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Sun

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

                                                          CAPTION: The Sun

                                                                   The Sun
                                                          Observation data
                                                        Mean distance from
                                                     Earth 149.6×10^6  km
                                                          (92.95×10^6 mi)
                                      (8.31 minutes at the speed of light)
                                           Visual brightness (V) −26.8^m
                                                  Absolute magnitude 4.8^m
                                               Spectral classification G2V
                                                   Orbital characteristics
                                                        Mean distance from
                                             Milky Way core ~2.5×10^17 km
                                               (26,000-28,000 light-years)
                                         Galactic period 2.25-2.50×10^8 a
         Velocity 217 km/ s orbit around the center of the Galaxy, 20 km/s
      relative to average velocity of other stars in stellar neighbourhood
                                                  Physical characteristics
                                              Mean diameter 1.392×10^6 km
                                                     (109 Earth diameters)
                                              Circumference 4.373×10^6 km
                                                     (342 Earth diameters)
                                                     Oblateness 9×10^−6
                                            Surface area 6.09×10^12  km²
                                                           (11,900 Earths)
                                                  Volume 1.41×10^18  km³
                                                        (1,300,000 Earths)
                                                 Mass 1.988 435×10^30  kg

                                                          (332,946 Earths)
                                                      Density 1.408 g/cm³
                                             Surface gravity 273.95 m s^-2

                                                                  (27.9 g)
                                                           Escape velocity
                                              from the surface 617.54 km/s

                                                               (55 Earths)
                                               Surface temperature 5785  K
                                               Temperature of corona 5  MK
                                                 Core temperature ~13.6 MK
                                       Luminosity (L[sol]) 3.827×10^26  W
                                                          ~3.75×10^28  lm
                                                       (~98 lm/W efficacy)
                          Mean Intensity (I[sol]) 2.009×10^7 W m^-2 sr^-1
                                                  Rotation characteristics
                                                         Obliquity 7.25 °
                                                         (to the ecliptic)
                                                                   67.23°
                                                   (to the galactic plane)
                                                           Right ascension
                                                    of North pole 286.13°
                                                         (19 h 4 min 30 s)
                                                               Declination
                                                    of North pole +63.87°
                                                           (63°52' North)
                                                           Rotation period
                                                   at equator 25.3800 days
                                                     (25 d 9 h 7 min 13 s)
                                                         Rotation velocity
                                                      at equator 7174 km/h
                                        Photospheric composition (by mass)
                                                          Hydrogen 73.46 %
                                                            Helium 24.85 %
                                                             Oxygen 0.77 %
                                                             Carbon 0.29 %
                                                               Iron 0.16 %
                                                               Neon 0.12 %
                                                           Nitrogen 0.09 %
                                                            Silicon 0.07 %
                                                          Magnesium 0.05 %
                                                            Sulphur 0.04 %

   The Sun is the star of our solar system. The Earth and other matter
   (including other planets, asteroids, meteoroids, comets and dust) orbit
   the Sun, which by itself accounts for more than 99% of the solar
   system's mass. Energy from the Sun—in the form of insolation from
   sunlight—supports almost all life on Earth via photosynthesis, and
   drives the Earth's climate and weather.

   The Sun is sometimes referred to by its Latin name Sol or by its Greek
   name Helios. Its astrological and astronomical symbol is a circle with
   a point at its centre: . Some ancient peoples of the world considered
   it a planet before the acceptance of heliocentrism.

Overview

   The sun as it appears through a camera lens from the surface of Earth.
   Enlarge
   The sun as it appears through a camera lens from the surface of Earth.

   About 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is
   made up of trace quantities of heavier elements. The Sun has a spectral
   class of G2V. "G2" means that it has a surface temperature of
   approximately 5,500 K, giving it a white colour, which because of
   atmospheric scattering appears yellow. Its spectrum contains lines of
   ionized and neutral metals as well as very weak hydrogen lines. The "V"
   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 balance,
   neither contracting nor expanding over time. There are more than 100
   million G2 class stars in our galaxy. Because of logarithmic size
   distribution, the Sun is actually brighter than 85% of the stars in the
   Galaxy, most of which are red dwarfs.

   The Sun orbits the centre of the Milky Way galaxy at a distance of
   approximately 25,000 to 28,000 light-years from the galactic centre,
   completing one revolution in about 225–250 million years. The orbital
   speed is 217 km/s, equivalent to one light-year every 1,400 years, and
   one AU every 8 days.

   The Sun is a third generation star, whose formation may have been
   triggered by shockwaves from a nearby supernova. 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 the main source of energy near the surface of Earth. 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 of area 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.

