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{| border="2" cellpadding="4" cellspacing="0" style="margin: 0 0 1em 1em; border: 1px #aaa solid; border-collapse: collapse;" align="right" width=280px
|+
|+ '''The Sun''' &nbsp; <math> \bigodot</math>
|+
|-
| colspan="2" align="center" | ]
|-
! bgcolor="#ffffc0" colspan="2" align="center" | '''Observation data'''
|-
! align="left" | Mean distance from<br>]
| ]{{e|6}} ]<br>(92.95{{e|6}} ]) <br>(8.31 minutes at the ])
|-
! align="left" | ] (''V'')
| &minus;26.8<sup>m</sup>
|-
! align="left" | ]
| 4.8<sup>m</sup>
|-
! bgcolor="#ffffc0" colspan="2" align="center" | ''']al characteristics'''
|-
! align="left" | Mean distance from<br>] core
| ~2.5{{e|17}} km <br>(26,000-28,000 ]s)
|-
! align="left" | ] period
| 2.25-2.50{{e|8}} ]
|-
! align="left" | Velocity
| 217 km/] orbit about the center of the Galaxy, 20 km/s relative to average velocity of other stars in neighborhood
|-
! bgcolor="#ffffc0" colspan="2" align="center" | '''Physical characteristics'''
|-
! align="left" | Mean diameter
| ]{{e|6}} km<br>(109 ]s)
|-
! align="left" | Circumference
| ]{{e|6}} km<br>(109 ]s)
|-
! align="left" | Oblateness
| 9{{e|&minus;6}}
|-
! align="left" | Surface area
| ]{{e|12}} ]<br>(11,900 Earths)
|-
! align="left" | Volume
| ]{{e|18}} ]<br>(1,300,000 Earths)
|-
! align="left" | Mass
| ]{{e|30}} ]<br>
(332,950 Earths)
|-
! align="left" | Density
| 1.408 g/cm³
|-
! align="left" | Surface ]
| 273.95 m s<sup>-2</sup><br>
(27.9 ])
|-
! align="left" | ]<br> from the surface
| 617.54 km/s
|-
! align="left" | Surface temperature
| 5780 ]
|-
! align="left" | Temperature of ]
| 5 ]K
|-
! align="left" | Core temperature
| ~13.6 MK
|-
! align="left" | ] (''L<sub>sol</sub>'')
| 3.827{{e|26}} ]<br>3.9{{e|28}} ]<br>or 100 lm/W ]
|-
! align="left" | Mean ] (''I<sub>sol</sub>'')
| 2.009{{e|7}} W m<sup>-2</sup> sr<sup>-1</sup>
|-
! bgcolor="#ffffc0" colspan="2" align="center" | '''] characteristics'''
|-
! align="left" | ]
| 7.25] <br>(to the ]) <br>67.23° <br>(to the ])
|-
! align="left" | ]<br>of North pole <sup></sup>
| 286.13° <br>(19 h 4 min 30 s)
|-
! align="left" | ]<br>of North pole
| +63.87°<br>(63°52' North)
|-
! align="left" | ]<br>at equator
| 25.3800 ]s <br>(25 d 9 h 7 min 13 s)
|-
! align="left" | Rotation velocity<br>at equator
| 7174 km/h
|-
! bgcolor="#ffffc0" colspan="2" align="center" | '''] composition (by mass)'''<br>(All in the ] state)
|-
! align="left" | ]
| 73.46 %
|-
! align="left" | ]
| 24.85 %
|-
! align="left" | ]
| 0.77 %
|-
! align="left" | ]
| 0.29 %
|-
! align="left" | ]
| 0.16 %
|-
! align="left" | ]
| 0.12 %
|-
! align="left" | ]
| 0.09 %
|-
! align="left" | ]
| 0.07 %
|-
! align="left" | ]
| 0.05 %
|-
! align="left" | ]
| 0.04 %
|}
The '''Sun''' is the ] G2V yellow ] at the center of the ], in the ] galaxy. The ] and other matter (including other ]s, ]s, ]s, ]s and ]) ] the Sun, which accounts for more than 99% of the solar system's ]. Different ]s of the Sun rotate at different rates; a point on the ] takes 25 days to complete a rotation, while a point at a pole takes 36 days. The resultant ] creates the Sun's very strong ] with its 11-year ] of activity. Energy from the Sun has supported almost all ] on Earth through ] since the appearance of the first organisms. Humans use ] to grow ]s and power ]s.

