# how does sun get light?

Sun generates its light energy by nuclear fusion of hydrogen nuclei into helium.In this process hydrogen nuclei collide, fuse into heavier helium atoms and release tremendous amounts of energy in the process.

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Main article: Solar core
Cross-section of a solar-type star (NASA)

The core of the Sun is considered to extend from the center to about 0.2 to 0.25 of the solar radius.[32] It has a density of up to 150 g/cm3[33][34] (about 150 times the density of water) and a temperature of close to 13.6 million kelvin (K). By contrast, the Sun's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone.[32] Through most of the Sun's life, energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium.[35] Less than 2% of the helium generated in the Sun comes from the CNO cycle.

The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; inside 24% of the Sun's radius, 99% of the power has been generated, and by 30% of the radius, fusion has stopped nearly entirely. The rest of the star is heated by energy that is transferred outward from the core and the layers just outside. The energy produced by fusion in the core must then travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.[36][37]

The proton–proton chain occurs around 9.2×1037 times each second in the core of the Sun. Since this reaction uses four free protons (hydrogen nuclei), it converts about 3.7×1038 protons to alpha particles (helium nuclei) every second (out of a total of ~8.9×1056 free protons in the Sun), or about 6.2×1011 kg per second.[37] Since fusing hydrogen into helium releases around 0.7% of the fused mass as energy,[38] the Sun releases energy at the mass-energy conversion rate of 4.26 million metric tons per second, 384.6 yottawatts (3.846×1026 W),[1] or 9.192×1010 megatons of TNT per second. This mass is not destroyed to create the energy, rather, the mass is carried away in the radiated energy, as described by the concept of mass-energy equivalence.

The power production by fusion in the core varies with distance from the solar center. At the center of the Sun, theoretical models estimate it to be approximately 276.5 watts/m3,[39] a power production density that more nearly approximates reptile metabolism than a thermonuclear bomb.[note 2] Peak power production in the Sun has been compared to the volumetric heats generated in an active compost heap. The tremendous power output of the Sun is not due to its high power per volume, but instead due to its large size.

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.[40][41]

The gamma rays (high-energy photons) released in fusion reactions are absorbed in only a few millimeters of solar plasma and then re-emitted again in random direction and at slightly lower energy. Therefore it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years.[42]

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 photons of visible light before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 23 of them because the neutrinos had changed flavor by the time they were detected.[43]

From about 0.25 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.[44] This zone is free of thermal convection; while the material gets cooler from 7 to about 2 million kelvin with increasing altitude, this temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection.[34] Energy is transferred by radiationions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.[44] The density drops a hundredfold (from 20 g/cm3 to only 0.2 g/cm3) from the bottom to the top of the radiative zone.[44]

The radiative zone and the convection form a transition layer, the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear—a condition where successive horizontal layers slide past one another.[45] The fluid motions found in the convection zone above, slowly disappear from the top of this layer to its bottom, matching the calm characteristics of the radiative zone on the bottom. Presently, it is hypothesized (see Solar dynamo), that a magnetic dynamo within this layer generates the Sun's magnetic field.[34]

### Convective zone

In the Sun's outer layer, from its surface down to approximately 200,000 km (or 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the thermal energy of the interior outward through radiation; in other words it is opaque enough. 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 downward to the base of the convection zone, to receive more heat from the top of the radiative zone. At the visible surface of the Sun, the temperature has dropped to 5,700 K and the density to only 0.2 g/m3 (about 1/6,000th the density of air at sea level).[34]

The thermal columns in the convection zone form an imprint on the surface of the Sun as the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior causes a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.[34] The Sun's thermal columns are Bénard cells and therefore tend to be hexagonal prisms.[46]

### Photosphere

The effective temperature, or black body temperature, of the Sun (5777 K) is the temperature a black body of the same size must have to yield the same total emissive power.
Main article: Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light.[47] 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.[47] Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H ions.[48][49] The photosphere is tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening.[47] Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of ~1023 m−3 (this is about 0.37% of the particle number per volume of Earth's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy).[44]

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.[50]

### Atmosphere

During a total solar eclipse, the solar corona can be seen with the naked eye, during the brief period of totality.

