The amount of energy radiated from the Sun varies very slightly (depending on the amount of solar activity) and this is known as the Solar Constant. At the distance of the Earth from the Sun, (150 million kilometres) this energy is spread out over about 70 000 million square kilometres, reducing its intensity to about 1.3 kW per square metre at the top of the atmosphere, on a surface directly facing the Sun. The atmosphere further reduces the Suns power to a maximum of about 1 kW per square metre on the surface of the Earth (Figure 16). This reduction is due to absorption and scattering, particularly by dust and water vapour. Even on the clearest day, at least 10% of the Suns energy comes from scattered light. This component cannot be focused by concentrating collectors, but can be absorbed by flat surfaces.

 

 

Figure 16 Atmospheric reduction of sunlight.

The intensity of the Suns energy (power) falling on a surface will vary depending on a number of factors:

• The amount of cloud cover: Cloud cover will reduce the amount of sunlight available. Some of the radiation is scattered, and can be absorbed by a flat surface, but not focused.
• The amount of dust and optically active pollutants in the atmosphere: These will also reduce the total radiation, as some is scattered or diffused.
• The thickness of the atmosphere: The amount of sunlight absorbed and scattered by the atmosphere depends on its thickness. For example, the maximum intensity at the top of mountains is higher than at sea level, and is greater at the equator than at higher latitudes (that is, towards the poles). Even at a particular latitude, the intensity will vary with the seasons as the Sun varies in its maximum angular altitude.
• The angle between the Sun and the absorbing surface: In a given situation, the maximum sunlight intensity will be on a surface directly facing the Sun. The variation with angle is illustrated in Figures 17 and 18.

 

Figure 17

Because the Sun produces so much energy in the visible spectrum, scientists must use different wavelengths to examine the surface and the atmosphere of the Sun. When astronomers look at the surface of the Sun in white light, or the visible part of the spectrum, they use special filters, which block all but approximately 0.0001% of the light from the Sun. On the surface we can see unusual dark and light texture, which is called solar granulation (see Figure 12). This granulation is a result of the convection of energy from the core through the interior, and each granule is only about 1000 km across. The bright areas are the hottest areas, where energy has just arrived at the surface as part of the convective cycle, and the darker areas are cooler zones, which are about to descend into the interior for warming.

Figure 12 Close up image of the solar surface showing granulation.

 

Figure 13 Doppler shift colour image of Super Granulation, red supergranules are heading
towards the interior, blue towards the surface.

Super granulation is a larger scale version of the granulation seen on the surface, except each supergranule is approximately 35 times larger than the granules. Images such as Figure 13 are taken using instruments, which can measure the Doppler shift of the supergranules, hence the colour of the image.

About 5.5 billion years ago, a passing star or galaxy disturbed a calm and placid cloud of gas and dust, called a nebula. The star or galaxy caused the cloud to swirl around, causing small eddies to form. The swirl caused the gas to start to coalesce together. Gravity, one of the universes four fundamental forces, caused more and more gas and dust to gather onto these masses. The masses kept getting bigger and bigger. At this stage, they were called protostars. As gravity caused the material to pile on, it also caused them to condense, which increased their gravitational force. The condensation caused the pressure in their cores to rise, and their internal heat increased. When the heat reached a temperature of 10,000,000 °C, nuclear fusion started, and our Sun was born (Case Western Reserve University, 2006).

After a lifetime of 9 billion years as a main-sequence star, approximately 10% of the hydrogen in the Suns core will have been converted into helium, and nuclear fusion reactions will cease producing energy. The equilibrium between the total pressure force directed outwards and the gravitational force directed towards the centre of the Sun will be disturbed. The core of the Sun starts to slowly collapse under its own gravity and the fusion reactions move out towards shells surrounding the core, where hydrogen-rich material is still present. The gravitational energy from the collapse will be converted into heat causing the shell to burn vigorously and the Suns outer layers swell. The surface will be far removed from the central energy source, and it will cool and appear to glow red. The Sun will then have evolved into the stage of a red giant (Encyclopedia of Planetary Sciences, 1997).