The Sun is a ball of ] with a diameter of 1.392 million km (864,950 mi) and a mass of about 2.0{{e|30}} ], which is somewhat greater than that of an average star in our galaxy. About 74% of its mass is ], with 25% ], and the rest made up of trace quantities of heavier elements. The Sun is about 4.6 ] (10<sup>9</sup>) years old and is about halfway through its ] evolution, during which ] reactions in its core fuse hydrogen into helium. About 5 million tons of ] are converted into ] within the Sun's core every second, producing ]s and ]. In about 5 billion years, the Sun will evolve into a ] and then a ], creating a ] in the process.

The Sun is a magnetically active star: it supports a strong, changing ] that varies year-to-year and reverses direction about every eleven years. The magnetic field gives rise to many effects that are collectively called ], including ]s on the surface of the Sun, ], and variations in the ] that carries material through the solar system. Effects of solar activity on the Earth include ]s at moderate to high latitudes, and disruption of ] communications and ]. Solar activity is thought to have played a large role in the ] and evolution of our ], and strongly affects the structure of Earth's ].

Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 10<sup>6</sup> ] when its visible surface (the ]) has a temperature of just 6,000 K. Current topics of scientific study include the sun's regular cycle of ] activity, the physics and origin of ]s and ], the magnetic interaction between the ] and the ], and the origin of the ].


==General information==
] from the surface of Earth.]]

The Sun has a ] of G2V: the G2 means that it has a surface temperature of about 5,500&nbsp;K, giving it a yellow color, and its spectrum contains ]s of ionized and neutral metals as well as very weak hydrogen lines, and the V indicates that it, like most stars, is a ] star. This means it generates its energy by ] of ] nuclei into ] and is in a state of ], neither contracting nor expanding over time.

The Sun will spend a total of approximately 10 ] years as a ]. Its current age, determined using ] of ] and ], is thought to be about 4.57 billion years.{{ref|Bonanno}} The Sun orbits the center of the ] ] at a distance of about 25,000 to 28,000 ]s from the ], completing one revolution in about ]. The ] is 220&nbsp;km/s, equivalent to one light-year every 1,400 years, and one ] every 8 days.{{ref|Kerr}}

The Sun is thought to be a second-generation star, whose formation may have been triggered by shockwaves from a nearby ]. This is suggested by a high ] of ] such as ], ] and ] in the solar system; the most plausible ways that these elements could be produced are by ] nuclear reactions during a supernova, or by ] via ] absorption inside a massive first-generation star.

The Sun does not have enough mass to explode as a supernova, and its mass is below the ]. Instead, in 4–5 ] years, it will enter a ] 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 about 3{{e|8}}&nbsp;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 ] phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a ]. The Sun will then evolve into a ], slowly cooling over eons. This ] scenario is typical of low- to medium-mass stars.{{ref|future-sun}}

Sunlight is the main source of energy near the surface of Earth. The ] 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&nbsp;]s per square meter of area at a distance of one ] from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is ] by the Earth's atmosphere so that less power arrives at the surface&mdash;closer to 1,000&nbsp;watts per directly exposed square meter in clear conditions when the Sun is near its ]. This energy can be harnessed via a variety of natural and synthetic processes&mdash;] 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 ] are used by ] equipment to generate ] or to do other useful work. The energy stored in ] was originally converted from sunlight by photosynthesis in the distant past.

Observed from Earth, the path of the Sun 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 ], is called the ] 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 (due to the 23.5-degree tilt of the Earth with respect to the Sun), there is an East/West component as well. The North/South swing in apparent angle is the main source of ] on Earth.

The Sun is sometimes referred to by its ] name '']''. Its ] is a ]: <math> \bigodot</math>. Some ancient peoples of the world considered it a ].