The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere.[47] 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.[47] 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.[47] The reason has not been conclusively proven; evidence suggests that Alfvén waves may have enough energy to heat the corona.[51]

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,100 K.[47] 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.[52]

Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines.[47] It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun.[44] The temperature in the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top.[47] In the upper part of chromosphere helium becomes partially ionized.[53]

Taken by Hinode's Solar Optical Telescope on January 12, 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.

Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K.[54] The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma.[53] 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.[44] The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum.[55]

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona continuously expands into space forming the solar wind, which fills all the Solar System.[56] The low corona, which is very near the surface of the Sun, has a particle density around 1015–1016 m−3.[53][note 3] The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K.[54] 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.[54][56]

The heliosphere, which is the cavity around the Sun filled with the solar wind plasma, 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.[57] 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,[56] 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.[58]

### 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.[59]

The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum.[60] The Sun's magnetic field leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System.[61] Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth's outer atmosphere.[62]

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 creates the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years.[63][64]

The solar magnetic field extends well beyond the Sun itself. The magnetized solar wind plasma carries Sun's magnetic field into the space forming what is called the interplanetary magnetic field.[56] Since the plasma can only move along the magnetic field lines, the interplanetary magnetic field is initially stretched radially away from the Sun. Because the fields above and below the solar equator have different polarities pointing towards and away from the Sun, there exists a thin current layer in the solar equatorial plane, which is called the heliospheric current sheet.[56] At the large distances the rotation of the Sun twists the magnetic field and the current sheet into the Archimedean spiral like structure called the Parker spiral.[56] The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun's 50–400 μT (in the photosphere) magnetic dipole field reduces with the cube of the distance to about 0.1 nT at the distance of the Earth. However, according to spacecraft observations the interplanetary field at the Earth's location is about 100 times greater at around 5 nT.[65]

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The light is emitted as a product of the atomic reactions taking place within the Sun.

"The Sun is made up of gases not as we think of volcanoes or something melt.
It's full of Hydrogen. Hydrogen forms Helium by nuclear fusion. This make more energy than any nuclear reactor or any Nuclear Bomb. Light is a part of energy.
If you talk about heat, then the main source of heat is Infra Red Rays which is a part of sun light."

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The sun itself is a luminous object.This means the sun  emits light .

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The sun itself is a luminous object.This means the sun emits light .

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SARYAT SAHOO ,I WANT TO TELL YOU THAT PAWAN IS NOT A ROBOT THAT HE WILL READ ALL OF IT

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because of thegases present in it

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I DONT UNDERSTAND LIGHT

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thats a complete dffrnt qveschn

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sun does not get it's  light  it emit it's own light

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Main article: Solar core
Cross-section of a solar-type star (NASA)

The core of the Sun is considered to extend from the center to about 0.2 to 0.25 of the solar radius. [32] It has a density of up to 150 g/cm3[33] [34] (about 150 times the density of water) and a temperature of close to 13.6 million kelvin (K). By contrast, the Sun 's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone. [32] Through most of the Sun 's life, energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium. [35] Less than 2% of the helium generated in the Sun comes from the CNO cycle.

The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; inside 24% of the Sun 's radius, 99% of the power has been generated, and by 30% of the radius, fusion has stopped nearly entirely. The rest of the star is heated by energy that is transferred outward from the core and the layers just outside. The energy produced by fusion in the core must then travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles. [36] [37]

The proton–proton chain occurs around 9.2�1037 times each second in the core of the Sun. Since this reaction uses four free protons (hydrogen nuclei), it converts about 3.7�1038 protons to alpha particles (helium nuclei) every second (out of a total of ~8.9�1056 free protons in the Sun), or about 6.2�1011 kg per second. [37] Since fusing hydrogen into helium releases around 0.7% of the fused mass as energy, [38] the Sun releases energy at the mass-energy conversion rate of 4.26 million metric tons per second, 384.6 yotta watts (3.846�1026 W), [1] or 9.192�1010 megatons of TNT per second. This mass is not destroyed to create the energy, rather, the mass is carried away in the radiated energy, as described by the concept of mass-energy equivalence.

The power production by fusion in the core varies with distance from the solar center. At the center of the Sun, theoretical models estimate it to be approximately 276.5 watts/m3, [39] a power production density that more nearly approximates reptile metabolism than a thermonuclear bomb. [note 2] Peak power production in the Sun has been compared to the volumetric heats generated in an active compost heap. The tremendous power output of the Sun is not due to its high power per volume, but instead due to its large size.

