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The
Sun () is the
star at the center of the
Solar System. The
Earth and other matter (including other
planets,
asteroids,
meteoroids, comets and Cosmic 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, supports almost all life on Earth via
photosynthesis, and drives the Earth's climate and weather.
The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume),
helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a stellar classification of G2V.
G2 implies that it has a surface temperature of approximately 5,780 Kelvin, giving it a color temperature color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the Rayleigh 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
spectral lines of ionized and neutral metals as well as very weak hydrogen lines. The
V (Roman numerals) 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. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the
Milky Way, most of which are red dwarfs.
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. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one
Astronomical unit every 8 days.
It 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 arm and
Sagittarius arms of the galaxy. Of the 50
Nearest stars within 17 light years from the Earth, the sun ranks 4th in
absolute magnitude as a fourth magnitude star (M=4.83).
Overview
The Sun is a metallicity, or third generation, star whose formation may have been triggered by shockwaves from one or more nearby supernovae.
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 1 million
Kelvin 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 flare and solar prominence, the magnetic interaction between the
chromosphere and the
corona, and the origin (propulsion source) of solar wind.
Life cycle
The Sun's current
main sequence age, determined using computer simulation of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.
Structure
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, to as little as 17,000 years.{{cite web|url=http://www.badastronomy.com/bitesize/solar_system/sun.html|first=Phil|last=Plait|publisher=Bad Astronomy|title=Bitesize Tour of the Solar System: The Long Climb from the Sun's Core|year=1997|accessdate=2006-03-22--> After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space.
Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were
Solar neutrino problem 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.
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 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.
Convection zone
In the Sun's outer layer (down to approximately 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal 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 radiant zone.
Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiant zone.
The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the
granule (solar physics) 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 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.{{cite book] on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or
limb of the solar disk, in a phenomenon known as limb darkening. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000
kelvin, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 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.{{cite web|url=http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html|work=Solar and Magnetospheric MHD Theory Group|publisher=University of St Andrews|title=Discovery of Helium|accessdate=2006-03-22-->
Atmosphere
, the solar corona can be seen with the naked eye.
The parts of the Sun above the photosphere are referred to collectively as the
solar atmosphere. They can be viewed with telescopes operating across the
electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the
temperature minimum, the
chromosphere, the
solar transition region, the corona, and the
heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of
Pluto to the heliopause, where it forms a sharp shock wave 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
Kelvin. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.
Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the
chromosphere from the Greek root
chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of solar eclipse. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.
'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 solar transition region in which the temperature rises rapidly from around 100,000
kelvin to coronal temperatures closer to one million K. The increase is because of a
phase transition as
helium within the region becomes fully
ionization by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of Halo (optical phenomenon) around chromospheric features such as Spicule (solar physics)s and
Solar filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from outer space by instruments sensitive to the ultraviolet portion of the
electromagnetic spectrum.
The
corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the
solar wind that fills the
solar system and
heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 1014–1016 m−3. (Earth's atmosphere near sea level has a particle density of about 2 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
Alfven wave. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a
Parker spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the
Voyager program 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.{{cite web|url=http://www.spaceref.com/news/viewpr.html?pid=16394|title=The Distortion of the Heliosphere: our Interstellar Magnetic Compass|month=March 15|year=2005|author=European Space Agency|accessdate=2006-03-22-->
Solar cycles
Sunspots and the sunspot 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.
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 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
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.
Theoretical problems
Solar neutrino problem
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
neutrino oscillation—that is, change into undetectable
tau neutrino and muon neutrinos as they traveled between the Sun and the Earth. 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, 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.
Magnetic field
extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the
Plasma (physics) in the interplanetary medium.
All
matter in the Sun is in the form of gas and
plasma (physics) 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
solar rotation of the Sun's latitudes causes its
magnetic field lines to become twisted together over time, causing Coronal loop 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 sunspot 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 (unit) 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
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, and Sun 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, Chichen Itza 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
One of the first people to offer a scientific explanation for the Sun was the
Ancient Greece philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the
Peloponnese, and not the
chariot of
Helios. For teaching this heresy, he was imprisoned by the authorities and
capital punishment, 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
Common Era, 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 of Samos 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 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.{{cite web] 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 (optics), and showed that it was made up of light of many colors,{{cite web|url=http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml|title=Sir Isaac Newton (1643–1727)|publisher=BBC|accessdate=2006-03-22--> while in 1800
William Herschel discovered infrared radiation beyond the red part of the solar spectrum.{{cite web] 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.
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.{{cite web] 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.{{cite web]. 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. The paper demonstrated convincingly that most of the elements in the universe had been
nucleosynthesis by nuclear reactions inside stars, some like our Sun. This revelation stands today as one of the great achievements of science.
Solar space missions
" in sequence as recorded in November 2000 by four instruments onboard the Solar and heliospheric observatory spacecraft.
The first satellites designed to observe the Sun were
NASA's
Pioneer programs 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, 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.{{cite web] 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 United States-Federal Republic of Germany probe that studied the solar wind from an orbit carrying the spacecraft inside
Mercury (planet)'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 reentry the Earth's atmosphere in June 1989.{{cite web|url=http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html|title=Solar Maximum Mission Overview|first=Chris|last=St. Cyr|coauthors=Joan Burkepile|accessdate=2006-03-22|year=1998-->
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 solar eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric reentry in 2005.{{cite web|url=http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html|title=Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere|year= 2005|author=Japan Aerospace Exploration Agency|accessdate=2006-03-22-->
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 2007). 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.{{cite web
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 Shoemak
The
Sun () is the
star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets and
Cosmic 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, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.