For a few hundred million years, the expansion of the outer solar layers will continue, and the Sun, as a red giant, will engulf the planet Mercury. The temperature on Venus and Earth will rise tremendously. Hydrogen fusion in the shell continues to deposit helium "ash" onto the core, which becomes even hotter and more massive. In the Suns core nuclear fusion of helium into carbon and oxygen will start to trigger even further expansion of its outer layers. The helium-rich core, unable to lose heat fast enough becomes unstable. In a very short time of a few hours, the core will get too hot and is forced to expand explosively. Outer layers of the Sun will absorb the core explosion but the core will no longer be able to produce energy by thermonuclear burning. Helium fusion will then continue in a shell and the structure of the Sun will look like an onion: An outer, hydrogen-fusion layer and an inner, helium-fusion layer, which surrounds an inert core of carbon and oxygen. The old Sun may repeat the cycle of shrinking and swelling several times. In this stage of evolution the Sun is called an asymptotic giant branch star. Finally enough carbon will accumulate in the core to prevent the core explosion. Helium-shell burning will add heat to the outer layers of the Sun, mainly containing hydrogen and helium. The asymptotic giant Sun will eventually generate an intense wind that will begin to carry off its outer envelope. The precise mechanism behind this phenomenon is not yet well understood. The Sun will expand a final time, and after about 30 million years it will swallow Venus and Earth, outer layers will keep expanding outward and as much as half of the Sun mass will be lost into space (Encyclopedia of Planetary Sciences, 1997).

The solar core will keep shrinking, and because it is not able to produce radiation by fusion, the further evolution of the Sun will be governed by gravitation. All matter will collapse into a small body about the size of the Earth. Thus, the Sun will have become a white dwarf. This is a dense-matter configuration, having radiated away the energy of its collapse. Then the white dwarf rapidly begins to cool. The final stage of solar evolution will be the black dwarf stage. The white dwarf will emit yellow light and then red light in the course of its evolution, drawing from the stars reservoir of thermal energy. Its nuclei will be packed as tightly as physically possible and no further collapse will be possible. The body will progressively cool down and finally becomes as cold as the interstellar space around it, emitting no light at all. As a carbon-oxygen-rich black dwarf it will continue its journey through the galaxy (milky way) and may eventually encounter another giant gas cloud to become involved in the birth of a new star (Encyclopedia of Planetary Sciences, 1997). For a schematic of the Suns luminosity and temperature as it evolves, see Figure 2.

The most important series of fusion reactions are those converting hydrogen to helium in a process known as hydrogen burning. The chances of four protons fusing together to form helium in one go are completely negligible. Instead, the reaction must proceed through a series of steps. The two main hydrogen-burning reaction chains are the proton-proton (PP) chain and the carbon-nitrogen (CNO) cycle. The PP chain divides into three main branches, which are called the PPI, PPII and PPIII chains. The first reaction is the interaction of two protons (p or 1H) to form a nucleus of heavy hydrogen (deuteron, d, or 2H), consisting of one proton and one neutron, with the emission of a positron (e⁺) and a neutrino (n). The deuteron then captures another proton and forms the light isotope of helium with the emission of a gamma ray (Dhillon, 1999). The PPI reaction occurs 69% of the time, the PPII reaction occurs around 30% of the time, and the PPIII reaction is very rare, only representing 0.093% of the proton-proton reactions in the Sun. The huge release of gamma rays from these processes is what we call “sunlight”. For a pictorial representation of all three proton-proton chains, see Figure 3.

Figure 3 The three proton-proton reactions
(courtesy of Professor Vik Dhillon).