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. [40] [41]

The gamma rays (high-energy photons) released in fusion reactions are absorbed in only a few millimeters of solar plasma and then re-emitted again in random direction and at slightly lower energy. Therefore it takes a long time for radiation to reach the Sun 's surface. Estimates of the photon travel time range between 10,000 and 170,000 years. [42]

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 photons of visible light before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 23 of them because the neutrinos had changed flavor by the time they were detected. [43]

From about 0.25 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. [44] This zone is free of thermal convection; while the material gets cooler from 7 to about 2 million kelvin with increasing altitude, this temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection. [34] Energy is transferred by radiation—ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions. [44] The density drops a hundredfold (from 20 g/cm3 to only 0.2 g/cm3) from the bottom to the top of the radiative zone. [44]

The radiative zone and the convection form a transition layer, the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear—a condition where successive horizontal layers slide past one another. [45] The fluid motions found in the convection zone above, slowly disappear from the top of this layer to its bottom, matching the calm characteristics of the radiative zone on the bottom. Presently, it is hypothesized (see Solar dynamo), that a magnetic dynamo within this layer generates the Sun 's magnetic field. [34]

### Convective zone

In the Sun 's outer layer, from its surface down to approximately 200,000 km (or 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the thermal energy of the interior outward through radiation; in other words it is opaque enough. 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 downward to the base of the convection zone, to receive more heat from the top of the radiative zone. At the visible surface of the Sun, the temperature has dropped to 5,700 K and the density to only 0.2 g/m3 (about 1/6,000th the density of air at sea level). [34]

The thermal columns in the convection zone form an imprint on the surface of the Sun as the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior causes a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun. [34] The Sun 's thermal columns are B�nard cells and therefore tend to be hexagonal prisms. [46]

### Photosphere

The effective temperature, or black body temperature, of the Sun (5777 K) is the temperature a black body of the same size must have to yield the same total emissive power.
Main article: Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. [47] 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. [47] Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H ions. [48] [49] The photosphere is tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening. [47] Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of ~1023 m−3 (this is about 0.37% of the particle number per volume of Earth 's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy). [44]

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. [50]

### Atmosphere

During a total solar eclipse, the solar corona can be seen with the naked eye, during the brief period of totality.

The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. [47] They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: thetemperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. [47] 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. [47] The reason has not been conclusively proven; evidence suggests that Alfv�n waves may have enough energy to heat the corona.[51]

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,100 K .[47] 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. [52]

Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. [47] It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun. [44] The temperature in the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. [47] In the upper part of chromosphere helium becomes partially ionized. [53]

Taken by Hinode 's Solar Optical Telescope on January 12, 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.

Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K. [54] The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. [53] 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.[44] The transition region is not easily visible from Earth 's surface, but is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum. [55]

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona continuously expands into space forming the solar wind, which fills all the Solar System. [56] The low corona, which is very near the surface of the Sun, has a particle density around 1015–1016 m−3. [53] [note 3] The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K. [54] 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. [54] [56]

The heliosphere, which is the cavity around the Sun filled with the solar wind plasma, 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 becomessuperalfv�nic—that is, where the flow becomes faster than the speed of Alfv�n waves. [57] 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 Alfv�n waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, [56] 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. [58]

### 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. [59]

The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum. [60] The Sun 's magnetic field leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System. [61]Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth 's outer atmosphere. [62]

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 creates the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun 's magnetic field reverses itself about every 11 years. [63] [64]

The solar magnetic field extends well beyond the Sun itself. The magnetized solar wind plasma carries Sun 's magnetic field into the space forming what is called the interplanetary magnetic field. [56] Since the plasma can only move along the magnetic field lines, the interplanetary magnetic field is initially stretched radially away from the Sun. Because the fields above and below the solar equator have different polarities pointing towards and away from the Sun, there exists a thin current layer in the solar equatorial plane, which is called the heliospheric current sheet. [56] At the large distances the rotation of the Sun twists the magnetic field and the current sheet into the Archimedean spiral like structure called the Parker spiral. [56] The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun 's 50–400 μT (in the photosphere) magnetic dipole field reduces with the cube of the distance to about 0.1 nT at the distance of the Earth. However, according to spacecraft observations the interplanetary field at the Earth 's location is about 100 times greater at around 5 nT. [65]

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Sorry I dont have time to tell about it but i tell you a short cut sun is a big ball of heat

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sun is made of many types of  gases  like -  hydrogen, nitrogen  and many other gases  when these gases combine [mix] together that gases creat some nuclear reacts and sun light up .

