Sol - Our nearest Star
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From n3kl.org
 
The sun's gravitational attraction holds the planets in their orbits, and its heat and light are what make life possible on Earth. It's diameter (864 000 miles) means that its volume could hold more than one million Earths. The Sun is so massive that it contains 99.87% of all the matter in the Solar System.
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The Sun is a typical star, but is unique as far as we are concerned because of its proximity to Earth; it is about 150 million km away. A summary of basic solar properties is given in table 1. Solar energy provides virtually all the heat and light which our planet receives, and it therefore sustains every living entity. The character of our own environment is strongly influenced by solar radiation which has been a major factor in determining the course of natural evolution on Earth. The Sun is important to astronomers and physicists because it enables them to investigate physical conditions, which are typical of most stars, in detail. Historically, the study of the Sun led to significant advances in atomic physics, nuclear physics, magnetohydrodynamics, and plasma physics. An understanding of solar processes is therefore of biological and physical interest, and several fundamental areas of scientific research are furthered by observing this local astrophysical laboratory. Satellites have enabled space scientists to probe more closely the interaction between the Sun and the Earth, especially the influence of the Sun on the magnetic field of the Earth.

Table 1

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Before describing the Sun in detail we must issue an important warning about observing the Sun: intense solar radiation permanently damages the tissue of the human eye. The Sun must never be viewed directly with a telescope or binoculars, which would have the effect of concentrating a massive dose of radiation on to the delicate tissue. The solar filters sold with many cheap telescopes are not an adequate safeguard because they may admit a dangerous dose of invisible ultraviolet light or may shatter unexpectedly. They should be destroyed in order to remove the temptation to use them. With a little experimentation, good images of the Sun can be produced by projecting through an eyepiece on to a piece of stiff card. This arrangement is adequate for viewing sunspots or the progress of an eclipse. An eclipse may also be observed by the unaided eye by looking at the Sun through a really dark filter or looking at the reflection in a dark container of still water, but even these methods require caution. We repeat: to look directly at the Sun through binoculars or a telescope is a dangerous act that can cause permanent blindness.
 
The Sun condensed about 5 000 million years ago from a pocket of interstellar gas that contracted under the attractive forces of its own gravity. An important property of the Sun is its mass (330000 times the Earth's mass), which results in a pressure and temperature at the centre of the Sun sufficiently high to cause the nuclear reactions that sustain the prodigious output of energy. A rather small proportion of the total volume of the Sun, known as the core, contains most of the mass and is responsible for the entire luminosity: within the central sphere of one-quarter the solar radius (1.5 per cent of the volume) is concentrated half the mass, and it is here that 99 percent of the energy is generated. Originally the core consisted of about 75 per cent hydrogen, almost 25 per cent helium and around 1 per cent of heavier elements. Although the outer part of the Sun still has this original composition, nuclear burning has altered the make-up of the core.
 
The source of solar energy is the proton-proton cycle, in which hydrogen nuclei are converted to helium nuclei. Today, after more than 4 500 million years of fusion in the core, the concentration by mass of hydrogen has been reduced from 75 per cent to about 35 per cent; consequently the helium abundance has risen to around 65 per cent. Fusion is accompanied by a mass loss, which appears as energy. To generate the observed solar luminosity of 4 x 1026 watts demands the destruction of mass at the rate of 5 X 109kgs-1. Even at this rate the change in the Sun's mass due to fusion while it is a main-sequence star will be below 0.1 per cent. The Sun can maintain essentially its present output for about 5000 million years. After this the exhaustion of hydrogen at the centre will induce structural changes that turn the Sun into a red giant star. Ultimately the Sun will become a white dwarf.
 
Figures 1 and 2 illustrate the variations of the solar temperature and density with increasing distance from the centre. Themcentral temperature is about 1.5 X 107 K, and it decreases steadily, reaching a surface value of 5800 K. At the centre, the density is nearly 1.6 X 105 kgm-3, or 12 times the density of lead. The density falls rapidly with increasing distance; for example, it reaches the value for water half-way from the centre. These extreme physical conditions in the core cause the complete ionization of matter. Even nuclei of the heavy elements are unable to retain orbital electrons. Therefore the solar core consists primarily of hydrogen nuclei (protons), helium nuclei (alpha particles), and free electrons. Most of the energy released from the nuclear fusion reactions is in the form of gamma-ray photons, X-ray photons, and weird particles called neutrinos. The neutrinos have such a small probability of interacting with matter that they stream straight out of the Sun. For the photons a different situation prevails because the free electrons readily scatter photons, and the nuclei can participate in non-elastic collision with photons. These properties of the electrons and nuclei make the core essentially opaque to electromagnetic radiation. Consequently it takes about 106 years for electromagnetic energy to diffuse from the core to the surface of the Sun. This long diffusion time is a major contributor to the stability of the Sun.
Figs 1 (left) & 2 (right)
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Out to a distance of around 0.85 solar radii, energy is transported primarily by radiation. The absence of convection in this part of the Sun prevents the helium made in the nuclear reactions being carried out of the core. At the distance 0.85 solar radii and beyond, the temperature has fallen sufficiently to enable the heavier nuclei to recapture outer orbital electrons and so form partially ionized atoms. These outer electrons of the atoms can easily absorb photons, and this leads to a sharp increase in the opacity of the solar material to radiation. Convective instability is then triggered because the radiation streaming out of the core is suddenly blocked; the transport of energy is primarily by turbulent circulating currents of gas and each element of rising gas takes energy directly to the surface. This zone of convection extends from a depth of 150 000 km or so up to the visible surface itself. At the surface, radiation again predominates as the means of energy transport.
 