The average proton in the Sun will undergo the PPI reaction approximately once in the lifetime of the Sun, i.e. once every 10¹⁰ years. The subsequent reactions occur much more quickly, with the second step of the PP chain taking approximately 6 seconds and the third step approximately 106 years in the Sun. The relative importance of the PPI and PPII chains depend on the relative importance of the reactions of 3He with 3He in PPI, as compared to the reactions of 3He with 4He in PPII. For temperatures in excess of 1.4 x 10⁷ K, 3He prefers to react with 4He. At lower temperatures, the PPI chain is more important. The PPIII chain is never very important for energy generation, but it does generate abundant high-energy neutrinos (Dhillon, 1999).

The other hydrogen burning reaction of importance is the CNO cycle. For more massive stars than the Sun, the proton-proton chain can still occur, but there is another sequence of reactions that become favourable for converting hydrogen to helium. In stars, the primary constituents are hydrogen and helium, however there are relatively minute amounts of heavier elements. Carbon (C), Nitrogen (N), and Oxygen (O) ions, if they are present, can partake in the sequence of reactions illustrated in the figure below. The reaction starts with a carbon nucleus, to which are added four protons successively. In two cases the proton addition is followed immediately by beta decay, with the emission of a positron and a neutrino, and at the end of the cycle a helium nucleus is emitted and a nucleus of carbon remains (Dhillon, 1999).  The reactions of the CNO cycle are shown pictorially in Figure 4.

Figure 4 The CNO cycle
(courtesy of Professor Vik Dhillon).

The Sun is made up of different layers, which can broadly be described as the Corona, Chromosphere, Photosphere, the Radiative and Convective zones, and the Core, as shown in Figure 5.

The Core
The core of the Sun contains the innermost 10% of the Suns mass and 25% of the diameter. As discussed previously, fusion is a process whereby lighter nuclei are fused or joined together into more massive nuclei. All of the elements present in the Sun are plasmas, as a result of the extremely high temperature of the core (around 15 million degrees Celsius) and the density of the core is about 150 grams per cubic centimetre, more than 20 times that of iron. Unlike the Earth, there is no molten rock or other liquid in the core.

The Interior
The interior of the Sun consists of three zones; the radiative zone, the interface layer and the convective zone.

Radiative Zone
In the radiative zone, (or radiative envelope) heat and energy is transported from the extremely hot core to the convective zone by the movement of photons (light) from the core towards the surface. It extends from the top of the core to about 70% of the Suns diameter. The density in the radiative zone drops from 20 grams per cubic centimetre (about that of gold) near the core, to 0.2 grams per cubic centimetre (much less than water). In the same distance, the temperature drops from 7 million degrees Celsius to 2 million degrees Celsius.

Interface Layer
As the name suggests, this thin layer lies between the radiative zone and the convective zone. In recent years, this layer has become the focus of much research, as it is now believed to be home to a magnetic dynamo, which is the source of the Suns magnetic field. As an intermediary between the turbulent convective zone and the calm radiative zone, there are significant changes in the velocity of the fluid, which result in a large shear and a stretching in the magnetic field lines, which increases the field strength.

Convective Zone
The convective zone (or convective envelope) extends for about the last 200 000 km up to the start of the photosphere. Energy in the outer convective zone is transported by a process of convection. At a temperature of about 2 million degrees Celsius at the innermost edge, the heavy ions of elements such as C, N, O, Ca and Fe retain some of their electrons which makes it more difficult for the radiative transfer of energy, so convection processes become more important.

The Photosphere
The photosphere is the deepest part of the Sun that we can see and is analogous to the Earths crust (see Figure 2). It is only about 100 km thick. Unlike the Earth, there is no solid foundation, and the photosphere consists of approximately 94% H, 6% He and 0.13% other gases at a temperature of about 5700 K (or ~ 6000 degrees Celsius). The term photosphere comes from the Greek word photos - meaning light.

 

Energy is radiated away from the Sun mainly as electromagnetic radiation - that is light and heat (see Figure 8), which originate in the photosphere. In the Sun, the maximum amount of electromagnetic radiation is emitted in the visible part of the spectrum, which is a direct consequence of the temperature of the photosphere. The photosphere actually makes up the lowest part of the solar atmosphere and can be thought of as the Suns surface.

