Elements with more protons in their nuclei require still higher temperatures. For instance, fusing carbon requires a temperature of about one billion degrees! Most of the heavy elements, from oxygen up through iron, are thought to be produced in stars that contain at least ten times as much matter as our Sun. Our Sun is currently burning, or fusing, hydrogen to helium. This is the process that occurs during most of a star's lifetime. After the hydrogen in the star's core is exhausted, the star can burn helium to form progressively heavier elements, carbon and oxygen and so on, until iron and nickel are formed.
Up to this point the process releases energy. The formation of elements heavier than iron and nickel requires the input of energy. Supernova explosions result when the cores of massive stars have exhausted their fuel supplies and burned everything into iron and nickel. The nuclei with mass heavier than nickel are thought to be formed during these explosions.
It is composed of four distinct parts: the chromosphere , the transition region , the corona and the heliosphere. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments , and is in constant, chaotic motion. The corona is the next layer of the Sun.
A flow of plasma outward from the Sun into interplanetary space is the solar wind. The heliosphere , the tenuous outermost atmosphere of the Sun, is filled with the solar wind plasma. 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 , the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. High-energy gamma-ray photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only a few millimeters.
Re-emission happens in a random direction and usually at a slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,, years.
This is the time it would take the Sun to return to a stable state, if the rate of energy generation in its core were suddenly changed. Neutrinos are also released by the fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. The Sun has a magnetic field that varies across the surface of the Sun. Its polar field is 1—2 gauss 0. The magnetic field also varies in time and location.
The quasi-periodic year solar cycle is the most prominent variation in which the number and size of sunspots waxes and wanes. Sunspots are visible as dark patches on the Sun's photosphere , and correspond to concentrations of magnetic field where the convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, and, so, they appear dark. At a typical solar minimum , few sunspots are visible, and occasionally none can be seen at all.
Those that do appear are at high solar latitudes. The largest sunspots can be tens of thousands of kilometers across. An year sunspot cycle is half of a year Babcock —Leighton dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum , the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength.
At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east—west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle. During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size.
At solar-cycle minimum , the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next year sunspot cycle, differential rotation shifts magnetic energy back from the poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field.
The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the interplanetary magnetic field. As a result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind.
The Sun's magnetic field leads to many effects that are collectively called solar activity. Solar flares and coronal-mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from coronal holes at the photospheric surface. Both coronal-mass ejections and high-speed streams of solar wind carry plasma and interplanetary magnetic field outward into the Solar System.
Solar activity is thought to have played a large role in the formation and evolution of the Solar System. With solar-cycle modulation of sunspot number comes a corresponding modulation of space weather conditions, including those surrounding Earth where technological systems can be affected. Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance,  which, in turn, might influence Earth's long-term climate.
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This coincided in time with the era of the Little Ice Age , when Europe experienced unusually cold temperatures. A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41, or , years. These could provide a better explanation of the ice ages than the Milankovitch cycles. The Sun today is roughly halfway through the most stable part of its life.
It has not changed dramatically for over four billion [a] years, and will remain fairly stable for more than five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo dramatic changes, both internally and externally. The Sun formed about 4. This indicates that one or more supernovae must have occurred near the location where the Sun formed. A shock wave from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity.
Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion. HD and HD are hypothesized stellar siblings of the Sun, having formed in the same molecular cloud. The Sun is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation.
At this rate, the Sun has so far converted around times the mass of Earth into energy, about 0. The Sun will spend a total of approximately 10 billion years as a main-sequence star. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force, according to the inverse-square law. This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up as the core gradually becomes denser.
The Sun does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5 billion years and start to turn into a red giant. Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Earth will receive as much sunlight as Venus receives today. Once the core hydrogen is exhausted in 5. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous.
This then starts the red-giant-branch phase where the Sun will spend around a billion years and lose around a third of its mass. After the red-giant branch the Sun has approximately million years of active life left, but much happens. It will then have reached the red clump or horizontal branch , but a star of the Sun's mass does not evolve blueward along the horizontal branch. Instead, it just becomes moderately larger and more luminous over about million years as it continues to burn helium in the core.
When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the asymptotic-giant-branch phase, and the Sun is alternately burning hydrogen in a shell or helium in a deeper shell. After about 20 million years on the early asymptotic giant branch, the Sun becomes increasingly unstable, with rapid mass loss and thermal pulses that increase the size and luminosity for a few hundred years every , years or so.