The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a stellar classification of G2V.
G2 implies that it has a surface temperature of approximately 5,780
Kelvin, giving it a color temperature color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the
Rayleigh 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
spectral lines of ionized and neutral metals as well as very weak hydrogen lines. The
V (Roman numerals) 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. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the
Milky Way, most of which are red dwarfs.
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. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one
Astronomical unit every 8 days.
It 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 arm and
Sagittarius arms of the galaxy. Of the 50 Nearest stars within 17 light years from the Earth, the sun ranks 4th in
absolute magnitude as a fourth magnitude star (M=4.83).
Overview
The Sun is a metallicity, or third generation, star whose formation may have been triggered by shockwaves from one or more nearby
supernovae.
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 1 million
Kelvin 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 flare and
solar prominence, the magnetic interaction between the chromosphere and the
corona, and the origin (propulsion source) of solar wind.
Life cycle
The Sun's current
main sequence age, determined using computer simulation of
stellar evolution and
nucleocosmochronology, is thought to be about 4.57 billion years.
Structure
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, to as little as 17,000 years.{{cite web|url=http://www.badastronomy.com/bitesize/solar_system/sun.html|first=Phil|last=Plait|publisher=Bad Astronomy|title=Bitesize Tour of the Solar System: The Long Climb from the Sun's Core|year=1997|accessdate=2006-03-22--> After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space.
Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were Solar neutrino problem 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.
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 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.
Convection zone
In the Sun's outer layer (down to approximately 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as
thermal 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 radiant zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiant zone.
The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the granule (solar physics) 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 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.{{cite book] on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or
limb of the solar disk, in a phenomenon known as
limb darkening. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 kelvin, interspersed with atomic
absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 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.{{cite web|url=http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html|work=Solar and Magnetospheric MHD Theory Group|publisher=University of St Andrews|title=Discovery of Helium|accessdate=2006-03-22-->
Atmosphere
, the solar corona can be seen with the naked eye.
The parts of the Sun above the photosphere are referred to collectively as the
solar atmosphere. They can be viewed with telescopes operating across the
electromagnetic spectrum, from radio through
visible light to gamma rays, and comprise five principal zones: the
temperature minimum, the
chromosphere, the solar transition region, the
corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the
heliopause, where it forms a sharp shock wave 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 Kelvin. This part of the Sun is cool enough to support simple molecules such as
carbon monoxide and water, which can be detected by their absorption spectra.
Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the
chromosphere from the Greek root
chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of
solar eclipse. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.
'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
solar transition region in which the temperature rises rapidly from around 100,000 kelvin to coronal temperatures closer to one million K. The increase is because of a phase transition as
helium within the region becomes fully ionization by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of
Halo (optical phenomenon) around chromospheric features such as Spicule (solar physics)s and
Solar filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from
outer space by instruments sensitive to the
ultraviolet portion of the
electromagnetic spectrum.
The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the
solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 1014–1016 m−3. (Earth's atmosphere near sea level has a particle density of about 2 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
Alfven wave. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a Parker spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the
Voyager program 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.{{cite web|url=http://www.spaceref.com/news/viewpr.html?pid=16394|title=The Distortion of the Heliosphere: our Interstellar Magnetic Compass|month=March 15|year=2005|author=European Space Agency|accessdate=2006-03-22-->
Solar cycles
Sunspots and the sunspot 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.
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 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
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.
Theoretical problems
Solar neutrino problem
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
neutrino oscillation—that is, change into undetectable
tau neutrino and muon neutrinos as they traveled between the Sun and the Earth. 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, 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.
Magnetic field
extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the
Plasma (physics) in the
interplanetary medium.
All matter in the Sun is in the form of
gas and
plasma (physics) 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 solar rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing Coronal loop 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
sunspot 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 (unit) 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
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, and Sun 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, Chichen Itza 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
One of the first people to offer a scientific explanation for the Sun was the Ancient Greece
philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the
Peloponnese, and not the
chariot of Helios. For teaching this
heresy, he was imprisoned by the authorities and
capital punishment, 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
Common Era, 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 of Samos 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 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.{{cite web] 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 (optics), and showed that it was made up of light of many colors,{{cite web|url=http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml|title=Sir Isaac Newton (1643–1727)|publisher=BBC|accessdate=2006-03-22--> while in 1800
William Herschel discovered infrared radiation beyond the red part of the solar spectrum.{{cite web] 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.
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.{{cite web] 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.{{cite web]. 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. The paper demonstrated convincingly that most of the elements in the universe had been
nucleosynthesis by nuclear reactions inside stars, some like our Sun. This revelation stands today as one of the great achievements of science.
Solar space missions
" in sequence as recorded in November 2000 by four instruments onboard the Solar and heliospheric observatory spacecraft.
The first satellites designed to observe the Sun were
NASA's
Pioneer programs 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, 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.{{cite web] 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 United States-Federal Republic of Germany probe that studied the solar wind from an orbit carrying the spacecraft inside
Mercury (planet)'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 reentry the Earth's atmosphere in June 1989.{{cite web|url=http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html|title=Solar Maximum Mission Overview|first=Chris|last=St. Cyr|coauthors=Joan Burkepile|accessdate=2006-03-22|year=1998-->
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
solar eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric reentry in 2005.{{cite web|url=http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html|title=Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere|year= 2005|author=Japan Aerospace Exploration Agency|accessdate=2006-03-22-->
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 2007). 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.{{cite web
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 Shoemak
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