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For other uses, see Sun (disambiguation) and Sol (disambiguation).
The Sun (Latin: Sol) is the star at the center of the Solar System. It is a medium size star. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for about 99.8% of the solar system\'s mass. Energy from the Sun, in the form of sunlight and heat, supports almost all life on Earth via photosynthesis, and drives the Earth\'s climate and weather.
The surface composition of the Sun consists of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 24-25% of mass,Basu, Sarbani; Antia, H. M. (2007). "Helioseismology and Solar Abundances". Physics Reports. Retrieved on 2007-12-09. 7% of volume), and trace quantities of other elements, including Fe, Ni, O, Si, S, Mg, C, Ne, Ca, and Cr.Manuel O. K. and Hwaung Golden (1983), Meteoritics, Volume 18, Number 3, 30 September 1983, pp 209-222. Online: http://web.umr.edu/~om/archive/SolarAbundances.pdf (retrieved 7 December 2007 20:21 UTC). The Sun has a spectral class of G2V. G2 implies that it has a surface temperature of approximately 5,780 K, giving it a white color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of blue photons (causing the sky color) removes enough blue light to leave a residual reddishness that is perceived as yellow. (When low enough in the sky, the Sun appears orange or red, due to this scattering.)
Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) suffix indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic equilibrium, neither contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now known to be brighter than 85% of the stars in the galaxy, most of which are red dwarfs.Than, Ker. "Astronomers Had it Wrong: Most Stars are Single", SPACE.com, January 30, 2006. Retrieved on 2007-08-01.
The Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 light-years from the galactic center, completing one revolution in about 225–250 million years. Its approximate orbital speed is 220 kilometers per second, plus or minus 20 km/s. This is equivalent to about one light-year every 1,400 years, and about one AU every 8 days. These measurements of galactic distance and speed are as accurate as we can get given our current knowledge, but will change as we learn more.Kerr, F. J.; Lynden-Bell D. (1986). "Review of galactic constants" (PDF). Monthly Notices of the Royal Astronomical Society 221: 1023–1038.
The Sun is currently traveling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 light years from the Earth, the sun ranks 4th in absolute magnitude as a fourth magnitude star (M=4.83).
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The Sun is a Population I, or third generation, star whose formation may have been triggered by shockwaves from one or more nearby supernovae.Falk, S. W.; Lattmer, J. M., Margolis, S. H. (1977). "Are supernovae sources of presolar grains?". Nature 270: 700-701. This is suggested by a high abundance of heavy elements such as gold and uranium in the solar system. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.
Sunlight is Earth\'s primary source of energy. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,370 watts per square meter at a distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by the Earth\'s atmosphere so that less power arrives at the surface—closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. This energy can be harnessed via a variety of natural and synthetic processes—photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.
Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly attenuated by Earth\'s ozone layer, so that the amount of UV varies greatly with latitude and has been responsible for many biological adaptations, including variations in human skin color in different regions of the globe.Barsh G.S., 2003, What Controls Variation in Human Skin Color?, PLoS Biology, v. 1, p. 19
Observed from Earth, the Sun\'s path across the sky varies throughout the year. The shape described by the Sun\'s position, considered at the same time each day for a complete year, is called the analemma and resembles a figure 8 aligned along a north/south axis. While the most obvious variation in the Sun\'s apparent position through the year is a north/south swing over 47 degrees of angle (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an east/west component as well, caused by the acceleration of the Earth as it approaches its perihelion with the sun, and the reduction in the Earth\'s speed as it moves away to approach its aphelion. The north/south swing in apparent angle is the main source of seasons on Earth.
The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum. The Sun\'s magnetic field gives rise to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System. Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth\'s outer atmosphere.
Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered. Current topics of scientific inquiry include the Sun\'s regular cycle of sunspot activity, the physics and origin of flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin (propulsion source) of solar wind.
The Sun\'s current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.Bonanno, A.; Schlattl, H.; Patern, L. (2002). "The age of the Sun and the relativistic corrections in the EOS" (PDF). Astronomy and Astrophysics 390: 1115–1118.