==Structure==
]

The Sun is a near-perfect ], with an ] estimated at about 9 millionths, {{ref|Godier}} which means that its polar diameter differs from its equatorial diameter by only 10&nbsp;km. While the Sun does not rotate as a solid body (the rotational period is 25 days at the ] and about 35 days at the ]), it takes approximately 28 days to complete one full rotation; the ] effect of this slow ] is 18 million times weaker than the surface gravity at the Sun's equator. Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the ] of the solar system, which is located nearly a solar radius away from the center of the Sun mostly because of the large mass of ].

The Sun does not have a definite boundary as rocky planets do; the density of its gases drops approximately ] 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 ]. This is simply the layer below which the gases are thick enough to be ] but above which they are ]; the photosphere is the surface most readily visible to the ]. Most of the Sun's mass lies within about 0.7 radii of the center.

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

===Core===
At the center of the Sun, where its ] reaches up to 150,000 kg/m<sup>3</sup> (150 times the density of ] on Earth), thermonuclear reactions (]) convert ] into ], producing the energy that keeps the Sun in a state of equilibrium. About 8.9{{e|37}} ] (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 ] (9.15{{e|16}} tons of ] per second). The fusion rate in the core is in a self-correcting ]: a slightly higher rate of fusion would cause the core to heat up more and ] slightly against the ] of the outer layers, reducing the fusion rate and correcting the ]; and a slightly lower rate would cause the core to shrink slightly, increasing the fusion rate and again reverting it to its present level.

The core extends from the center of the Sun to about 0.2 solar radii, and is the only part of the Sun in which an appreciable amount of heat is produced by fusion; the rest of the star is heated by energy that is transferred outward. All of the energy produced by interior fusion must travel through many successive layers to the solar photosphere before it escapes into space.

The high-energy ]s (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years{{ref|Lewis}} to as little as 17,000 years . After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as ]. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. ]s are also released by the fusion reactions in the core, but unlike photons they very 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 ], a problem which was recently resolved through a better understanding of the effects of ].

===Radiation 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 ]; while the material grows cooler as altitude increases, this temperature ] is slower than the ] and hence cannot drive convection. Heat is transferred by ]&mdash;] of hydrogen and helium emit ], which travel a brief distance before being reabsorbed by other ].

===Convection zone===
]

From about 0.7 solar radii to 1.0 solar radii, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, ] occurs as ] 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. ] is thought to occur at the base of the convection zone, carrying turbulent down flows 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 ] and ]. 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.

===Photosphere===
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<sup>-</sup> ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with ] atoms to produce H<sup>-</sup> ions. Sunlight has approximately a ] spectrum that indicates its temperature is about 6,000 ], interspersed with atomic ] from the tenuous layers above the photosphere. The photosphere has a particle density of about 10<sup>23</sup>/m<sup>3</sup> (this is about 1% of the particle density of ] at sea level).

During early studies of the ] of the photosphere, some absorption lines were found that did not correspond to any ]s then known on Earth, and ] hypothesized that they were due to a new element which he dubbed "]", after the Greek Sun god ]. It was not until 25 years later that helium was isolated on Earth .

===Atmosphere===

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 ], from ] through ] to ], and comprise five principal zones: the ''temperature minimum'', the ], the ], the ], and the ]. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of ] to the ], where it forms a sharp ] boundary with the ]. The chromosphere, transition region, and corona are much hotter than the surface of the Sun; the reason why is not yet known.

The coolest layer of the Sun is a temperature minimum region about 500&nbsp;km above the photosphere, with a temperature of about 4,000&nbsp;]. This part of the Sun is cool enough to support simple molecules such as ] and ], which can be detected by their absorption spectra. Above the temperature minimum layer is a thin layer about 2,000&nbsp;km thick, dominated by a spectrum of emission and absorption lines. It is called the '']'' from the Greek root ''chroma'', meaning color, because the chromosphere is visible as a colored flash at the beginning and end of ]. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000&nbsp;] near the top.

Above the chromosphere is a ] in which the temperature rises rapidly from around 100,000&nbsp;] to coronal temperatures closer to one million ]. The increase is due to a ] as ] within the region becomes fully ] by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of ] around chromospheric features such as ]s and ]s, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from ] by instruments sensitive to the ] portion of the ].