The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5, times the current level and the radius to over 1 AU. Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the red-giant branch produce smaller, less luminous stars at the tip of the asymptotic giant branch, perhaps only 2, times the luminosity and less than times the radius.
By the end of that phase—lasting approximately , years—the Sun will only have about half of its current mass. The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionized into a planetary nebula as the exposed core reaches 30, K. The final naked core, a white dwarf , will have a temperature of over , K, and contain an estimated The Apex of the Sun's Way , or the solar apex , is the direction that the Sun travels relative to other nearby stars.
This motion is towards a point in the constellation Hercules , near the star Vega. Within It is estimated that a further systems within this range have not yet been identified. Out to The number of substellar objects in that volume are expected to be comparable to the number of stars. The Sun orbits the center of the Milky Way, and it is presently moving in the direction of the constellation of Cygnus. A simple model of the motion of a star in the galaxy gives the galactic coordinates X , Y , and Z as:. We take X 0 and Y 0 to be zero and Z 0 is estimated to be 17 parsecs.
In the X, Y coordinates, the Sun describes an ellipse around the point, whose length in the Y direction is. The oscillation in the Z direction takes the Sun. The Sun's orbit around the Milky Way is perturbed due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events. 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.
Currently, it is unclear whether waves are an efficient heating mechanism. Current research focus has therefore shifted towards flare heating mechanisms. Theoretical models of the Sun's development suggest that 3. Such a weak star would not have been able to sustain liquid water on Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today.
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One theory among scientists is that the atmosphere of the young Earth contained much larger quantities of greenhouse gases such as carbon dioxide , methane than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching it. However, examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations.
Instead, the moderate temperature range may be explained by a lower surface albedo brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output. The Sun has been an object of veneration in many cultures throughout human history. 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 entity. Worship of the Sun was central to civilizations such as the ancient Egyptians , the Inca of South America and the Aztecs of what is now Mexico. In religions such as Hinduism , the Sun is still considered a god. The Egyptians portrayed the god Ra as being carried across the sky in a solar barque, accompanied by lesser gods, and to the Greeks, he was Helios , carried by a chariot drawn by fiery horses. From the reign of Elagabalus in the late Roman Empire the Sun's birthday was a holiday celebrated as Sol Invictus literally "Unconquered Sun" soon after the winter solstice, which may have been an antecedent to Christmas.
Regarding the fixed stars , the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac , and so Greek astronomers categorized it as one of the seven planets Greek planetes , "wanderer" ; the naming of the days of the weeks after the seven planets dates to the Roman era. In the early first millennium BC, Babylonian astronomers observed that the Sun's motion along the ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an elliptic orbit around the Sun, with Earth moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.
One of the first people to offer a scientific or philosophical explanation for the Sun was the Greek philosopher Anaxagoras. He reasoned that it was not the chariot of Helios , but instead a giant flaming ball of metal even larger than the land of the Peloponnesus and that the Moon reflected the light of the Sun. In the 1st century AD, Ptolemy estimated the distance as 1, times the radius of Earth , approximately 7. The theory that the Sun is the center around which the planets orbit was first proposed by the ancient Greek Aristarchus of Samos in the 3rd century BC, and later adopted by Seleucus of Seleucia see Heliocentrism.
This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century by Nicolaus Copernicus. Observations of sunspots were recorded during the Han Dynasty BC—AD by Chinese astronomers , who maintained records of these observations for centuries. Averroes also provided a description of sunspots in the 12th century.
Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun. Arabic astronomical contributions include Albatenius ' discovery that the direction of the Sun's apogee the place in the Sun's orbit against the fixed stars where it seems to be moving slowest is changing. Ibn Yunus observed more than 10, entries for the Sun's position for many years using a large astrolabe.
From an observation of a transit of Venus in , the Persian astronomer and polymath Avicenna concluded that Venus is closer to Earth than the Sun. In , Isaac Newton observed the Sun's light using a prism , and showed that it is made up of light of many colors. 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 is a gradually cooling liquid body that is radiating an internal store of heat. Not until was a documented 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. The theoretical concept of fusion was developed in the s 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 first satellites designed for long term observation of the Sun from interplanetary space were NASA 's Pioneers 6, 7, 8 and 9, which were launched between and These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field.
