The Sun

[sardarzada11]

Introduction
By far, the Sun is the most massive body in our solar system. The mass of all the planets combined is only about 0.2% of the Sun's mass. The Sun is also the only object whose internal temperature is high enough to produce nuclear reactions. If Jupiter had been 100 times more massive, or 1/10 of the mass of the Sun, ours would have been a binary star system. While gas giant planets such as Jupiter do emit more energy than they receive from the Sun, only the Sun owes its internal pressure to nuclear fusion.
Nuclear fusion generates all the power emitted by our star. This energy heats up the gas to very high temperatures. The Sun shines because it is made of incandescent gas, with a surface temperature of about 5,800 K. Because of its high temperature, the Sun emits light in a wide spectrum of wavelengths, with a peak in what we consider the 'visible' part of the spectrum.
The fact that our eyes are sensitive to light of wavelengths corresponding to the Sun's peak emission is no coincidence, of course. Most of the other light from our Sun fortunately does not reach the ground, since our atmosphere absorbs it. If ult raviolet and X-ray radiation reached the Earth's surface, they would be devastating to on our planet.
The portion of the light that we receive from the Sun powers all atmospheric phenomena, and ultimately life itself. Far from having a uniform surface and from emitting a constant amount of energy per unit time, the Sun is very dynamic and displays activity cycles. The best known is the eleven-year cycle, during which the number of sunspots and other disturbances of the solar atmosphere greatly change in number and intensity.
The eleven-year cycle is intimately connected with the intensity of the solar wind, a stream of charged particles emitted by our star that continuously collides with the Earth's magnetosphere. At times, solar eruptions give rise to ejections of gas t hat stream out of the Sun and reach the Earth. The strong flow of particles thus generated can be quite dangerous for the network of communication satellites orbiting our planet

The Sun through History
The Sun has been an essential part of human culture and mythology since prehistoric times. The obvious reason is that the Sun's position in the sky is linked with the seasonal changes on Earth, and seasons have had a great importance both for agricultural and pre-agricultural societies. This point is clearly illustrated by the tremendous effort that ancient people put into building structures like Stonehenge at a time when no technology other than ropes was available to transport boulders weighing several tons. It is now believed that the orientation of the temple/observatory at Stonehenge and other such monuments was chosen so as to mark the Sun's solstices, and to celebrate the change of the seasons.
In classical Greece, and throughout the Renaissance, the Sun was believed to be made of 'ethereal' matter, i.e. perfect and devoid of any blemishes. The same substance was believed to make up all planets and the Moon as well, and the uneven tint of the Moon was explained away by our satellite's vicinity to the Earth. The Earth, contrary to celestial objects, was supposed to be made of corruptible elements.
Given this premise, Galileo's detailed telescopic observation of the Sun in 1610 caused quite a stir. Galileo showed that the Sun has spots on its surface and rotates with a period of about 27 days. Although Chinese astronomers had already observed sunspots with the naked eye, this fact was not known in the West. Galileo's observation, together with the others he made of the solar system, were instrumental in the acceptance of the modern view of the universe, where the same physics applies to the Sun as to any other object, and laboratory experiments on Earth can have universal application.
In the 19th century another debate ensued centering on the reach of scientific knowledge, and once again the Sun was the protagonist. The French philosopher Auguste Comte claimed that, given that we cannot access stars and other astronomical bodies directly, there could be no chance of humanity ever being able to know what exactly they are made of. As it often turns out in the history of science, one should never say never.
Around the same period that Comte made his sweeping statement, it was discovered that different elements, when in gaseous form, absorb light passing through them in a very particular way: only light of particular wavelengths get absorbed, and such wavelengths depend on the element making up the gas. Armed with this knowledge, based on Earth lab experiments, Kirchhoff and Bunsen showed in 1859 that the atmosphere of the Sun was made of hydrogen as well as other known elements.