It is thought that about 4.59 billion years ago, the rapid collapse of a hydrogen molecular cloud led to the formation of a third generation T Tauri Population I star, the Sun. The nascent star assumed a nearly circular orbit about 26,000 light-years from the centre of the Milky Way Galaxy.According to isotopicAges, the Ca-Al-I\'s (= Ca-Al-rich inclusions) here formed in a proplyd (= protoplanetary disk]). The page Protoplanetary disk says that proplyds are never older than 25 Ma. If 4567 Ma is given for the age of the Earth, then 4567 + 25 = 4592. But 25 Ma is the "maximum age" of proplyds. If proplyds slowly decay from the influence of the Sun and from planetesimal formation, then most Ca-Al-I\'s must have been formed some time within the range of 0 Ma and 25 Ma after the formation of the proplyd. If the median of Ca-Al-I ages are about 10 Ma after the proplyd formation, then we get 4565 + 10 = 4575, but this figure is created by speculating twice. Since it is assumed that planetary formation occurs over a period of about 100,000 years, that is the date given here
The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun\'s core, producing neutrinos and solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.
The Sun does not have enough mass to explode as a supernova. Instead, in 5–6 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 MK, and will produce carbon and oxygen, entering the asymptotic giant branch of a planetary nebula phase in about 7.8 billion years, during which instabilities in interior temperature will lead the surface of the sun to shed mass. Earth\'s fate is not clear. The sun will certainly expand to envelop Earth\'s current orbit, but due to mass loss of the sun as a red giant, the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the sun due to tidal interactions.Klaus-Peter Schroeder, Robert C. Smith (2008). "Distant future of the Sun and Earth revisited". ArXiv Astrophysics e-prints. Even if Earth escaped incineration in the sun, its water would be boiled away and most of its atmosphere would escape into space. In fact, even during its life in the main sequence the sun is gradually becoming more luminous, its surface temperature slowly rising. The increase in solar temperatures is such that in about 900 million years, the surface of the Earth will become too hot for the survival of life as we know it.Guillemot, H.; Greffoz, V. (Mars 2002). "Ce que sera la fin du monde" (in French). Science et Vie N° 1014. After another billion years the surface water will have completely disappeared.Carrington, Damian. "Date set for desert Earth", BBC News, February 21, 2000. Retrieved on 2007-03-31.
Life-cycle of the Sun; sizes are not drawn to scale.
Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.Pogge, Richard W. (1997). The Once and Future Sun (lecture notes). New Vistas in Astronomy. The Ohio State University (Department of Astronomy). Retrieved on 2005-12-07.Sackmann, I.-Juliana; Arnold I. Boothroyd; Kathleen E. Kraemer (11 1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457.
An illustration of the structure of the Sun
The Sun is a yellow dwarf star. It comprises approximately 99% of the total mass of the solar system. The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths,Godier, S.; Rozelot J.-P. (2000). "The solar oblateness and its relationship with the structure of the tachocline and of the Sun\'s subsurface" (PDF). Astronomy and Astrophysics 355: 365–374. which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it undergoes differential rotation as it spins on its axis (i.e. the Sun rotates faster at its equator than at its poles). 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 Sun 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. Also, the tidal effect from the planets does not significantly affect the shape of the Sun.
The Sun does not have a definite boundary as rocky planets do; in its outer parts the density of its gases drops approximately exponentially with increasing distance from the center of the Sun. Nevertheless, the Sun 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; the photosphere is 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.Hannah Cohen (2007-05-16). From Core to Corona: Layers of the Sun. Princeton Plasma Physics Laboratory (PPPL).
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 Sun\'s inner structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.
Cross-section of a solar-type star. (NASA)
The 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 kelvins (by contrast, the surface of the Sun is around 5,800 kelvins). Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone.Garcia R. A. et al. "Tracking Solar Gravity Modes: The Dynamics of the Solar Core", Science, 316, 5831, 1591 - 1593 (2007) Through most of the Sun\'s life, energy is produced by nuclear fusion through a series of steps called the p–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 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9×1056 total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (3.83×1026 W) or 9.15×1010 megatons of TNT per second. This actually corresponds to a surprisingly low rate of energy production in the Sun\'s core—about 0.3 µW/cm³ (microwatts per cubic cm), or about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, millions of 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; 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 fusion reactions are absorbed in only few millimetres 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.The 8-minute travel time to Earth by sunlight hides a thousand-year journey that actually began in the core.
