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EDE71610 sun poster:sun poster AW

5/1/09

17:18

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a guide to THE SUN
THE SUN'S VITAL STATISTICS
Mean distance from the earth: 92.95 million miles. Diameter: about 800,000 miles. Diameter of core: about 250,000 miles. Rate at which mass is converted into energy: 4.3 million tons per second. Surface temperature: 5,500 oC. Core temperature: 15.6 million oC. Approximate age: 4.57 billion years.

THE SUN - OUR NEAREST STAR
The Sun is at the centre of the Solar System; all the other Solar System bodies revolve around it in their orbits. It is also a star: a typical, average dwarf G star. Ninety-nine per cent of the Solar System's mass is accounted for by the Sun, and virtually all life on Earth is directly or indirectly supported by it. The Sun consists of about 70 per cent hydrogen and 28 per cent helium. The remainder is mostly oxygen and carbon. As with all stars, the Sun's energy is generated by nuclear fusion reactions that take place under extreme conditions in the core. Energy released from the core, which is about 250,000 miles across, passes through the radiative zone, which is nearly 200,000 miles thick, by a process of successive absorptions and re-emissions. It then passes through the 125,000-mile-thick convective zone to the surface ­ the photosphere ­ from where it is radiated into space. To put these figures in context, the Sun is roughly 100 times the diameter of our planet and its light and heat take around 8 minutes to reach us. The Sun radiates as much energy in 15 minutes as the entire human race consumes, in all forms, during a year.

SUNSPOTS & SOLAR FLARES
Sunspots are areas where the Sun's light output is restricted to a certain extent by magnetic fields at that point in the photosphere. Sunspots emit a considerable amount of light, but in comparison with the rest of the Sun's surface they appear dark. An individual spot has a dark centre, known as the umbra, surrounded by a lighter region, the penumbra. Spots often appear in pairs or groups. Small ones may last for only an hour or two, while bigger ones may last for weeks. The Sun rotates every 27 days or so as seen from the Earth's point of view, so spots appear to move from one side of the solar disk to the other, changing day by day. The number of sunspots visible depends on the stage in a solar cycle of roughly 11 years. Solar flares ­ violent releases of magnetic energy in the inner solar atmosphere above active regions ­ are common at times of high sunspot activity.

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SOLAR ECLIPSES
A solar eclipse occurs when the Sun, Moon and Earth are very nearly in a straight line. A total solar eclipse is seen at places where the umbra of the Moon's shadow cone falls on and moves over the Earth's surface; at the same time the eclipse will appear partial to observers on either side of the central track of totality. The overall duration of a solar eclipse can be as much as four hours, but totality only lasts at most seven and a half minutes. There are between two and five solar eclipses each year. Even during a total eclipse, it is vital that you never look directly at the Sun.

THE SOHO MISSION
SOHO (Solar and Heliospheric Observatory) a joint European Space Agency and Nasa Mission, was launched in 1995. It has made the largest and most detailed database ever of solar surface features, viewing and investigating the Sun from its deep core, through its outer atmosphere - the corona - and the the solar wind, the stream of highly ionized gas that blows continuously outward through the Solar System, out to a distance ten times beyond the Earth's orbit. Although not part of its mission, it has become the most prolific discoverer of comets in the history of astronomy.

GLOBAL WARMING
Short-wave radiation from the Sun readily penetrates Earth's atmosphere to reach the ground. Radiation re-emitted from the ground is trapped by atmospheric constituents such as carbon dioxide, raising the planetary temperature. This is the so called "greenhouse effect". The past 150 years or so has seen a significant increase in levels of greenhouse gasses (such as carbon dioxide and methane). If the proportion of solar energy trapped in our atmosphere continues to rise, global temperature will do likewise, with potentially catastrophic effects, such as the loss of Polar ice.

THE SUN AND OUR HEALTH
Ultraviolet radiation from the Sun is the main cause of skin cancer, which kills more than 1,000 Britons every year. Sunburn, the first stage of UV damage, can result from as little as 30 minutes of exposure to the Sun. High exposure to sunlight can also increase your susceptibility to cataracts ­ the world's main cause of blindness. On the postitive side, sunlight is our most important source of vitamin D, which helps strengthen bones and muscles and boosts the immune system. 10 minutes of sunshine a day is sufficient for most health needs.


EDE71610 sun poster:sun poster AW

5/1/09

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OBSERVING THE SUN
The Sun must never be viewed directly through any optical instrument, as there is a risk that you may be blinded permanently. There are two ways for amateurs to observe the Sun's disk. A full aperture filter (which must fit snugly over the aperture of the telescope ­ that is, over the top, not the eyepiece) has a special metallic coating which cuts out harmful radiation before the Sun's light enters the telescope. More simply, the Sun's image can be projected onto a white screen. Point the telescope at the Sun without looking through it, by using its shadow as a guide; use a low-power eyepiece and hold a piece of white card about 30cm behind the eyepiece, in place of your eye. You should see a bright circle of light on the card, probably out of focus. Refocus until the disk of light is sharp ­ this should be the disk of the Sun. Professional instruments for solar work include the coronagraph and the spectroheliograph. Many probes and satellites have been launched to study the Sun ­ notably the Solar and Heliospheric Observatory, launched in 1995. Its images are available daily on the internet at http://sohowww.nascom.nasa.gov.