In fact, the analysis of the solar spectrum soon led to the discovery of helium. Nowadays taking the spectrum of astronomical objects is an essential step in determining their nature. As discovered by the young astronomer Cecilia Payne in 1925, the compo sition of the Sun is very close to the average in the rest of the Universe, and very different from the Earth's. Hydrogen makes up 70.5% of the Sun's mass, followed by 27.5% helium and only 2% of all the other elements. The composition is almost constant th roughout the Sun, although the percentage of helium is higher in the Sun's core, where helium is being formed by nuclear fusion.

The Sun's Interior
The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. These two forces combat and neutralize each other. The temperature and pressure inside the Sun reflect such a balance. Gravity ten ds to squeeze the Sun towards its center, while the pressure of the gas would dismember the Sun into the surrounding vacuum if left unchecked.
For the sake of discussion, think of the Sun's interior as comprised of several little cubic volumes of unit size. The matter inside each volume is attracted to the center of the Sun, therefore weighing onto the underlying gas. No actual motion occurs b ecause the weight of the gas in the unit volume is neutralized by an equal and opposing force due to the pressure of the gas elements around the chosen cubic volume (think of it as a box).
There are six faces to the cube, and the force vector of the pressure on the side faces simply totals zero. However, the two faces, facing downward and upward, correspond to a slightly different pressure, since the pressure of the gas is a function of th e distance from the center of the Sun. The face oriented downward is pushed upward by the pressure of the gas below, while the opposite happens to the face oriented upward.
The difference in the gas pressure at the two locations means that the total force acting on this volume of gas is upward. In a situation of equilibrium, gravity exactly balances this 'pressure gradient,' and the gas can be considered as approximately sta tic. From the laws of thermodynamics and the hydrostatic equilibrium we can, using a computer, reconstruct the density, the pressure and the temperature inside the Sun, as a function of the distance from its center. This can be done by starting from the surface of the Sun, using the conditions we measure directly, and virtually 'peeling off' the Sun layer by layer in a computer simulation.
Though we do not have direct access to the interior of the Sun, plenty of indirect evidence supports the results we get from the computer models. In particular, the Sun and the other stars evolve over time in a way consistent with our expectations. More over, a detailed study of a new discipline called 'helioseismology' is giving us direct insights about the internal structure of the Sun that strongly corroborate the computer models.
Energy Production
The closer to the center of the Sun, the higher are the values of density, temperature and pressure. At about 40% the radius from the center of the Sun (0.4 Rsun), the temperature is so high that the hydrogen nuclei overcome their electrostatic repulsion and smash into each other. At very short distances nuclear forces become important, and cause the hydrogen (H) nuclei to fuse into deuterium (D). Deuterium can further fuse, and the net result of this chain of nuclear reactions is the fusion of four hydrogen nuclei into one nucleus of helium (He).
This process is only qualitatively similar to the nuclear fusion taking place in H-bombs, but in both cases fusion liberates large quantities of energy. Three such reaction chains are active in the Sun. The most common, producing about 85% of the energy, is the PPI chain: 1H + 1H→2D + e+ + ν 2D + 1H→3He + γ 3He + 3He→4He + 1H + 1H In this chain of reaction, 41H nuclei combine to form one 4He, and the reactions also produce one antielectron e+, a neutrino ν and a γ-ray. Both the e+ and the ν are generated by the nuclear process that transmutes a proton (hydrogen nucleus) into a neutron, while forming deuterium. Gamma rays are high-energy photons--higher in energy visible light or even X-rays.
The other two reaction chains are less common. The PPII chain, which produces about 15% of the total energy, reads as follows: 1H + 1H→2D + e+ + ν 2D + 1H→3He + γ 3He + 4He→7Be + γ 7Be→7Li + e+ + ν 7Li + 1H→4He + 4He After being liberated in the reaction, the e+ immediately gets annihilated by colliding with ordinary electrons in the plasma, e+ + e-→γ, thus producing more γ-rays. The reaction 1H + 1H→2D is the one with the smallest probability of occurring in a collision between nuclei: two H nuclei will transform into one D nucleus on a typical time scale of more than a billion years. However, by the end of the Sun's 1010 years lifetime, about 1056 hydrogen nuclei will have undergone fusion.