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 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.
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 sheer.
Structure of the Sun
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 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 visible surface of the Sun, the photosphere, is the layer below which the Sun becomes 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- ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H- ions.Gibson, Edward G. (1973). The Quiet Sun. NASA. Shu, Frank H. (1991). The Physics of Astrophysics. University Science Books. 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 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023 m−3 (this is 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.Discovery of Helium. Solar and Magnetospheric MHD Theory Group. University of St Andrews. Retrieved on 2006-03-22.
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 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 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.De Pontieu, Bart; et al (2007-12-07). "Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind". Science 318 (5856): 1574 - 77. doi:10.1126/science.1151747. Retrieved on 2008-01-22.
The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 K. 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 total eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.
Taken by Hinode\'s Solar Optical Telescope on January 12, 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.
Above the chromosphere is a transition region in which the temperature rises rapidly from around 100,000 K 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 nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth\'s surface, but is readily observable from space by instruments sensitive to the far ultraviolet portion of the 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 particle density of 1014–1016 m−3. (Earth\'s atmosphere near sea level has a particle density of about 2×1025 m−3.) The temperature of the corona is several million kelvins. 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 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.European Space Agency (2005-03-15). The Distortion of the Heliosphere: our Interstellar Magnetic Compass. Retrieved on 2006-03-22.
The Sun is composed of chemical elements. Various scientists have analysed these elements to find out their abundances, their relations to planetary elements, and their diffusion (distribution) within the solar interior.
According to Bahcal (1990)Bahcall, J. N. 1990, Neutrino Astrophysics (Cambridge University Press, Cambridge) cited in Thoul (1993:15),Thoul et al 1993: http://arxiv.org/abs/astro-ph/9304005 the characteristic mass fractions of some elements are:
In 1968, a Belgian academic found that the abundances of lithium, beryllium, and boron are higher than previously thought (Grevesse 1968Nicolas Grevesse 1968, Solar abundances of lithium, beryllium and boron, Solar Physics Journal, Volume 5, Number 2 / October, 1968, DOI 10.1007/BF00147963, pp 159-180, Springer Netherlands, ISSN 0038-0938 (Print) ISSN 1573-093X (Online), http://www.springerlink.com/content/l37qghqnm7345247/).
In 2005, three academics claimed that the neon abundance in the Sun may be higher than previously thought, based on helioseismological observations (Bahcall et al 2005Bahcall John N., Basu Sarbani, Sereneli Aldo M. 2005: What Is the Neon Abundance of the Sun?, The Astrophysical Journal, 631:1281–1285, 2005 October 1, DOI: 10.1086/431926, The American Astronomical Society (USA), http://www.journals.uchicago.edu/doi/abs/10.1086/431926).
It is also interesting to note that until at least 1986 the generally accepted initial helium content of the Sun was Y=0.25, but two academics in 1986 claimed that the value Y=0.279 is more correct (Lebreton and Maeder 1986:119Lebreton, Y. & Maeder, A. (1986), The evolution and helium content of the sun, Astronomy and Astrophysics (ISSN 0004-6361), vol. 161, no. 1, June 1986, p. 119-124., http://articles.adsabs.harvard.edu//full/1986A%26A...161..119L/0000119.000.html).
In 1970s, much research focused on the abundances of iron group elements in the Sun(Biemont 1978;Biemont Emile, 1978: Abundances of singly-ionized elements of the iron group in the sun, Royal Astronomical Society, Monthly Notices, vol. 184, Sept. 1978, p. 683-694, http://adsabs.harvard.edu/abs/1978MNRAS.184..683B and Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977, cited in Biemont 1978).
The first largely complete set of gf values of singly-ionised iron group elements were made available first by Corliss and Bozman (1962 cited in Biemont 1978) and Warner (1967 cited in Biemont 1978), and improved f values were computed by Smith (1976 cited in Biemont 1978). In 1978 the abundances of singly-ionised elements of the iron group were derived by Biemont (1978).