The ] is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the ] that fills the ] and ]. The low corona, which is very near the surface of the Sun, has a particle density of 10<sup>14</sup>/m<sup>3</sup>&ndash;10<sup>16</sup>/m<sup>3</sup>. (Earth's atmosphere near sea level has a particle density of about 2x10<sup>25</sup>/m<sup>3</sup>.) The temperature of the corona is several ]s. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be due to ].

The ] extends from approximately 20 solar radii (0.1&nbsp;]) to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the ] becomes ''superalfvénic''&mdash;that is, where the flow becomes faster than the speed of ]. 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 ] shape, until it impacts the ] more than 50 ] from the Sun. In ] ], the ] passed through a ] 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.

==Solar activity==

===Sunspots and the solar cycle===

]
]
When observing the Sun with appropriate filtration, the most immediately visible features are usually its ]s, which are well-defined surface areas that appear darker than their surroundings due to lower temperatures. Sunspots are regions of intense magnetic activity where energy transport is inhibited by strong magnetic fields. They are often the source of intense flares and ]. The largest sunspots can be tens of thousands of kilometres across.

The number of sunspots visible on the Sun is not constant, but varies over a 10-12 year cycle known as the ]. 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 ]. Sunspots usually exist as pairs with opposite magnetic polarity. The 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.

The solar cycle has a great influence on ], and seems also to have a strong influence on the Earth's climate. Solar minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 1600s, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during the period. During this era, which is known as the ] or ], Europe experienced very cold temperatures.{{ref|Lean}} Earlier extended minima have been discovered through analysis of ]s and also appear to have coincided with lower-than-average global temperatures.

===Effects on Earth===

Solar activity has several effects on the Earth and its surroundings. Because the Earth has a magnetic field, charged particles from the solar wind cannot impact the atmosphere directly, but are instead deflected by the magnetic field and aggregate to form the ]. The Van Allen belts consist of an inner belt composed primarily of ]s and an outer belt composed mostly of ]s. Radiation within the Van Allen belts can occasionally damage ]s passing through them.

The Van Allen belts form arcs around the Earth with their tips near the north and south poles. The most energetic particles can 'leak out' of the belts and strike the Earth's upper atmosphere, causing ], known as ''aurorae borealis'' in the ] and ''aurorae australis'' in the ]. In periods of normal solar activity, aurorae can be seen in oval-shaped regions centred on the ]s and lying roughly at a ] of 65&deg;, but at times of high solar activity the auroral oval can expand greatly, moving towards the equator. Aurorae borealis have been observed from locales as far south as ].

===Predicting the solar cycle===

Because the sunspot cycle is slightly ] it is difficult to determine in advance how strong a solar cycle will be or when it will occur. In ] ], ] and collaborators at ]/] made the world's first physics-based prediction of the next solar cycle, based on ]s of the ] in the manner of terrestrial ]. The next solar cycle (called 'Cycle 24' because it will be the 24th since regular sunspot observations began in Europe) is predicted to arrive around a year late (in late ] or early ]) and to be 30%-50% stronger than the previous solar cycle (which began in ] and peaked in ]) {{ref|Dikpati}}.

==Theoretical problems==
===Solar neutrino problem===
]).]]

For many years the number of solar ]s detected on Earth was only a third of the number expected, according to theories describing the nuclear reactions in the Sun. This anomalous result was termed the ]. 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 ], that is, change into undetectable ] and ]s as they traveled between the Sun and the Earth.{{ref|Haxton}} Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the ] and ]. Results from these observatories eventually led to the discovery that neutrinos have a very small ] and can indeed oscillate.{{ref|Schlattl}}

===Coronal heating problem===
The optical surface of the Sun (the ]) is known to have a temperature of approximately 6,000 ]. Above it lies the solar ] at a temperature of 1,000,000&nbsp;K. The high temperature of the corona shows that it is heated by something other than the ].

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 ] 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 ] heating, in which magnetic energy is continuously built up by photospheric motion and released through ] in the form of large ]s and myriad similar but smaller events.{{ref|Alfven}}

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except ]s have been found to dissipate or refract before reaching the corona.{{ref|Sturrock}} In addition, Alfven 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,{{ref|Parker2}} but this remains an open topic of investigation.