The reaction chains that we have described are the predominant ways in which fusion occurs in the Sun. They are not the only ways, however. Another, more complicated chain, called the CNO cycle, also contributes some energy. In stars with a mass larger than that of the Sun, the CNO cycle is dominant because the stellar core has a higher temperature. The CNO cycle involves nuclei of carbon (C), nitrogen (N) and oxygen (O) as catalysts.
As we are going to discuss further, neutrinos easily escape from the Sun. The γ-rays, however, move only a few centimeters before being intercepted and scattered by the particles in the plasma. The energy carried by the photons is efficiently redistributed to the plasma and thermalized. Photons liberated in the fusion reactions and the e+ + e- annihilations eventually escape the Sun, after about 10 million years of wandering in the star's interior. Most of the photon energy becomes kinetic energy of the plasma, helping to keep the plasma at a high temperature.
Energy Transport: Radiation and Convection Regions
On a macroscopic scale, the process of photon scattering we have just described is the main mechanism of energy transport within much of the Sun's interior. It is customarily called 'radiative energy transport,' and it is efficient in regions of the Sun where the change of temperature as a function of the radius is not too large. Where the temperature differential of different layers becomes too great, 'convective energy transport' becomes a more efficient mechanism of energy exchange between hotter and colder regions of the Sun.
Convection is best illustrated by the motions of the water in a boiling pot, or the by the weather phenomena in the Earth's atmosphere. The basic idea is that cells of hot gas at the bottom rise up to the top, where they cool down before plunging back. In the case of the Earth's atmosphere, the cells cool by radiating, since at the top the radiative transport becomes efficient. In the case of the Sun, the bottom of the convective zone is situated at about 0.7 Rsun, and the top is at the base of the photosphere: in complete analogy with Earth, the cells rise up to the top of the convective zone where they cool down by radiational cooling.
The internal structure of the Sun is known by using computer models, as we previously mentioned, but the predictions of the models are confirmed by the direct observations of a new branch of solar physics, called 'helioseismology.' Broadly speaking, the various motions in the Sun's interior, and the phenomena at the level of the photosphere generate sound waves, which travel in the Sun in a way that completely resembles earthquakes.
Delicate measurements of the vibrations and the way they propagate across the Sun can be used to scan the structure of the Sun's interior, providing us with a profile of the physical conditions and of the motion of the gas. Such measurements indicate that the Sun's convection zone indeed extends down to 0.7 Rsun. In the convection region the Sun rotates differentially, just like at its surface. In the core and the radiative zone, however, the Sun rotates like a rigid body, with a period of rotation of about 27 days.
The Solar Neutrino Problem
As we discussed previously, the nuclear reactions in the Sun's core generate neutrinos ν. Neutrinos are weakly interacting electrically neutral particles with little or no mass. They travel long distances before being scattered or absorbed. In other words, the Sun is transparent to neutrinos, and these particles simply escape from it at the speed of light. It is immediately apparent that neutrino detection could be an excellent and direct way to check our theoretical models of Sun's interior.
In the 1960s Raymond Davis designed an experiment to detect solar neutrinos. This was a tricky experiment, since the 1011 neutrinos/sec hitting each cm2 of surface area on Earth mostly pass through our planet without being deflected. Davis's experiment was set up in the depth of a gold mine, so that the mountain rock would shield other particles, which would have confused the reading. In Davis's experiment, chlorine37 atoms contained in a pool of C2Cl4 were struck by neutrinos and transformed into argon37 Ar atoms. The reaction rate was very small, but over time the argon gas built up to detectable levels.
The results of Davis's experiment and of later ones designed to detect neutrinos coming from other nuclear processes in the Sun are puzzling. Invariably, the experiments detect one third to one half of the expected number of neutrinos, and the question arises: are our models of the Sun incorrect, or does some physical process change the neutrinos during their flight and render them undetectable?
Today's favorite candidate theories involve the transmutation of the neutrinos during flight to a variety (the exact term is 'flavor') which is undetectable by our experiments. This transmutation could naturally occur if the neutrinos had a small mass, rather than being completely massless. There are hints, from other neutrino experiments, that neutrinos do have a small mass, but the scientific debate on the solar neutrino problem is still raging.