The abundance determination of some iron group elements is difficult because of their hyperfine structures, eg cobalt and manganese (Biemont 1978).
Various authors have considered the existence of a mass fractionation relationship between the isotopic compositions of solar and planetary noble gases (Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and Manuel 1970; Srinivasan and Manuel 1971, all cited in Manuel and Hwaung 1983), for example correlations between isotopic compositions of planetary and solar Ne and Xe (Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7). Nevertheless, the belief that the whole Sun has the same composition as the solar atmosphere was still widespread, at least until 1983 (Manuel and Hwaung 1983:7).
In 1983, two academics claimed that it was the fractionation in the Sun itself that caused the fractionation relationship between the isotopic compositions of planetary and solar wind implanted noble gases (Manuel and Hwaung 1983:7).
The Sun is composed of chemical elements. Of particular scientific interest is the diffusion of these elements inside the Sun, i.e. their distribution inside the star\'s interior. The diffusion of solar elements is determined by many variables, including gravity, which causes the heavier elements (e.g. helium in absence of other heavier elements) to stick to the centre of the solar mass while the non-heavy elements (e.g. hydrogen) diffuse towards the exterior of the Sun (Thoul et al 1993:3).
Of specialist scientific interest is the diffusion of helium in the solar interior. It has been found that the diffusion process of helium speeds up with time (Noerdlinger 1977Noerdlinger, P. D., Diffusion of helium in the Sun, Astronomy and Astrophysics, vol. 57, no. 3, May 1977, p. 407-415, online: http://adsabs.harvard.edu/full/1977A&A....57..407N).
The composition of the photosphere, ie the surface layers of the Sun, is usually taken as representative of the chemical composition of the primordial solar system, except for deuterium, lithium, boron, and berrylium (Aller 1968Aller L. H. (1968): The chemical composition of the Sun and the solar system, Proceedings of the Astronomical Society of Australia, Vol. 1, p.133, http://adsabs.harvard.edu/full/1968PASAu...1..133A).
Measurements of solar cycle variation during the last 30 years.
When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometers across.
The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer\'s law. Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.
History of the number of observed sunspots during the last 250 years, which shows the ~11-year solar cycle.
The solar cycle has a great influence on space weather, and is a significant influence on the Earth\'s climate. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures.Lean, J.; Skumanich A.; White O. (1992). "Estimating the Sun\'s radiative output during the Maunder Minimum". Geophysical Research Letters 19: 1591–1594. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.
A recent theory claims that there are magnetic instabilities in the core of the Sun which cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles. Like many theories in astrophysics, this theory cannot be tested directly. Ehrlich, Robert (2007). "Solar Resonant Diffusion Waves as a Driver of Terrestrial Climate Change". Journal of Atmospheric and Solar-Terrestrial Physics. (27 Jan. 2007) "Sun\'s fickle heart may leave us cold". New Scientist 2588: 12.
For many years the number of solar electron neutrinos detected on Earth was one third to one half of the number predicted by the standard solar model. This anomalous result was termed the solar neutrino problem. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun\'s interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate—that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth.Haxton, W. C. (1995). "The Solar Neutrino Problem" (PDF). Annual Review of Astronomy and Astrophysics 33: 459–504. Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and do indeed oscillate.Schlattl, H. (2001). "Three-flavor oscillation solutions for the solar neutrino problem". Physical Review D 64 (1). Moreover, in 2001 the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun\'s total neutrino emission rate agreed with the Standard Solar Model, although depending on the neutrino energy as few as one-third of the neutrinos seen at Earth are of the electron type. This proportion agrees with that predicted by the Mikheyev-Smirnov-Wolfenstein effect (also known as the matter effect), which describes neutrino oscillation in matter. Hence, the problem is now resolved.
The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K. Above it lies the solar corona at a temperature of 1,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.
It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events.Alfvén, H. (1947). "Magneto-hydrodynamic waves, and the heating of the solar corona". Monthly Notices of the Royal Astronomical Society 107: 211.
Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.Sturrock, P. A.; Uchida, Y. (1981). "Coronal heating by stochastic magnetic pumping" (PDF). Astrophysical Journal 246: 331. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales,Parker, E. N. (1988). "Nanoflares and the solar X-ray corona" (PDF). Astrophysical Journal 330: 474. but this remains an open topic of investigation.
Theoretical models of the Sun\'s development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth\'s surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than it is today. The consensus among scientists is that the young Earth\'s atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide, methane and/or ammonia) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.Kasting, J. F.; Ackerman, T. P. (1986). "Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere". Science 234: 1383–1385.
The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun\'s rotating magnetic field on the plasma in the interplanetary medium.The Mean Magnetic Field of the Sun. The Wilcox Solar Observatory. Retrieved on 2007-08-01.
All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 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). This twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun\'s magnetic field reverses itself about every 11 years.
The influence of the Sun\'s rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun\'s magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun\'s 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.
The Trundholm Sun chariot pulled by a horse is a sculpture believed to be illustrating an important part of Nordic Bronze Age mythology.
Humanity\'s most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon. Worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt, and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes. With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so Greek astronomers considered it to be one of the seven planets (Greek planetes, "wanderer"), after which the seven days of the week are named in some languages.
One of the first people to offer a scientific explanation for the Sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this heresy, he was imprisoned by the authorities and sentenced to death, though he was later released through the intervention of Pericles. Eratosthenes might have been the first person to have accurately calculated the distance from the Earth to the Sun, in the 3rd century BCE, as 149 million kilometers, roughly the same as the modern accepted figure.
The theory that the Sun is the center around which the planets move was apparently proposed by the ancient Greek Aristarchus and Indians (see Heliocentrism). This view was revived in the 16th century by Nicolaus Copernicus. In the early 17th century, the invention of the telescope permitted detailed observations of sunspots by Thomas Harriot, Galileo Galilei and other astronomers. Galileo made some of the first known Western observations of sunspots and posited that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun.Galileo Galilei (1564–1642). BBC. Retrieved on 2006-03-22. Sunspots were also observed since the Han dynasty and Chinese astronomers maintained records of these observations for centuries. In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun. Isaac Newton observed the Sun\'s light using a prism, and showed that it was made up of light of many colors,Sir Isaac Newton (1643–1727). BBC. Retrieved on 2006-03-22. while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum.Herschel Discovers Infrared Light. Cool Cosmos. Retrieved on 2006-03-22. The 1800s saw spectroscopic studies of the Sun advance, and Joseph von Fraunhofer made the first observations of absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. When expanding the spectrum of light from the Sun, there are large number of missing colors can be found.
In the early years of the modern scientific era, the source of the Sun\'s energy was a significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat.Thomson, Sir William (1862). "On the Age of the Sun’s Heat". Macmillan\'s Magazine 5: 288–293. Kelvin and Hermann von Helmholtz then proposed the Kelvin-Helmholtz mechanism to explain the energy output. Unfortunately the resulting age estimate was only 20 million years, well short of the time span of several billion years suggested by geology. In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.Lockyer, Joseph Norman (1890). The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems. London and New York: Macmillan and Co..
Not until 1904 was a substantiated solution offered. Ernest Rutherford suggested that the Sun\'s output could be maintained by an internal source of heat, and suggested radioactive decay as the source.Darden, Lindley (1998). The Nature of Scientific Inquiry. However it would be Albert Einstein who would provide the essential clue to the source of the Sun\'s energy output with his mass-energy equivalence relation E = mc².
In 1920 Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass.Studying the stars, testing relativity: Sir Arthur Eddington. ESA Space Science (June 15, 2005). Retrieved on 2007-08-01. The preponderance of hydrogen in the sun was confirmed in 1925 by Cecilia Payne. The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.Bethe, H. (1938). "On the Formation of Deuterons by Proton Combination". Physical Review 54: 862–862. Bethe, H. (1939). "Energy Production in Stars". Physical Review 55: 434–456.
Finally, a seminal paper was published in 1957 by Margaret Burbidge, entitled "Synthesis of the Elements in Stars".E. Margaret Burbidge; G. R. Burbidge; William A. Fowler; F. Hoyle (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics 29 (4): 547–650.