   Sunlight has several interesting biological properties. Ultraviolet
   light from the Sun has antiseptic properties and can be used to
   sterilize tools. It also causes sunburn, and has other medical effects
   such as the production of Vitamin D. Ultraviolet light is strongly
   attenuated by Earth's atmosphere, so that the amount of UV varies
   greatly with latitude because of the longer passage of sunlight through
   the atmosphere at high latitudes. This variation is responsible for
   many biological adaptations, including variations in human skin colour
   in different regions of the globe.

   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 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. 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. 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 the solar wind that
   carry material through the solar system. The 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, and strongly affects 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,
   such as why its outer atmosphere has a temperature of over a million K
   while its visible surface (the photosphere) has a temperature of less
   than 6,000 K. Current topics of scientific inquiry include the sun's
   regular cycle of sunspot activity, the physics and origin of solar
   flares and prominences, the magnetic interaction between the
   chromosphere and the corona, and the origin of the solar wind.

Life cycle

   The Sun's current age, determined using computer models of stellar
   evolution and nucleocosmochronology, is thought to be about 4.57
   billion years.
   Life-cycle of the Sun
   Enlarge
   Life-cycle of the Sun

   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.
   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 4-5 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 about 3×10^8 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. However, Earth's
   water and most of the atmosphere will be boiled away.

   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 remains 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.

Structure

   The Sun's diameter is about 110 times that of the Earth.
   Enlarge
   The Sun's diameter is about 110 times that of the Earth.

   While the Sun is an averaged-sized star, it contains 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, which means
   that its polar diameter differs from its equatorial diameter by only
   10 km. While the Sun does not rotate as a solid body (the rotational
   period is 25 days at the equator and about 35 days at the poles), it
   takes approximately 28 days to complete one full rotation; the
   centrifugal effect of this slow rotation 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 centre of mass of the solar system, which is
   located nearly a solar radius away from the centre of the Sun mostly
   because of the large mass of Jupiter.

   The Sun does not have a definite boundary as rocky planets do; 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 centre to the edge of the photosphere. This is simply
   the layer below which the gases are thick enough to be opaque but above
   which they are transparent; the photosphere is the surface most readily
   visible to the naked eye. Most of the Sun's mass lies within about 0.7
   radii of the centre.

   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.

Core

   The core of the Sun is considered to extend from the centre to about
   0.2 solar radii. It has a density of up to 150,000 kg/m^3 (150 times
   the density of water on Earth) and a temperature of close to 15,000,000
   Kelvins (by contrast, the surface of the Sun is close to 6,000
   Kelvins). Energy is produced by exothermic thermonuclear reactions (
   nuclear fusion) that mainly convert hydrogen into helium, helium into
   carbon, carbon into iron. 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 8.9×10^37 protons (hydrogen nuclei) are converted into helium
   nuclei every second, releasing energy at the matter-energy conversion
   rate of 4.26 million tonnes per second, 383 yottawatts (383×10^24 W) or
   9.15×10^10 megatons of TNT per second. The rate of nuclear fusion
   depends strongly on density, 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 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 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 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 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 much lower than theories predicted,
   a problem which was recently resolved through a better understanding of
   the effects of neutrino oscillation.

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
   convection; while the material grows cooler as altitude increases, this
   temperature gradient is slower than the 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.

Convection zone

   Structure of the Sun
   Enlarge
   Structure of the Sun

   From about 0.7 solar radii to the Sun's visible surface, 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, 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.

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 because of the
   decreasing overall particle density: the photosphere is actually tens
   to hundreds of kilometers thick, being slightly less opaque than air on
   Earth. Sunlight has approximately a black-body spectrum that indicates
   its temperature is about 6,000 K (10,340°F / 5,727 °C), interspersed
   with atomic absorption lines from the tenuous layers above the
   photosphere. The photosphere has a particle density of about 10^23 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.

Atmosphere

   During a total solar eclipse, the sun's atmosphere is more apparent to
   the eye.
   Enlarge
   During a total solar eclipse, the sun's atmosphere is more apparent to
   the 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 is not yet
   known.

   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 colour,
   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.

   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 10^14 m^−3–10^16 m^−3. (Earth's atmosphere near sea level
   has a particle density of about 2×10^25 m^−3.) 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 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.

Solar activity

Sunspots and the solar cycle

   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.
   Measurements of solar cycle variation during the last 30 years
   Enlarge
   Measurements of solar cycle variation during the last 30 years

   The number of sunspots visible on the Sun is not constant, but varies
   over a 10-12 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
   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.
   Enlarge
   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 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 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. Earlier extended minima have
   been discovered through analysis of tree rings 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 Van Allen
   belts. The Van Allen belts consist of an inner belt composed primarily
   of protons and an outer belt composed mostly of electrons. Radiation
   within the Van Allen belts can occasionally damage satellites 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 auroras,
   known as aurorae borealis in the northern hemisphere and aurorae
   australis in the southern hemisphere. In periods of normal solar
   activity, aurorae can be seen in oval-shaped regions centered on the
   magnetic poles and lying roughly at a geomagnetic latitude of 65°, 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 Mexico.