The Sun's Atmosphere
The atmosphere of the Sun and most of its interior are made mostly of hydrogen and helium. In the atmosphere, helium constitutes 73% of the mass while helium constitutes 25%, leaving only 2% for other elements. For the Sun as a whole (both atmosphere and interior), hydrogen averages at 70.5%, helium at 27.5%, and all other elements at 2%. The Sun is completely in a gaseous phase. The gas, made of the elements mentioned above, is either neutral or ionized depending on the atmospheric parameters at different locations. In an ionized gas, also called 'plasma,' some or all electrons orbiting the nuclei are stripped from the atoms, due either to violent collisions with other atoms or the absorption of light of sufficient energy. The higher the temperature of the gas the more favorable the conditions for the formation of a plasma.
Photosphere
The boundary between the atmosphere and the interior of the Sun is a region about 1,000 km thick, called the photosphere. Given that the radius of the Sun is 696,000 km, the photosphere is a relatively thin layer. Most of the light that we receive from the Sun comes from this boundary, which we customarily associate with the Sun's 'disk.' The existence of the photosphere is due to a drop in the 'opacity' of the gas in that region.
Opacity is an important concept, deserving a more detailed description. A gas is called opaque when propagating photons can only travel short distances before being deflected. The net effect of these many scatterings is a modification and randomization of the average wavelength of light in direct correspondence to the temperature of the gas. The light, in other words, gets 'thermalized' by its interaction with the gas.
Transparent gas represents the opposite situation: scattering and absorption of light happens seldom, allowing light to cover large distances without being deflected. While the interior of the Sun is opaque, its atmosphere is largely transparent. In fact, the transition between opaque and transparent layers is what defines the 'surface' of the Sun. The capacity of the gas to scatter light drastically diminishes at the base of the photosphere. One can make a loose comparison between the surface of the Sun and the surface of a cloud on Earth, where the 'border' of the cloud is defined by the opacity of the water droplets.
The spectrum of the Sun resembles pretty closely that of a black body at a temperature of 5,800 K. That is the temperature of the gas at the base of the photosphere. The light coming from the interior of the Sun is scattered many times below the photosphere, but from the base of the photosphere upward it is almost free to travel without deflection, keeping its spectrum almost unchanged.
The farther up one goes in the thin layer of the photosphere, the colder the gas becomes. The temperature actually drops to about 4,200 K. As light passes through the transparent and colder gas of the upper photosphere, dark lines appear in the solar spectrum in the foreground of an otherwise featureless black body spectrum. This phenomenon was first observed by Fraunhofer in the early 19th century.
The dark lines correspond to the specific wavelengths at which the various elements absorb light passing through the gas. The fact that one sees dark lines is correlated with the lower temperature of the gas in the upper photosphere, as compared to its base: if the temperature were increasing with height one would see bright lines superposed on the black body spectrum, as Kirchhoff and Bunsen showed in their laboratory.
The photosphere is far from being a homogeneous surface. It shows what is called 'granulation.' The granules are regions 1,500 km wide, on average. At the center of a granule the temperature of the photosphere is few hundreds Kelvin degrees higher than at its edge. The surface of the Sun appears coarse-grained because it is the outer edge of a vast convective region in the Sun's interior.
The granules are simply 'bubbles' of hotter gas coming from the interior. One also detects larger supercells, typically 30,000 km in size, which contain several granules. This supergranulation is probably related to large scale currents involving the whole convective zone of the Sun's interior (as we discuss later in the SparkNote).
Chromosphere
The layer of the atmosphere adjacent to the photosphere and extending outward to the corona is called chromosphere. Its boundary is defined by an increase in the atmospheric temperature with altitude, in contrast with the decrease seen in the photosphere. In about 2,000 km, the temperature of the chromosphere increases from 4,200 K to 25,000 K. Its density, though, is only about 10-4 that of the photosphere.