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unlight is a portion of the electromagnetic radiation given off by the Sun, particularly infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through the Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon.

When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light.

The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter.[1]

Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer.

Sunlight takes about 8.3 minutes to reach the Earth. On average, it takes energy between 10,000 and 170,000 years to leave the sun's interior and then be emitted from the surface as light.[2]

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface. Sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of ultraviolet radiation. At the top of the atmosphere sunlight is about 30% more intense with more than three times the fraction of ultraviolet (UV), with most of the extra UV consisting of biologically-damaging shortwave ultraviolet.[citation needed]

Sunlight is a key factor in photosynthesis, a process vital for many living beings on Earth.

## Composition and power

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K.[3] The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces Gamma rays as a result of the nuclear fusion process, these super high energy photons are converted to lower energy photons before they reach the Sun's surface and are emitted out into space. As a result, the Sun does not emit gamma rays. The Sun does, however, emit X-rays, ultraviolet, visible light, infrared, and even radio waves.[4]

Although, as mentioned, the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right) and will not be discussed further. The spectrum of nearly all solar electromagnetic radiation striking the Earth's atmosphere spans a range of 100 nm to about 1 mm. This band of significant radiation power can be divided into five regions in increasing order of wavelengths:[5]

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface. This spectrum of radiation has has germicidal properties, and is used in germicidal lamps.
• Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn.
• Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UV A is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and is able to cause cancer.[6]
• Visible range or light spans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye.
• Infrared range that spans 700 nm to 106 nm (1 mm). It is responsible for an important part of the electromagnetic radiation that reaches the Earth. It is also divided into three types on the basis of wavelength:
• Infrared-A: 700 nm to 1,400 nm
• Infrared-B: 1,400 nm to 3,000 nm
• Infrared-C: 3,000 nm to 1 mm.

Sunlight in space at the top of Earth's atmosphere at a power of 1366 watts/m2 is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.[7] At ground level this decreases to about 1120–1000 watts/m2, and by energy fractions to 44% visible light, 3% ultraviolet (with the Sun at the zenith, but less at other angles), and the remainder infrared.[8] Thus, sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of ultraviolet radiation.[9]

## Calculation

To calculate the amount of sunlight reaching the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the elliptical orbit by using the day number of the year (dn), is given by[10]

$E_{rm ext}= E_{rm sc} cdot left(1+0.033412 cdot cosleft(2pifrac{{rm dn}-3}{365}right)right),$

where dn=1 on January 1; dn=2 on January 2; dn=32 on February 1, etc. In this formula dn-3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and therefore the maximum Eext occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.

The solar illuminance constant (Esc), is equal to 128×103 lx. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:

$E_{rm dn}=E_{rm ext},e^{-cm},$

where c is the atmospheric extinction coefficient and m is the relative optical airmass.

## Solar constant

The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to the Earth). The "solar constant" includes all types of solar radiation, not just the visible light. Its average value was thought to be approximately 1.366 kW/m²,[11] varying slightly with solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1.361 kW/m² is more realistic.[12] This radiation is about 50% infrared, 40% visible, and about 10% ultraviolet at the top of the atmosphere.[7]

## Total (TSI) and spectral solar irradiance (SSI) upon Earth

Total Solar Irradiance (TSI) – the amount of solar radiation received at the top of the Earth’s atmosphere – was earlier measured by satellite to be roughly 1.366 kilo⁠watts per square meter (kW/m²),[11][13][14] but most recently NASA cites TSI as “1361 W/m² as compared to ~1366 W/m² from earlier observations [Kopp et al., 2005]”, based on regular readings from NASA's Solar Radiation and Climate Experiment(SORCE) satellite, active since 2003,[15] noting that this “discovery is critical in examining the energy budget of the planet Earth and isolating the climate change due to human activities.” Furthermore the Spectral Irradiance Monitor (SIM) has found in the same period that spectral solar irradiance (SSI) at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with earth's climate responses than earlier assumed, fueling broad avenues of new research in “the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate”.[15]

## Intensity in the Solar System

Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder.