===Faint young sun problem===
{{main|Faint young sun paradox}}

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the ], 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 general consensus among scientists is that the young Earth's atmosphere contained much larger quantities of ]es (such as ] and/or ]) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.{{ref|Kasting}}

==Magnetic field==
] extends to the outer reaches of the Solar System, and results from the influence of the ]'s rotating magnetic field on the ] in the ] (]) . (click to enlarge)]]

All ] in the Sun is in the form of ] and ] due to 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 ] of the Sun's latitudes causes its ] 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 ]s and ]s (see ]). This twisting action gives rise to the ] and an 11-year ] 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 ] creates the ], which separates regions with magnetic fields pointing in different directions. The plasma in the ] 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<sup>-4</sup> ] magnetic dipole field would reduce with the cube of the distance to about 10<sup>-11</sup> tesla. But satellite observations show that it is about 100 times greater at around 10<sup>-9</sup> tesla. ] (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 ].

==History of solar observation==

===Early understanding of the Sun===

] pulled by a horse is a sculpture believed to be illustrating an important part of ] mythology.]]

Mankind's most fundamental understanding of the Sun is as the luminous disk in the ], whose presence above the ] creates ] and whose absence causes ]. In many prehistoric and ancient cultures, the Sun was thought to be a ] or other ] phenomenon, and ] of the Sun was central to civilisations such as the ] of ] and the ]s of what is now ]. Many ancient monuments were constructed with solar phenomena in mind&mdash;examples include ]s that accurately mark the ], in ], ], and also ] in ]; also the pyramid of ] at ] in ] is designed so that shadows give the illusion of serpents climbing the pyramid at the vernal and autumn ]es. With respect to the ], the Sun appears from Earth to revolve once a ] along the ] through the ], and so the Sun was considered by Greek astronomers to be one of the seven ] (Greek ''planetes'', "wanderer"), after which the seven days of the ] are named in some languages.

===Scientific understanding===
The concept of ], with the Sun at the centre of the solar system, was first suggested in ] by ] (circa ]) in his '']'', which referred to the Earth as a sphere and the Sun as the "centre of spheres". Based on this heliocentric model, the distance of the Sun from the Earth was accurately measured as 108 times the diameter of the Sun, very close to the modern measurement of 107.6 times the Sun's diameter. The calendar described in the text corresponds to an average ] of 365.2467 days, which was close to the modern value of 365.2422 days.

One of the first people in the Western world to offer a scientific explanation for the sun was the ] ] ], who reasoned that it was a giant flaming ball of metal even larger than the ], and not the ] of ]. For teaching this ], he was imprisoned by the authorities and ] (though later released through the intervention of ]).

The ]-] ] in the ], in his magnum opus ''Aryabhatiya'', propounded a heliocentric model where the Earth was taken to be spinning on its axis, and the periods of the planets were given with respect to a stationary Sun. He was also the first to discover that the light from the Moon and the planets were reflected from the Sun, and that the planets follow an ] orbit around the Sun; thus he propounded an ] elliptical model of the planets, whereby he accurately calculated many astronomical constants, such as the time of the ]. ] (]-]) expanded on Aryabhata's heliocentric model in his treatise ''Siddhanta-Shiromani'', where he mentioned the law of gravity, and discovered that the planets don't orbit the Sun at a uniform ]. ] translations of Aryabhata's ''Aryabhatiya'' were available from the ], while ] translations were available from the ], before the time of ], so it is quite possible that Aryabhata's work had an influence on Copernicus' ideas.

], another scientist to run afoul of the authorities, in the 16th century also developed the theory that the Earth orbited the Sun, rather than the other way around. In the early 17th century, ] pioneered ] observations of the Sun, making some of the first known observations of sunspots and positing that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun . ] observed the Sun's light using a ], and showed that it was made up of light of many colours , while in 1800 ] discovered ] radiation beyond the red part of the solar spectrum . The 1800s saw spectroscopic studies of the Sun advance, and ] made the first observations of ] in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. The first ] observations of the Sun were made in ]{{ref|Ryle}}, and these were soon followed by ].