Due to its low density, seen against the backdrop of the photosphere, the chromosphere is all but invisible. Hence, it was only discovered when astronomers observed the Sun during solar eclipses. During such eclipses, the disk of the Moon covers the photosphere and permits the view of the upper layers of the atmosphere of the Sun, i.e. the chromosphere and the corona.
The chromosphere owes its name to its bright red color, against the dark background of the sky during solar eclipses. Under these circumstances, its spectrum is made of several emission lines (no black body component is expected from a transparent gas). Given the temperature and the composition of the gas, much of the light comes from the red Balmer-α spectral line of the hydrogen, a fact that explains the prevalent color of the chromosphere.
Corona
Sun's corona extends to distances comparable to our star's radius. At wavelengths in the visible spectrum, the corona is only visible during solar eclipses. It can also be seen by using 'coronagraphs,' which block the sunlight from the photosphere directly within telescopes, thus simulating solar eclipses. The corona is irregularly shaped and extends farther where there are disturbances in the underlying layers of the atmosphere. The corona is very hot and extremely diluted. It can reach temperatures of few million Kelvins, and a density 10-12 that of the photosphere.
At the shorter UV and X-ray wavelengths, only accessible by telescopes orbiting above Earth's atmosphere, the irregular shape of the corona is strongly correlated with the distribution of the Sunspots and of the solar eruptions. The corona shines brightly in the X-ray region of the spectrum, against the dark background of the photosphere: the photosphere emits as a black body at 5,800 K, which tapers off at wavelengths in the ultraviolet region of the spectrum.
The transparent hot gas of the corona emits a line spectrum, just like the spectrum of fluorescent light bulbs. The emission is strong in the X-rays because of the extreme temperature of the gas. It is still not certain why the corona is so hot. It seems likely that the gas is heated up by colliding with the particle streams generated by the photosphere during solar eruptions. This would explain why the corona emits the strongest radiation in correspondence with eruptions and sunspots.
Because of its temperature, the corona is a highly ionized gas. Oxygen, for instance, is often stripped of two of its eight electrons. As a direct consequence of the ionization, the corona is electrically charged and its gas particles are deviated in their motion when subjected to the Sun's strong magnetic field. The magnetic field is a very important component of solar atmospheric activity. The temperature of the corona is so high that the gravitational attraction of the Sun is not strong enough to keep the corona from escaping the Sun. The gas is bound to the star mainly because of the trapping action of the star's magnetic field.
Sunspots, Prominences and Flares
As was noticed long ago through naked eye observation made by Chinese astronomers, and then by Galileo using the telescope, the Sun is dotted by several spots. Spots are transitory phenomena that appear as darker patches in the photosphere. They are irregular and their size can easily reach more than 10,000 km in diameter. Spots are usually found in groups, and quite often the groups form pairs, oriented along the Sun's parallels. Each spot is composed of a central dark region, called 'umbra,' surrounded by a lighter region, called 'penumbra.'
The cause of the darkness is simply related to the temperature of the gas. Temperatures at the center of the umbra are usually around 3,000 K. The lower the temperature, the weaker the blackbody emission of the photosphere. As an analogy, think of what happens when one turns down the voltage applied to an incandescent bulb, thereby lowering the temperature of is filament. The lower the voltage, the dimmer and redder the black body radiation emitted by the light bulb, in complete analogy with the sunspot being just a colder region of the photosphere.
Flares and prominences are phenomena of the chromosphere and of the corona. They are associated with the sunspot groups, and they are part of the same physical phenomenon. While spots are at times detectable even without the aid of a telescope, flares and prominences are best seen either during solar eclipses or using special filters that highlight their emission in the backdrop of the emission from Sun's photosphere. Flares are localized eruptions that can emit great amounts of energy. They appear brightest in the X-ray portion of the spectrum (at which the background of the photosphere is weaker), and are associated with the sunspots. An individual flare can emit up to 1033 ergs of energy.
