ABOUT THE SUN

The Sun is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for about 99.8% of the solar system's mass. Energy from the Sun in the form of sunlight supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

Overview of the Sun:
About 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. The Sun has a spectral class of G2V. "G2" means that it has a surface temperature of approximately 5,500 K, giving it a white color, which, because of atmospheric scattering, appears yellow. This is a subtractive effect, as the preferential scattering of blue photons (that 'color' the sky) removes enough blue light to leave a residual reddishness that is perceived as yellow. (See Rayleigh scattering for detail and references.) Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The "V" suffix indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic balance, neither contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the Galaxy, most of which are red dwarfs. The Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 light-years from the galactic center, completing one revolution in about 225-250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days. The Sun is a third generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as gold and uranium in the solar system. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star. Sunlight is the main source of energy to the surface of Earth. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,370 watts per square meter of area at a distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. This energy can be harnessed via a variety of natural and synthetic processes photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.

Ultraviolet light from the Sun has antiseptic properties and can be used to sterilize tools. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly attenuated by Earth's atmosphere, so that the amount of UV varies greatly with latitude because of the longer passage of sunlight through the atmosphere at high latitudes. This variation is responsible for many biological adaptations, including variations in human skin color in different regions of the globe [3]. Observed from Earth, the path of the Sun across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma and resembles a figure 8 aligned along a north/south axis. While the most obvious variation in the Sun's apparent position through the year is a north/south swing over 47 degrees of angle (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an east/west component as well. The north/south swing in apparent angle is the main source of seasons on Earth. The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years. The Sun's magnetic field gives rise to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in the solar wind that carry material through the solar system. The effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the solar system, and strongly affects the structure of Earth's outer atmosphere. Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 1 million K while its visible surface (the photosphere) has a temperature of less than 6,000 K. Current topics of scientific inquiry include the Sun's regular cycle of sunspot activity, the physics and origin of solar flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin of the solar wind.

Life cycle of the Sun:
The Sun's current age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years. The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. The Sun will spend a total of approximately 10 billion years as a main sequence star. The Sun does not have enough mass to explode as a supernova. Instead, in 4-5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 MK, and will produce carbon and oxygen. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. However, Earth's water will be boiled away and most of its atmosphere will escape into space. Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.

Structure of the Sun:
While the Sun is an averaged-sized star, it contains approximately 99% of the total mass of the solar system. The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 km. While the Sun does not rotate as a solid body (the rotational period is 25 days at the equator and about 35 days at the poles), it takes approximately 28 days to complete one full rotation; the centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. Tidal effects from the planets do not significantly affect the shape of the Sun. The Sun does not have a definite boundary as rocky planets do; in its outer parts the density of its gases drops approximately exponentially with increasing distance from the center of the Sun. Nevertheless, the Sun has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light; the photosphere is the surface most readily visible to the naked eye. Most of the Sun's mass lies within about 0.7 radii of the center. The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the Sun's inner structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.



Core of the Sun:
The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150,000 kg/m3 (150 times the density of water on Earth) and a temperature of close to 13,600,000 kelvins (by contrast, the surface of the Sun is close to 5,785 kelvins (1/2350th of the core)). 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. The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles. About 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every second (out of about ~8.9×1056 total amount of free protons in Sun), releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (383×1024 W) or 9.15×1010 megatons of TNT per second. This corresponds to extremely low rate of energy production in the Sun's core - about 0.3 µW/cm³, or about 6 µW/kg. For comparison, ordinary candela produces heat at the rate 1 W/cm³, and human body - at the rate 1.2 W/kg. Use of plasma with similar parameters as solar interior plasma for energy production on Earth is completely impractical - as even modest 1 GW fusion power plant would require about 170 billion tonnes of plasma occupying almost one cubic mile. The rate of nuclear fusion depends strongly on density (and extremely strongly on temperature), so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level. The high-energy photons (gamma and X-rays) released in fusion reactions are absorbed in only few millimeters of solar plasma and then re-emitted again in random direction (and at slightly lower energy) - so it takes a long time for radiation to reach the Sun's surface. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years. After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.

Radiation zone of the Sun:
From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone there is no thermal convection; while the material grows cooler as altitude increases, this temperature gradient is slower than the adiabatic lapse rate and hence cannot drive convection. Heat is transferred by radiation ions of hydrogen and helium emit photons, which travel a brief distance before being reabsorbed by other ions. Light in this layer takes millions of years to escape and from there takes about 8 minutes to reach Earth.

Convection zone of the Sun:
Structure of the SunFrom about 0.7 solar radii to the Sun's visible surface, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone. The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.

Photosphere of the Sun:
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H- ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H- ions. The photosphere is actually tens to hundreds of kilometres 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 is known as limb darkening. 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 about 1023 m-3 (this is about 1% of the particle density of Earth's atmosphere at sea level). During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. It was not until 25 years later that helium was isolated on Earth.

Astronomy Hacks: Tips and Tools for Observing the Night Sky (Hacks)

Atmosphere of the Sun:
The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun; the reason why is not yet known. Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top. Above the chromosphere is a transition region in which the temperature rises rapidly from around 100,000 K to coronal temperatures closer to one million K. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the far ultraviolet portion of the spectrum. The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 1014 m-3-1016 m-3. (Earth's atmosphere near sea level has a particle density of about 2×1025 m-3.) The temperature of the corona is several million kelvin. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection. The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the solar wind becomes superalfvénic that is, where the flow becomes faster than the speed of Alfvén waves. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.

Sunspots and the sunspot cycle of the Sun:
When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometres across. The number of sunspots visible on the Sun is not constant, but varies over a 11 year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity. The polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next. History of the number of observed sunspots during the last 250 years, which shows the ~11 year solar cycleThe solar cycle has a great influence on space weather, and seems also to have an influence on the Earth's climate. Solar minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.

Possible long term cycle of the Sun:
A recent theory claims that there are magnetic instabilities in the core of the Sun which cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles. Like many theories in astrophysics, this theory cannot be tested directly.

Theoretical problems of the Sun:
Solar neutrino problem:
For many years the number of solar electron neutrinos detected on Earth was one third to one half of the number predicted by the standard solar model. This anomalous result was termed the solar neutrino problem. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate - that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth. Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and do indeed oscillate. Moreover, in 2001 the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun's total neutrino emission rate agreed with the Standard Solar Model, although depending on the neutrino energy as few as one-third of the neutrinos seen at Earth are of the electron type. This proportion agrees with that predicted by the MSW effect which describes neutrino oscillation in matter. Hence, the problem is completely resolved.

Coronal heating problem of the Sun:
The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K. Above it lies the solar corona at a temperature of 1,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events. Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.[20] In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales, but this remains an open topic of investigation.

Faint young Sun paradox of the Sun:
Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than it is today. The consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide and/or ammonia) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.

Magnetic field of the Sun:
The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium. All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences (see magnetic reconnection). This twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years. The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.



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