Theoretical problems

Solar neutrino problem

   Extremely high resolution spectrum of the Sun showing thousands of
   elemental absorption lines (Fraunhofer lines).
   Enlarge
   Extremely high resolution spectrum of the Sun showing thousands of
   elemental absorption lines ( Fraunhofer lines).

   For many years the number of solar electron neutrinos 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 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. 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 can
   indeed oscillate.. Moreover, 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 only one-third of the neutrinos seen at Earth were of
   the electron type.

Coronal heating problem

   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.

   Currently, it is unclear whether waves are an efficient heating
   mechanism. All waves except Alfven waves have been found to dissipate
   or refract before reaching the corona. 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, but this
   remains an open topic of investigation.

Faint young sun problem

   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 general consensus among
   scientists is that the young Earth's atmosphere contained much larger
   quantities of greenhouse gases (such as carbon dioxide and/or ammonia)
   than are present today, which trapped enough heat to compensate for the
   lesser amount of solar energy reaching the planet.

Magnetic field

   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 [1]
   Enlarge
   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

   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.

History of solar observation

Early understanding of the Sun

   The Trundholm sun chariot pulled by a horse is a sculpture believed to
   be illustrating an important part of Nordic Bronze Age mythology.
   Enlarge
   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 heavens, 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, and 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 in England); 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 the Sun was considered by Greek
   astronomers to be one of the seven planets (Greek planetes,
   "wanderer"), after which the seven days of the week are named in some
   languages.

Development of modern scientific understanding

   Comparison between the sun and the red supergiant Antares. The black
   circle is the size of the orbit of Mars. Arcturus is also included in
   the picture for comparison.
   Enlarge
   Comparison between the sun and the red supergiant Antares. The black
   circle is the size of the orbit of Mars. Arcturus is also included in
   the picture for comparison.
   The sun compared with the red supergiant VV Cephei A (sun can only be
   seen when image is clicked on twice)
   Enlarge
   The sun compared with the red supergiant VV Cephei A (sun can only be
   seen when image is clicked on twice)

   One of the first people in the Western world 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
   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.

   Another scientist to challenge the accepted view was Nicolaus
   Copernicus, who in the 16th century developed the theory that the Earth
   orbited the Sun, rather than the other way around. In the early 17th
   century, Galileo pioneered telescopic 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. Isaac Newton observed the Sun's light
   using a prism, and showed that it was made up of light of many colors,
   while in 1800 William Herschel discovered infrared radiation beyond the
   red part of the solar spectrum. 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.

   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. 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, the discoverer of helium in the solar spectrum, proposed a
   meteoritic hypothesis for the formation and evolution of the sun.
   Another proposal was that the Sun extracted its energy from friction of
   its gas masses.

   It would be 1904 before a potential solution was offered. Ernest
   Rutherford suggested that the energy could be maintained by an internal
   source of heat, and suggested radioactive decay as the source. However
   it would be Albert Einstein who would provide the essential clue to the
   source of a Sun's energy with his mass-energy 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 into helium, resulting in a production of energy from the net
   change in mass. This theoretical concept 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.

   Finally, in 1957, a paper titled Synthesis of the Elements in Stars was
   published that demonstrated convincingly that most of the elements in
   the universe had been created by nuclear reactions inside stars like
   the Sun.

Solar space missions

   Solar "fireworks" in sequence as recorded in November 2000 by four
   instruments onboard the SOHO spacecraft.
   Enlarge
   Solar " fireworks" in sequence as recorded in November 2000 by four
   instruments onboard the SOHO spacecraft.

   The first satellites designed to observe the Sun were NASA's Pioneers
   5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These
   probes orbited the Sun at a distance similar to that of the Earth's
   orbit, and made the first detailed measurements of the solar wind and
   the solar magnetic field. Pioneer 9 operated for a particularly long
   period of time, transmitting data until 1987.

   In the 1970s, Helios 1 and the Skylab Apollo Telescope Mount provided
   scientists with significant new data on solar wind and the solar
   corona. The Helios 1 satellite was a joint U.S.- German probe that
   studied the solar wind from an orbit carrying the spacecraft inside
   Mercury's orbit at perihelion. The Skylab space station, launched by
   NASA in 1973, included a solar observatory module called the Apollo
   Telescope Mount that was operated by astronauts resident on the
   station. Skylab made the first time-resolved observations of the solar
   transition region and of ultraviolet emissions from the solar corona.
   Discoveries included the first observations of coronal mass ejections,
   then called "coronal transients", and of coronal holes, now known to be
   intimately associated with the solar wind.