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Among the proposals were that the Sun extracted its energy from friction of its gas masses, or that its energy was derived from ] released as it continuously contracted. Either of these sources of energy could only power the Sun for a few million years at most, but ] were showing that the Earth's age was several billion years. Nuclear fusion was first proposed as the source of solar energy only in the 1930s, when ] calculated the details of the two main energy-producing nuclear reactions that power the Sun {{ref|Bethe}}<sup>,</sup>{{ref|Bethe2}}.

===Solar space missions===

The first satellites designed to observe the Sun were ]'s ]s 5, 6, 7, 8 and 9, launched between 1959 and 1968. These probes orbited the Sun at a similar distance to the Earth's orbit, and made the first detailed measurements of the solar wind and the solar magnetic field, and Pioneer 9 transmitted data until 1987 . The ] satellite, launched in 1974, was a joint ]-] probe which studied the solar wind from an orbit which took it to within ]'s orbit. Also during this period, the ] ] included a solar ] module (the ]) that was operated by the astronauts on-board. Skylab made the first time-resolved observations of the solar transition region and of the ultraviolet emissions from the solar corona. Discoveries included the first observations of ]s, then called "coronal transients", and of ]s, now known to be intimately associated with the ].

1980 saw the launch of the ], designed to observe ]s, ]s and ] radiation from ]s during a time of high solar activity. Just a few months after launch, though, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this state. In 1984 ] mission STS-41C retrieved the satellite and repaired its electronics before releasing it into orbit again, and it took thousands of images of the solar corona over the next five years before ] the Earth's atmosphere in June 1989 .

]'s ] (''Sunbeam'') satellite was launched in 1991 and observed solar flares at X-ray wavelengths. It discovered several different types of flares, and found that the corona away from active regions was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an ] in 2001 caused it to lose its lock on the Sun. It reentered the atmosphere and burnt up in 2005 .

One of the most important solar missions to date has been the ], jointly built by the ] and ] and launched on ], ]. Originally a two-year mission, SOHO is now over ten years old (as of late 2005). It has proved so useful that a follow-on mission, the ], is planned for launch in 2008. Situated at the ] between the Earth and Sun where the gravitational pull from both is equal, SOHO has provided a constant view of the Sun at many wavelengths ever since its launch. Besides solar observations, it has also discovered enormous numbers of ]s, mostly very tiny ]s which have burnt up as they passed the Sun .

All these satellites have observed the Sun from the plane of the ], and so have only observed its equatorial regions in detail. The ] was launched in 1990 to study the Sun's polar regions. It first travelled to ], to 'slingshot' past the planet into an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was well placed to observe the collision of ] with Jupiter in 1994. Once it was in its scheduled orbit, Ulysses began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s (slower than expected), and that there were large magnetic waves emerging from high latitudes which scattered galactic ]s .

Elemental abundances in the photosphere are well known from ] studies, but the composition of the interior of the Sun is much less well known. A ] sample return mission, ], was designed to allow astronomers to directly measure the composition of solar material. It returned to ] in ] and is undergoing analysis, but it was damaged by crash landing when its ] failed to deploy on reentry to ].

==Sun and eye damage==
]/EIT ] using ] light from the ] ] at ]&nbsp;].]]

Sunlight is very bright, and looking directly at the Sun with the ] is painful but generally safe.{{ref|White}} Looking directly at the Sun causes ] visual artifacts and ''temporary'' partial blindness. It also delivers about 4&nbsp;milliwatts of sunlight to the retina, slightly heating it and potentially (though not normally) damaging it. ] exposure gradually yellows the lens of the eye over a period of years and can cause cataracts, but those depend on general exposure to solar UV, not on whether one looks directly at the Sun.

Viewing the Sun through light-concentrating ] such as ] is hazardous without an ] to dim the sunlight. Using a proper filter is important as some improvised filters pass UV rays that can damage the eye at high brightness levels. Unfiltered binoculars can deliver over 500 times more sunlight to the retina than does the naked eye, killing retinal cells almost instantly. Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness{{ref|Marsh}}. One way to view the Sun safely is by projecting an image onto a screen using binoculars or a small telescope. All other methods carry a risk of eye damage. Solar filters can become damaged, in which case harmful levels of radiation can enter the eye.