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun. A rough table comparing the amount of solar radiation received by each planet in the Solar System follows (from data in [1]):

Planet distance (AU) Solar radiation (W/m²)
Perihelion Aphelion maximum minimum
Mercury  0.3075  0.4667 14,446  6,272
Venus  0.7184  0.7282  2,647  2,576
Earth  0.9833  1.017  1,413  1,321
Mars  1.382  1.666    715    492
Jupiter  4.950  5.458     55.8     45.9
Saturn  9.048 10.12     16.7     13.4
Uranus 18.38 20.08      4.04      3.39
Neptune 29.77 30.44      1.54      1.47

The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example Venus' thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".[16]

Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses, and as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus it would give perceptions and "feel" very much like Earth daylight.

For comparison purposes, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise (see daylight for comparison table). Even on Pluto the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full moonlight on the Earth, a distance of about 500 AU (~69 light-hours) is needed; there are only a handful of objects in the solar system known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

## Surface illumination

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component from atmospheric scatter dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere.

According to Craig Bohren, "preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".[17]

See diffuse sky radiation for more details.

## Climate effects

On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body, the ground and other objects depends on the absorption (electromagnetic radiation) of the electromagnetic radiation in the form of heat.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant due to Kepler's second law,

$tfrac{2A}{r^2}dt = dtheta,$

where $A$ is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.

$int_{0}^{T} tfrac{2A}{r^2}dt = int_{0}^{2pi} dtheta = mathrm{constant}.$

If we assume the solar radiation power P as a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a constant.

But the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies.[18] For example, at latitudes of 65 degrees the change in solar energy in summer and winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles).

## Past variations in solar irradiance

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies with the 11-year sunspot solar cycle. When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done.[19][20][21][22] These studies show that solar irradiance does vary with distinct periodicities such as: 11 years (Schwabe), 88 years (Gleisberg cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle).

## Life on Earth

This short film explores the vital connection between Earth and the Sun.

The existence of nearly all life on Earth is fueled by light from the sun. Most autotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building blocks and in other synthetic pathways which allow the organism to grow.

Heterotrophs, such as animals, use light from the sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as cellular respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals which had previously only provided humans with meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans.

The more recent discoveries of coal, petroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g. alternative fuels, Malthusian catastrophe, new urbanism, peak oil).

## Cultural aspects

Claude Monet: Le déjeuner sur l'herbe

The effect of sunlight is relevant to painting, evidenced for instance in works of Claude Monet on outdoor scenes and landscapes.

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Cars, many helmets and caps are equipped with visors to block the sun from direct vision when the sun is at a low angle. Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, shutters or curtains, or by nearby shade trees.

In colder countries, many people prefer sunnier days and often avoid the shade. In hotter countries the converse is true; during the midday hours many people prefer to stay inside to remain cool. If they do go outside, they seek shade which may be provided by trees, parasols, and so on.

In Hinduism the sun is considered to be a god as it is the source of life and energy on earth.

### Sunbathing

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafés. Sunbathers typically wear limited amounts of clothing or some simply go nude. For some, an alternative to sunbathing is the use of a sunbed that generates ultraviolet light and can be used indoors regardless of outdoor weather conditions and amount of sunlight.

For many people with pale or brownish skin, one purpose for sunbathing is to darken one's skin color (get a sun tan) as this is considered in some cultures to be beautiful, associated with outdoor activity, vacations/holidays, and health. Some people prefer naked sunbathing so that an "all-over" or "even" tan can be obtained, sometimes as part of a specific lifestyle.

For people suffering from psoriasis, sunbathing is an effective way of healing the symptoms.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes and it is actually an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

## Effects on human health

The body produces vitamin D from sunlight (specifically from the UVB band of ultraviolet light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet.

Sunburn can have mild to severe inflammation effects on skin; this can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called skin photodamage), which produces a difficult to treat cosmetic effect. Some people are concerned that ozone depletion is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.[23]

A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) involve light therapy, replicating sunlight via lamps tuned to specific wavelengths of visible light, or full-spectrum bulbs.

A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS) later in life.[24]unlight is a portion of the electromagnetic radiation given off by the Sun, particularly infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through the Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon.

When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light.

The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter.[1]

Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer.