The light that is currently reaching the Earth was generated in the Sun approximately 100,000 years ago. It takes that long to get to the surface because the Sun is so dense making it very difficult for the energy to escape. Once light leaves the Suns surface, it takes approximately 8 minutes and 26 seconds to reach Earth (Case Western Reserve University, 2006).

The light from the Sun is made up of many colours, called the visible spectrum and many shorter and longer wavelengths of light, collectively called the electromagnetic spectrum (see Figure 8). These other wavelengths are invisible to humans, but they can be measured with special detectors. These other wavelengths consist of Infrared (IR), Ultraviolet (UV), Micro, Radio, X, and Gamma. IR rays heat up matter. (IR light rays can be produced by specially modified light bulbs, and are used in many places that sell food.) Our atmosphere acts as an "infrared shield," and keeps this light from reaching the surface. UV light has become an increasing concern over the past few years. It is a form of radiation, and the hole in the ozone layer is allowing some of the normally blocked UV light to get through. UV light causes tans, sunburns, and skin cancer. Microwaves are put to use in most peoples kitchens in the aptly named microwave oven. They are used to heat foods quickly, and are more effective at doing so than IR. Radio waves are used in a whole branch of astronomy, for they can penetrate clouds of gas and dust that visible light cant. They are also used for transmitting radio and television shows, with television having a slightly higher frequency. X-rays are a form of radiation that are more powerful than UV, and are normally blocked by our atmosphere. X-rays are mainly used for medical purposes. Since they are a form of higher energy, they can penetrate denser objects than visible light can. Gamma rays are the most energetic form of radiation, and can pass through the human body. In cells, they can cause mutations and other severe damage. Luckily, they are blocked by our atmosphere. If they werent, life as we know it, would be impossible (Case Western Reserve University, 2006).

Figure 8 The Electromagnetic Spectrum
(copyright Lawrence Berkeley National Laboratory)

Figure 9 shows how the strongest frequency of light that is emitted from an object changes with its temperature and Figure 10 shows that the Sun is brightest in the visible spectrum at a temperature of approximately 6000 K, where cooler objects emit their maximum amount of electromagnetic radiation at lower frequencies.

 

Figure 9 The temperature of an object can be ascertained from the frequency it emits most strongly in the electromagnetic spectrum. The Sun, at around 6000 degrees Kelvin, is brightest in the visual spectrum in relation to cooler bodies, which are brightest at lower frequencies (courtesy of the University of Washington Astronomy Department).

 

The solar atmosphere consists of two main regions, the Chromosphere and the Corona.

Chromosphere
The chromosphere, chromo meaning colour, is an irregular layer immediately above the photosphere where the temperature rises to about 20 000 degrees Celsius (see Figure 10). In 1997, researchers from the Stanford-Lockheed Institute for Space Research in America, announced that they believed that the dramatic increase in temperature between the photosphere at about 5700 degrees Celsius and the chromosphere was due to the loops of a magnetic carpet created in the interface layer between the radiative and convective zones. Read about the discovery here.

 

 

Figure 10 The Solar Chromosphere in red against the pale blue of the corona
(copyright Marshall Space Flight Centre)

Corona
The corona is the outermost part of the Solar atmosphere and it extends far out into the solar system. Despite the high temperature of the corona, of about 1 million degrees Celsius, it contains very little heat energy as a result of the tenuous nature of the corona. The gases in the corona are so hot (although there are only a few particles) that they emit mostly X Rays. The corona contains mainly super heated gases, particularly Hydrogen and Helium as well as a few other light elements such as carbon, nitrogen and oxygen, which are completely ionised to leave only a bare nucleus.
There are actually three different types of corona, the White Light Corona, which we see as the wispy halo around the Sun during total eclipses, the Emission Line Corona, due to the emission spectra produced by the highly ionised light elements and the X Ray Corona which emitted as a result of the high temperatures in the corona (see Figure 11a, 11b and 11c).