   In 1980, the Solar Maximum Mission was launched by NASA. This
   spacecraft was designed to observe gamma rays, X-rays and UV radiation
   from solar flares during a time of high solar activity. Just a few
   months after launch, however, an electronics failure caused the probe
   to go into standby mode, and it spent the next three years in this
   inactive state. In 1984 Space Shuttle Challenger mission STS-41C
   retrieved the satellite and repaired its electronics before
   re-releasing it into orbit. The Solar Maximum Mission subsequently
   acquired thousands of images of the solar corona before re-entering the
   Earth's atmosphere in June 1989.

   Japan's Yohkoh (Sunbeam) satellite, launched in 1991, observed solar
   flares at X-ray wavelengths. Mission data allowed scientists to
   identify several different types of flares, and also demonstrated that
   the corona away from regions of peak activity was much more dynamic and
   active than had previously been supposed. Yohkoh observed an entire
   solar cycle but went into standby mode when an annular eclipse in 2001
   caused it to lose its lock on the Sun. It was destroyed by atmospheric
   reentry in 2005.

   One of the most important solar missions to date has been the Solar and
   Heliospheric Observatory, jointly built by the European Space Agency
   and NASA and launched on December 2, 1995. Originally a two-year
   mission, SOHO has now operated for over ten years (as of 2006). It has
   proved so useful that a follow-on mission, the Solar Dynamics
   Observatory, is planned for launch in 2008. Situated at the Lagrangian
   point between the Earth and the Sun (at which the gravitational pull
   from both is equal), SOHO has provided a constant view of the Sun at
   many wavelengths since its launch. In addition to its direct solar
   observation, SOHO has enabled the discovery of large numbers of comets,
   mostly very tiny sungrazing comets which incinerate as they pass the
   Sun.

   All these satellites have observed the Sun from the plane of the
   ecliptic, and so have only observed its equatorial regions in detail.
   The Ulysses probe was launched in 1990 to study the Sun's polar
   regions. It first traveled to Jupiter, 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 Comet
   Shoemaker-Levy 9 with Jupiter in 1994. Once Ulysses was in its
   scheduled orbit, it 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 cosmic rays.

   Elemental abundances in the photosphere are well known from
   spectroscopic studies, but the composition of the interior of the Sun
   is more poorly understood. A solar wind sample return mission, Genesis,
   was designed to allow astronomers to directly measure the composition
   of solar material. Genesis returned to Earth in 2004 but was damaged by
   a crash landing after its parachute failed to deploy on reentry into
   Earth's atmosphere. Despite severe damage, some usable samples have
   been recovered from the spacecraft's sample return module and are
   undergoing analysis.

Sun observation and eye damage

   Sunlight is very bright, and looking directly at the Sun with the naked
   eye for brief periods can be painful, but is generally not hazardous.
   Looking directly at the Sun causes phosphene visual artifacts and
   temporary partial blindness. It also delivers about 4 milliwatts of
   sunlight to the retina, slightly heating it and potentially (though not
   normally) damaging it. UV 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 optics such as binoculars
   is very hazardous without an attenuating (ND) filter to dim the
   sunlight. 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. One way to view
   the Sun safely is by projecting an image onto a screen using
   binoculars. This should only be done with a small refracting telescope
   (or binoculars) with a clean eyepiece. Other kinds of telescope can be
   damaged by this procedure.

   Partial solar eclipses are hazardous to view because the eye's pupil 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 surface brightness as during a
   normal day. In the overall gloom, the pupil expands from ~2 mm to
   ~6 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. 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 sunrise and sunset, sunlight is attenuated through rayleigh and
   mie scattering of light by a particularly long passage through Earth's
   atmosphere, and the direct Sun is sometimes faint enough to be viewed
   directly without discomfort or safely with binoculars (provided there
   is no risk of bright sunlight suddenly appearing in a break between
   clouds). Hazy conditions, atmospheric dust, and high humidity
   contribute to this atmospheric attenuation.

   Attenuating filters to view the Sun should be specifically designed for
   that use: some improvised filters pass UV or IR rays that can harm the
   eye at high brightness levels. In general, filters on telescopes or
   binoculars should be on the objective lens or aperture rather than on
   the eyepiece, because eyepiece filters can suddenly shatter due to high
   heat loads from the absorbed sunlight. Welding glass is an acceptable
   solar filter, but "black" exposed photographic film is not (it passes
   too much infrared).

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