Sunlight takes about 8.3 minutes to reach the Earth. On average, it takes energy between 10,000 and 170,000 years to leave the sun's interior and then be emitted from the surface as light.[2]

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface. Sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of ultraviolet radiation. At the top of the atmosphere sunlight is about 30% more intense with more than three times the fraction of ultraviolet (UV), with most of the extra UV consisting of biologically-damaging shortwave ultraviolet.[citation needed]

Sunlight is a key factor in photosynthesis, a process vital for many living beings on Earth.

## Composition and power

Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K.[3] The Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces Gamma rays as a result of the nuclear fusion process, these super high energy photons are converted to lower energy photons before they reach the Sun's surface and are emitted out into space. As a result, the Sun does not emit gamma rays. The Sun does, however, emit X-rays, ultraviolet, visible light, infrared, and even radio waves.[4]

Although, as mentioned, the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right) and will not be discussed further. The spectrum of nearly all solar electromagnetic radiation striking the Earth's atmosphere spans a range of 100 nm to about 1 mm. This band of significant radiation power can be divided into five regions in increasing order of wavelengths:[5]

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface. This spectrum of radiation has has germicidal properties, and is used in germicidal lamps.
• Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn.
• Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis. However, UV A is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and is able to cause cancer.[6]
• Visible range or light spans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye.
• Infrared range that spans 700 nm to 106 nm (1 mm). It is responsible for an important part of the electromagnetic radiation that reaches the Earth. It is also divided into three types on the basis of wavelength:
• Infrared-A: 700 nm to 1,400 nm
• Infrared-B: 1,400 nm to 3,000 nm
• Infrared-C: 3,000 nm to 1 mm.

Sunlight in space at the top of Earth's atmosphere at a power of 1366 watts/m2 is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.[7] At ground level this decreases to about 1120–1000 watts/m2, and by energy fractions to 44% visible light, 3% ultraviolet (with the Sun at the zenith, but less at other angles), and the remainder infrared.[8] Thus, sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of ultraviolet radiation.[9]

## Calculation

To calculate the amount of sunlight reaching the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (Eext), corrected for the elliptical orbit by using the day number of the year (dn), is given by[10]

$E_{rm ext}= E_{rm sc} cdot left(1+0.033412 cdot cosleft(2pifrac{{rm dn}-3}{365}right)right),$

where dn=1 on January 1; dn=2 on January 2; dn=32 on February 1, etc. In this formula dn-3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and therefore the maximum Eext occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.

The solar illuminance constant (Esc), is equal to 128×103 lx. The direct normal illuminance (Edn), corrected for the attenuating effects of the atmosphere is given by:

$E_{rm dn}=E_{rm ext},e^{-cm},$

where c is the atmospheric extinction coefficient and m is the relative optical airmass.

## Solar constant

The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to the Earth). The "solar constant" includes all types of solar radiation, not just the visible light. Its average value was thought to be approximately 1.366 kW/m²,[11] varying slightly with solar activity, but recent recalibrations of the relevant satellite observations indicate a value closer to 1.361 kW/m² is more realistic.[12] This radiation is about 50% infrared, 40% visible, and about 10% ultraviolet at the top of the atmosphere.[7]

## Total (TSI) and spectral solar irradiance (SSI) upon Earth

Total Solar Irradiance (TSI) – the amount of solar radiation received at the top of the Earth’s atmosphere – was earlier measured by satellite to be roughly 1.366 kilo⁠watts per square meter (kW/m²),[11][13][14] but most recently NASA cites TSI as “1361 W/m² as compared to ~1366 W/m² from earlier observations [Kopp et al., 2005]”, based on regular readings from NASA's Solar Radiation and Climate Experiment(SORCE) satellite, active since 2003,[15] noting that this “discovery is critical in examining the energy budget of the planet Earth and isolating the climate change due to human activities.” Furthermore the Spectral Irradiance Monitor (SIM) has found in the same period that spectral solar irradiance (SSI) at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with earth's climate responses than earlier assumed, fueling broad avenues of new research in “the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth’s climate”.[15]

## Intensity in the Solar System

Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfinder.

