosphere===
The effective temperature, or black body temperature, of the Sun (5777 K) is the temperature a black body of the same size must have to yield the same total emissive power.

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opacity (optics)opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. 
The change in opacity is due to the decreasing amount of H<sup>-</sup> ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H<sup>-</sup> ions. The photosphere is actually tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or ''limb'' of the solar disk, in a phenomenon known as limb darkening. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 kelvin, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1% of the particle density of Earth's atmosphere at sea level.

During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. It was not until 25 years later that helium was isolated on Earth. 

===Atmosphere===
During a total solar eclipse, the solar corona can be seen with the naked eye.
The parts of the Sun above the photosphere are referred to collectively as the ''solar atmosphere''. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the ''temperature minimum'', the chromosphere, the solar transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock waveshock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason why has not been conclusively proven; evidence suggests that Alfvén waves may have enough energy to heat the corona. 

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 Kelvin. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.

Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the ''chromosphere'' from the Greek root ''chroma'', meaning color, because the chromosphere is visible as a colored flash at the beginning and end of solar eclipsetotal eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.

Above the chromosphere is a solar transition region in which the temperature rises rapidly from around 100,000 kelvin to coronal temperatures closer to one million K. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of Halo (optical phenomenon)nimbus around chromospheric features such as Spicule (solar physics)spicules and Solar filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from outer space by instruments sensitive to the far ultraviolet portion of the electromagnetic spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the Solar System and heliosphere. The low corona, which is very near the surface of the Sun, has a very low partical density compared to the particle density of Earth's atmosphere near sea level. The temperature of the corona is several million kelvin. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the Solar System. Its inner boundary is defined as the layer in which the flow of the solar wind becomes ''superalfvénic'';that is, where the flow becomes faster than the speed of Alfvén waves. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a Parker spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.