Pioneer 9 operated for a particularly long time, transmitting data until May In the s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 and 2 probes were U. This spacecraft was designed to observe gamma rays , X-rays and UV radiation from solar flares during a time of high solar activity and solar luminosity. 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 Space Shuttle Challenger mission STSC 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 Earth's atmosphere in June Launched in , Japan's Yohkoh Sunbeam satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and 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 caused it to lose its lock on the Sun.
It was destroyed by atmospheric re-entry in 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 to study the Sun's polar regions. It first traveled to Jupiter , to "slingshot" into an orbit that would take it far above the plane of the ecliptic. Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood.
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A solar wind sample return mission, Genesis , was designed to allow astronomers to directly measure the composition of solar material. Two identical spacecraft were launched into orbits that cause them to respectively pull further ahead of and fall gradually behind Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona. The brightness of the Sun can cause pain from looking at it with the naked eye ; however, doing so for brief periods is not hazardous for normal non-dilated eyes.
Viewing the Sun through light-concentrating optics such as binoculars may result in permanent damage to the retina without an appropriate filter that blocks UV and substantially dims the sunlight.
When using an attenuating filter to view the Sun, the viewer is cautioned to use a filter specifically designed for that use. Some improvised filters that pass UV or IR rays, can actually harm the eye at high brightness levels. The sunlight that is destined for the eyepiece is reflected from an unsilvered surface of a piece of glass.
Only a very small fraction of the incident light is reflected. The rest passes through the glass and leaves the instrument. If the glass breaks because of the heat, no light at all is reflected, making the device fail-safe.
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Simple filters made of darkened glass allow the full intensity of sunlight to pass through if they break, endangering the observer's eyesight. Unfiltered binoculars can deliver hundreds of times as much energy as using the naked eye, possibly causing immediate damage. It is claimed that even brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.
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. This can damage or kill those cells, resulting in small permanent blind spots for the viewer. During sunrise and sunset , sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere,  and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics provided there is no risk of bright sunlight suddenly appearing through a break between clouds.
Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation. An optical phenomenon , known as a green flash , can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon being bent usually through a temperature inversion towards the observer. Light of shorter wavelengths violet, blue, green is bent more than that of longer wavelengths yellow, orange, red but the violet and blue light is scattered more, leaving light that is perceived as green.
Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn , and has other biological effects such as the production of vitamin D and sun tanning. Ultraviolet light is strongly attenuated by Earth's ozone layer , so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color in different regions of the Earth. The Sun has eight known planets. The Solar System also has at least five dwarf planets , an asteroid belt , numerous comets , and a large number of icy bodies which lie beyond the orbit of Neptune.
Solar deities play a major role in many world religions and mythologies. From at least the 4th Dynasty of Ancient Egypt , the Sun was worshipped as the god Ra , portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent.
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In the New Empire period, the Sun became identified with the dung beetle , whose spherical ball of dung was identified with the Sun. In the form of the sun disc Aten , the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. In the Bible , Malachi mentions the "Sun of Righteousness" sometimes translated as the "Sun of Justice" ,  which some Christians have interpreted as a reference to the Messiah Christ.
It was adopted as the Sabbath day by Christians who did not have a Jewish background. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. In paganism, the Sun was a source of life, giving warmth and illumination to mankind. It was the center of a popular cult among Romans, who would stand at dawn to catch the first rays of sunshine as they prayed. The celebration of the winter solstice which influenced Christmas was part of the Roman cult of the unconquered Sun Sol Invictus.
Christian churches were built with an orientation so that the congregation faced toward the sunrise in the East. Tonatiuh , the Aztec god of the sun, was usually depicted holding arrows and a shield  and was closely associated with the practice of human sacrifice. From Wikipedia, the free encyclopedia. For other uses, see Sun disambiguation and The Sun disambiguation. Star at the center of the Solar System.
Sun with sunspots and limb darkening as seen in visible light with solar filter. False-color photo of the Sun as seen in ultraviolet light wavelength of Main article: Sunlight. See also: Molecules in stars. Main article: Standard solar model. Main article: Solar core. Main article: Radiative zone. Main article: Tachocline. Main article: Convection zone. Main article: Photosphere. See also: Corona and Coronal loop.
See also: Solar irradiance. See also: Stellar magnetic field , Sunspots , List of solar cycles , and Solar phenomena. Butterfly diagram showing paired sunspot pattern. Graph is of sunspot area. Main articles: Formation and evolution of the Solar System and Stellar evolution. Main article: Corona.
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