                                                        

The wispy tendrils of the white light corona and the chromosphere  (copyright Marshall Space Flight Centre)

 

Figure 11a, 11b & 11c The Corona as seen in the different wavelengths.

A sunspot is a region on the Suns surface (photosphere) that is marked by a lower temperature than its surroundings and intense magnetic activity, which inhibits convection, forming areas of low surface temperature. Although they are blindingly bright, at temperatures of roughly 4000-4500 K, the contrast with the surrounding material at some 5700 K leaves them clearly visible as dark spots (see Figure 14) (Wikipedia, 2008b).

 

Figure 14 The closest view yet of a sunspot and its neighbourhood has been made by the Swedish Solar Telescope
(Courtesy of The Remote Sensing Tutorial).

A solar flare is a violent explosion in the Suns atmosphere equivalent to tens of millions of hydrogen bombs (see Figure 15). Solar flares take place in the solar corona and chromosphere, heating plasma to tens of millions of Kelvin, and accelerating the resulting electrons, protons and heavier ions to near the speed of light. They produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths from long-wave radio to the shortest wavelength gamma rays. It is generally accepted that flares represent conversion of magnetic energy into kinetic energy of particles and radiation, via a process called magnetic reconnection. Most flares occur around sunspots, where intense magnetic fields emerge from the Suns surface into the corona. The energy associated with solar flares may take several hours or even days to build up, but most flares take only a matter of minutes to release their energy (Wikipedia, 2008a). Solar flares extending well beyond the photosphere occur during Sun storms.

  Figure 15 A Transition Region and Coronal Explorer (TRACE) image acquired in the summer of 2000 that shows solar flaring.
(courtesy of The Remote Sensing Tutorial).

The Suns (apparent) movement
When we see the Suns position changing in the sky it is of course the Earth that is moving, not the Sun. While recognising this, for convenience in this Portal file, we will refer to it as the Suns movement. By appreciating how the Suns movement throughout the day varies from season to season, we can predict the performance of solar equipment and buildings. We will know when the Sun is shining on them and for how long.

The seasonal variation in the times of sunrise and sunset, and the variation in the Suns altitude are caused by the Earths axis being tilted at a constant angle to the plane of its rotation around the Sun. In Figure 19, we see that the Earths axis of rotation is tilted (inclined) at 23.5 degrees to its plane of revolution around the Sun, and constantly points to one direction in space.

 

Figure 19 The direction of the Earths axis of rotation remains fixed at 23.5 degrees as it moves around the Sun (based on original by Sam Paolino)

 

Figure 20 The number of hours of daylight varies as the Earth revolves around the Sun.

In Figure 20, we can see that in December, the southern hemisphere of the Earth has longer days than the northern hemisphere. In June, the reverse occurs, with the days being shorter in the southern hemisphere. The Earths axis is still tilted at 23.5 degrees in September and March, so both the northern and southern hemispheres have the same length of day. We can also see from Figures 20 and 21 that the angle of the Sun above the horizon at a given time of day will also vary throughout the year.

 

Figure 21 The apparent movement of the Sun as seen from a point on the Earths surface.

 

Figure 22 The Sun Angle Chart for Perth, Western Australia.

Figure 22 shows that at noon on December 21 (the summer solstice), the Suns altitude at Perth will be 81.5 degrees, or almost directly overhead. It also shows that at noon on June 21 (the winter solstice), the Suns altitude at Perth will be 34.5 degrees or shining more directly into our face than the top of our head. Figure 22 also shows the position of the Sun on March 21 and September 21 (autumn and spring equinoxes), when the maximum altitude of the Sun will be 58.5 degrees in Perth. The seasonal variation in the point on the horizon at which the Sun rises and sets are generally less understood or appreciated. From Figure 22, we can see that the Sun only rises in the east and sets in the west at the spring and autumn equinoxes. From March to September in the southern hemisphere, it will rise and set to the north of the east-west line and from September to March is will rise and set to the south of the east-west line. Given a date and time, you can tell where the Sun will be in the sky using a Sun Angle Chart.