Partial ]s are hazardous to view because the eye's ] is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, ''not'' by the brightest object in the field. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same ] as during a normal day. In the overall gloom, the pupil expands from ~2&nbsp;mm to ~6&nbsp;mm, and each retinal cell exposed to the solar image receives about ten times more light than it would looking at the non-eclipsed sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer.{{ref|Espenak}} The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.

During ] and ], sunlight is attenuated through ] and ] of light by a particularly long passage through ], and the direct Sun is sometimes faint enough to be viewed directly without discomfort or safely with binoculars. Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.

==External links==
{{sisterlinks|Sun}}
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==References==
<div style="font-size:90%">
* {{note|Alfven}} Alfven, H. (1947), ''Magneto-hydrodynamic waves, and the heating of the solar corona'', Monthly Notices of the Royal Astronomical Society, 107, 211
* {{note|Bethe}} Bethe H. (1938), ''On the Formation of Deuterons by Proton Combination'', Physical Review, v. 54, p. 862-862
* {{note|Bethe2}} Bethe H (1939), ''Energy Production in Stars'', Physical Review, v. 55, p. 434-456
* {{note|Bonanno}} Bonanno, A., Schlattl, H., Paternò, L. (2002), '''' Astronomy and Astrophysics, v.390, p.1115-1118
* {{note|Dikpati}} Dikpati, M., de Toma, G., and Gilman, P.A. (2006), ''Predicting the strength of solar cycle 24 using a flux-transport dynamo-based tool'', Geophysical Review Letters, v. 33, No. 5, p. L05102
* {{note|Espenak}} Espenak, F., NASA RP 1383 ''Total Solar Eclipse of 1998 February 26'', April 1996, p. 17.
* {{note|Godier}} Godier S., Rozelot J.-P. (2000), '''' Astronomy and Astrophysics, v.355, p.365-374
* {{note|Haxton}} Haxton W.C. (1995), '''' Annual Review of Astronomy and Astrophysics, v. 33, p. 459-504
* {{note|Kasting}} Kasting, J.F., Ackerman, T.P., 1986, ''Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere'', Science, v. 234, p. 1383-1385
* {{note|Kerr}} Kerr F.J., Lynden-Bell D. (1986), '''' Monthly Notices of the Royal Astronomical Society, v.221, p. 1023-1038
* {{note|Lean}} Lean J., Skumanich A., White O. (1992), ''Estimating the Sun's radiative output during the Maunder Minimum'', Geophysical Research Letters, v.19, p.1591-1594
* {{note|Lewis}} Lewis, Richard (1983), ''The Illustrated Encyclopedia of the Universe'', Harmony Books, New York, p. 65
* {{note|Marsh}} Marsh, J. C. D., '''' J. Brit. Ast. Assoc., 1982, 92, 6
* {{note|Parker2}} Parker, E.N. (1988), '''' Astrophysical Journal, 330, 474
* {{note|future-sun}} {{cite web|author=Pogge, Richard W.|year=1997|url=http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html|title=The Once & Future Sun|format=lecture notes|work=|accessdate=2005-12-07}}
* {{note|Ryle}} Ryle, M. & Vonberg, D. (1946), ''Solar radiation on 175Mc/s'', Nature 158 pp 339
* {{note|Schlattl}} Schlattl, H. (2001), ''Three-flavor oscillation solutions for the solar neutrino problem'', Physical Review D, vol. 64, Issue 1
* {{note|Sturrock}} Sturrock, P.A., &amp; Uchida, Y. (1981), '''' Astrophysical Journal, 246, 331
* {{note|Thompson}} Thompson, M.J. (2004), ''Solar interior: Helioseismology and the Sun's interior'', Astronomy & Geophysics, v. 45, p. 4.21-4.25
* {{note|White}} White, T. J., M. A. Mainster, P. W. Wilson, and J. H. Tips, ''Chorioretinal temperature increases from solar observation'', Bulletin of Mathematical Biophysics 33, 1-17 (1971)
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Revision as of 11:46, 20 March 2006

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