Currents in the convection zone are thought to arrange themselves in three major tiers, as shown infigure 3. Deepest are the GIANT CELLS, each encompassing possibly 200000km. At an intermediate layer, SUPER-GRANULAR CELLS about 30000km in diameter are located. Finally, a layer of small currents roughly 1000km across and up to 2000km deep reaches to the surface. The tops of this upper layer make up the Sun's visible surface.

Fig 3

 
An important check on our knowledge of the solar interior has come from attempts to detect the abundant flux of neutrinos, which is released in the core to escape into space easily on account of the almost negligible interaction of neutrinos with matter. An experiment started in the late-1960s utilized a neutrino detector that contained 600 tonnes of liquid tetrachlorethylene (C2Cl4). This substance is relatively effective in recording certain of the neutrinos thought to be emerging from the Sun because they have the appropriate energy to interact with the chlorine atoms. The neutrino telescope was positioned deep in a gold mine to reduce spurious responses caused by cosmic rays. After the experiment had run for several years it consistently recorded the number of solar neutrinos to be less than the theoretical predictions. Subsequently, experimental and theoretical refinements reduced the discrepancy somewhat, but the reasons for the lack of agreement are not clear. The experiment has demonstrated that our understanding of solar physics is perhaps not as complete as the simplified outline given here would suggest. One possibility, among several, is that the Sun's luminosity may vary slightly over a time scale of 2 x 108 years; this might also account for some of the major ice epochs experienced on Earth.
 

The visible surface
The highly luminous surface of the Sun is called the PHOTOSPHERE The photosphere is the sharp disc as observed with the eye or a small telescope. Larger telescopes used under excellent observing conditions show that the photosphere is not uniformly bright. but has a mottled texture termed granulation (figure 4). Graduations in this structure exist, but the smallest granules consist of bright patches of light, about 1 000km across, with a dark border. The pattern is changing continuously as granules dissolve away and are replaced by new ones, so that the appearance changes completely in only a few minutes. Measurements of the Doppler shifts present in light from the photosphere have shown that the bright centre of a granule is moving upwards, whereas the boundary is cooler, descending gas. Almost certainly the solar granulation is associated with the highest convective tier in the Sun. Doppler-shift measuring techniques have also revealed large-scale motion of the photosphere. Within supergranular cells 30 000 km in diameter the gas moves predominantly horizontally from the centre to the edge of the cell. Disturbances deep in the convection zone may also be the cause of the rhythmic rising and falling of the photosphere on a cycle time of five minutes.A remarkable property of the photosphere is that its edge, or limb, appears sharp to the naked eye, rather than merging gradually into the blackness of space, which is how we might expect an incandescent ball of gas to appear. This indicates that the layer from which most of the light is coming is shallow in comparison to the solar radius. The reason for this is as follows: as photons move through the convection zone. the temperature, pressure and density fall steadily. At visible wavelengths the negative hydrogen ion, which is a hydrogen atom that has temporarily captured a second electron, is a major contributor to the emission and absorption of radiation. Above the convection zone the density of this ion decreases much more rapidly than the total density of the solar atmosphere because it is extremely sensitive to changes in the temperature. One consequence of this is that most of the radiation that we can see is emitted in a layer only 500km thick. Below this the Sun is opaque and above it is completely transparent.
 
Granulation
 
 
The photospheric disc appears slightly less bright at the edge when observed visually or photographically (figure 5); this phenomenon is termed limb-darkening. It arises because a line of sight to the centre of the visible disc penetrates the solar atmosphere vertically and enables us to view slightly deeper, and therefore hotter, more luminous layers, as illustrated in figure 6. Towards the edge of the disc, the line of sight passes obliquely through a greater thickness of cooler and partially opaque atmosphere. Consequently we see only to a slightly higher level; where the temperature is lower the material is less luminous, and therefore the limb appears darker. The magnitude of the effect is dependent on wavelength: it is most noticeable in blue light. There are several ways of deriving a temperature for the Sun's visible surface. The black-body temperature is 6000K. This is obtained by matching the spectrum of the continuum radiation from the Sun to theoretical curves derived from radiation laws.
 
 
 
 
Fig 6Fig 6: Limb darkening arises because the light received from different parts of the solar photosphere arises from different depths in the outer layers of the Sun. A light beam from the solar limb directed towards Earth has to traverse a much greater thickness of the solar atmosphere, and consequently only light from the higher layers of the atmosphere, where the temperature and light intensity are lower, reaches us. When we. look at the. centre of the disc we receive light that has ascended the solar atmosphere vertically, and it is therefore possible, to see to greater, hotter, depths. Consequently the centre appears brighter than the. edge.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The effective temperature is related to the Sun's surface luminosity, which is 6.44 x 107 watts m-2. According to the Stefan-Boltzmann law, an object radiating at 5800K would match this value. We see that the temperature obtained from the shape of the spectrum is greater than the temperature derived from the luminosity. This is because the radiation we receive is coming from a 500-km layer in the solar atmosphere, and the temperature varies in this layer. At the centre of the disc most of the energy is coming from a zone with a temperature' of 6 500 K, whereas at the limb lower values prevail. For this reason it is not possible to define a unique temperature for the Sun's surface.

At the temperature of 6 200 K all substances are entirely gaseous; tungsten is the most refractory of all known elements, melting at 3 643 K and boiling at 6 200 K.

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