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun. A rough table comparing the amount of solar radiation received by each planet in the Solar System follows (from data in [1]):

Planet distance (AU) Solar radiation (W/m²)
Perihelion Aphelion maximum minimum
Mercury  0.3075  0.4667 14,446  6,272
Venus  0.7184  0.7282  2,647  2,576
Earth  0.9833  1.017  1,413  1,321
Mars  1.382  1.666    715    492
Jupiter  4.950  5.458     55.8     45.9
Saturn  9.048 10.12     16.7     13.4
Uranus 18.38 20.08      4.04      3.39
Neptune 29.77 30.44      1.54      1.47

The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example Venus' thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".[16]

Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses, and as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus it would give perceptions and "feel" very much like Earth daylight.

For comparison purposes, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise (see daylight for comparison table). Even on Pluto the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full moonlight on the Earth, a distance of about 500 AU (~69 light-hours) is needed; there are only a handful of objects in the solar system known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

## Surface illumination

The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component from atmospheric scatter dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light photography where the principal source of illumination is sunlight as mediated by the atmosphere.

According to Craig Bohren, "preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".[17]

See diffuse sky radiation for more details.

## Climate effects

On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body, the ground and other objects depends on the absorption (electromagnetic radiation) of the electromagnetic radiation in the form of heat.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant due to Kepler's second law,

$tfrac{2A}{r^2}dt = dtheta,$

where $A$ is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.

$int_{0}^{T} tfrac{2A}{r^2}dt = int_{0}^{2pi} dtheta = mathrm{constant}.$

If we assume the solar radiation power P as a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a constant.

But the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies.[18] For example, at latitudes of 65 degrees the change in solar energy in summer and winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles).

## Past variations in solar irradiance

Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies with the 11-year sunspot solar cycle. When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done.[19][20][21][22] These studies show that solar irradiance does vary with distinct periodicities such as: 11 years (Schwabe), 88 years (Gleisberg cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle).

## Life on Earth

This short film explores the vital connection between Earth and the Sun.

The existence of nearly all life on Earth is fueled by light from the sun. Most autotrophs, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as photosynthesis. These sugars are then used as building blocks and in other synthetic pathways which allow the organism to grow.

Heterotrophs, such as animals, use light from the sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as cellular respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals which had previously only provided humans with meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans.

The more recent discoveries of coal, petroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g. alternative fuels, Malthusian catastrophe, new urbanism, peak oil).

## Cultural aspects

Claude Monet: Le déjeuner sur l'herbe

The effect of sunlight is relevant to painting, evidenced for instance in works of Claude Monet on outdoor scenes and landscapes.

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Cars, many helmets and caps are equipped with visors to block the sun from direct vision when the sun is at a low angle. Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, shutters or curtains, or by nearby shade trees.

In colder countries, many people prefer sunnier days and often avoid the shade. In hotter countries the converse is true; during the midday hours many people prefer to stay inside to remain cool. If they do go outside, they seek shade which may be provided by trees, parasols, and so on.

In Hinduism the sun is considered to be a god as it is the source of life and energy on earth.

### Sunbathing

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafés. Sunbathers typically wear limited amounts of clothing or some simply go nude. For some, an alternative to sunbathing is the use of a sunbed that generates ultraviolet light and can be used indoors regardless of outdoor weather conditions and amount of sunlight.

For many people with pale or brownish skin, one purpose for sunbathing is to darken one's skin color (get a sun tan) as this is considered in some cultures to be beautiful, associated with outdoor activity, vacations/holidays, and health. Some people prefer naked sunbathing so that an "all-over" or "even" tan can be obtained, sometimes as part of a specific lifestyle.

For people suffering from psoriasis, sunbathing is an effective way of healing the symptoms.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes and it is actually an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

## Effects on human health

The body produces vitamin D from sunlight (specifically from the UVB band of ultraviolet light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet.

Sunburn can have mild to severe inflammation effects on skin; this can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called skin photodamage), which produces a difficult to treat cosmetic effect. Some people are concerned that ozone depletion is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.[23]

A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) involve light therapy, replicating sunlight via lamps tuned to specific wavelengths of visible light, or full-spectrum bulbs.

A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS) later in life.[24]

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it has its own heat and light

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Sun is made up of a lot of gases.There are mainly two gases-HYDROGENand HELIUM.These two gases react with each other thus creating light and heat.

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CIRCLE OF ILLUMINATION DIVIDES THE EARTH IN 2 EQUAL HALVES

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OOPS! SORRY I MISSUNDERSTOOD THE QUESTION.

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