Prominences assume different shapes. They typically appear as arcs of gas following the magnetic field lines associated with the sunspot groups, and they are of comparable size. Often prominences extend to well within the Sun's corona, and sometimes some of the gas completely escapes the Sun's gravity--a phenomenon called 'coronal mass ejection.' The ejected gas is highly ionized, just as are the prominences that originate it. When the ions reach the Earth they often cause damage to our telecommunication satellites.
The Solar Activity Cycle
Much solar activity observes cycles, the best known of which lasts about eleven years. As was first noticed by H. Schwabe in 1843, the average number of sunspots changes over time. Spots are nearly absent at the 'solar minimum,' reaching a peak at the 'solar maximum.' At the maximum it is not uncommon to count up to ten groups of sunspots at one time. The distribution of the spots also changes. Near the solar minima, spots are confined to a latitude of about 30-40 degrees North and South on the Sun's surface. As the cycle progresses, the spots are gradually found closer to the Sun's equator, and they increase in number.
An explanation for the solar cycle, and all the phenomena associated with the sunspots, was given by H. Babcock in 1960. The cycle is correlated with the distribution of the magnetic field in the outer layers of the Sun's interior. At the solar minimum the field is roughly oriented along the meridians, and the Sun's magnetic poles are not far from the poles of its rotation axis. Gradually, as the cycle progresses, the field lines are stretched and deformed, winding more and more around the Sun, in a pattern resembling a winding coil.
The field lines gradually become more closely oriented with the Sun's parallels, and their distance from each other decreases. Since the distance between field lines is correlated with the strength of the magnetic field, as the cycle approaches a solar maximum the magnetic field increases in strength. As it turns out, the stretching of the field lines is due to the differential rotation of the Sun. Closer to the equator, the Sun's outer layers rotate with a period of about 25 days, while the period gradually increases to 27 days at mid-latitudes.
Much of the Sun's interior is plasma, i.e. charged particles. From studying the dynamics of charged fluids we know that the magnetic field lines tend to move together with the plasma. `Since the differential rotation of the Sun causes a lag in the rotation of regions farther away from the equator, one can intuitively see why the field lines get stretched. As the plasma finds itself immersed in an increasingly strong magnetic field, it becomes unstable, and it arches out of the Sun's surface forming the sunspot groups. Pairs of spot groups correspond to places where the magnetic field arches out to the Sun's corona, in the shape of an Ω.
The disruptions in the magnetic field lead to great differences in the field's strength from place to place around the sunspots. These disruptions are responsible for the motion of vast quantities of plasma that we see as prominences. Flares are due to the collision of high energy plasma particles, where the particles are accelerated to high speeds and kinetic energy by processes similar to those used in particle accelerator laboratories on Earth. Flares occur in the vicinity of sunspot groups, where we find strong variable magnetic fields.
The mechanism that heats the corona to more than 106 K is correlated with these phenomena as well, although the detailed processes responsible for the heating are not completely understood. Because of the extreme temperature of the corona, its particles are held in the proximity of the Sun only where the magnetic field lines form loops, causing the particles to move along an arched trajectory that takes them back to the Sun's surface. At the poles of the Sun's magnetic field the high energy charged particles do escape the Sun, which is why the corona is absent there.
The Sun continuously emits a steady stream of charged particles. This flow does not directly reach the Earth because it is deviated by the Earth's magnetic field. Some particles, though, are able to penetrate this barrier and hit the Earth's atmosphere, giving rise to the phenomenon called aurora, a colorful luminescence in the night sky at high latitudes (such as Northern Canada and Scandinavia). The auroras happen preferentially close to the magnetic poles of the Earth's magnetic field, since the charged particles tend to follow the magnetic field lines.
The eleven-year cycle has been studied in great detail. The long-term activity cycles are less known because they take place over periods of time for which we do not have a direct record. There is evidence indicating that the long-term cycles of the Sun give rise to important oscillations in the Earth's climate. Between 1645 and 1715 no sunspots were detected at all. This sudden solar inactivity coincided with a decrease in temperature on our planet, called the 'little ice age'. It is possible that the main ice ages are also correlated with the Sun's activity, although this conjecture has not been sufficiently corroborated, and the Sun may be just a co-factor in this equation.

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Sardarzada