HOW THE SUN WORKS
The sun has been `burning' for more than 4.5 billion years and will continue to do so for several billion more. It is a massive collection of gas, mostly hydrogen and helium. Because it is so massive, it has immense gravity, enough gravitational force to hold all of its hydrogen and helium together and to hold all of the planets in their orbits around the sun. The sun does not burn like wood burns. If it did, it would burn out in a few thousand years. Instead it is a gigantic nuclear reactor. The best way to see how the Sun works is to understand its composition. As a terrestrial planet like Earth has a Core, a Mantle and a Crust, so the sun has three surface areas; Core, Radiative zone, and Convective zone. It also has three parts to its atmosphere; the Photosphere, the Chromosphere and the Corona. THE SUN'S CORE Here is where the majority of the sun's energy production takes place, in the area that starts at the centre of the sun out to 25 percent of its radius. The gravity of the sun has two effects; first it holds the whole ball of gas together, and second, the immense pressure smashes atoms of hydrogen together, creating nuclear fusion. Some 85 percent of the sun's energy is created by this nuclear fusion at the core, which releases that energy in both a direct and a roundabout fashion, which eventually finds its way to Earth as visible sunlight.

CONVECTIVE ZONE The rest of the sun's surface radius is taken up by the Convective Zone, which gets its name by the convection currents that carry energy the rest of the way to the surface. Currents of hot gas rise while currents of cooler gas fall, creating a cycle that carries photons much faster than from the Radiative Zone about 100,000 years. THE PHOTOSPHERE The portion of the sun visible from Earth is the Photosphere, the lower part of the Sun's atmosphere that is around 250 miles thick. The average temperature of the Photosphere is around 5,800 Kelvin and appears to bubble and boil. Each `bubble' is the top of an upward moving convection current and can stretch to 700 miles wide. THE CHROMOSPHERE Above the Photosphere in the sun's atmosphere and reaching up 1,200 miles is the Chromosphere, where the temperature soars to as much as 9,700oC. Scientists are puzzled as to why the temperature is higher here than closer to the sun's surface, but some surmise that churning gases between the two spheres produce shock waves that add heat energy. THE CORONA The Corona is the uppermost area of the sun's atmosphere and reaches an amazing several million miles out from the Photosphere, and here the temperature goes off the charts to an average of almost 2 million oC. Some theories as to the temperature jump have to do with he sun's magnetic field, but no one is certain.

PHOTOSYNTHESIS
From the Greek words photo `light' and synthesis, `placing with', Photosynthesis is a metabolic pathway that converts light energy into chemical energy. Its initial substrates are carbon dioxide and water; the energy source is sunlight (electromagnetic radiation) and the end-products are oxygen and energy containing carbohydrates, such as sucrose, glucose or starch. This process is one of the most important biochemical pathways, since nearly all life on Earth either directly or indirectly depends on it as a source of energy. It is a complex process occurring in plants and algae, as well as bacteria such as cyanobacteria. Photosynthetic organisms are also referred to as photoautotrophs. Human beings photosynthesize a class of chemicals usually referred to as the vitamin Ds in their skin through the action of ultra-voilet light. DISCOVERY Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800s. Jan van Helmont began the research of the process in the mid 1600s when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate as much of the gained mass also comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's biomass comes from the inputs of photosynthesis, not the soil itself. In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterwards, Nicolas-ThИodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO2, but also to the incorporation of water. Thus the basic reaction by which photosynthesis is used to produce food.

SUNSPOTS
Sunspots are areas where the magnetic field is about 2,500 times stronger than Earth's, much higher than anywhere else on the Sun. Because of the strong magnetic field, the magnetic pressure increases while the surrounding atmospheric pressure decreases. This in turn lowers the temperature relative to its surroundings because the concentrated magnetic field inhibits the flow of hot, new gas from the Sun's interior to the surface. Sunspots tend to occur in pairs that have magnetic fields pointing in opposite directions. A typical spot consists of a dark region called the umbra, surrounded by a lighter region known as the penumbra. Sunspots are on average about the same size as the Earth. Sunspots, Solar Flares and Coronal Mass Ejections can directly influence events on Earth. They occur near sunspots, usually at the dividing line between areas of oppositely directed magnetic fields. Hot matter called plasma interacts with the magnetic field sending a burst of plasma up and away from the Sun in the form of a flare. Solar flares emit x-rays and magnetic fields which bombard the Earth as geomagnetic storms. If sunspots are active, more solar flares will result creating an increase in geomagnetic storm activity for the Earth. Therefore during sunspot maximums, the Earth will see an increase in the Northern and Southern Lights and a disruption in power grids and radio transmissions. The storms can even change polarity in satellites which can damage sophisticated electronics.

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THE SUN

SUN WORSHIP
When man became a farmer, and thus dependent upon daily and seasonal changes of weather, he often turned to worship the great force that regulated these changes, the light and heat of the sun. The worship of the sun, although not peculiar to any one time or place, received its greatest prominence in ancient Egypt. There, the daily birth, journey, and death of the sun was the dominating feature of life. One of the most important gods of Egyptian religion was Ra, the sun-god, who was considered the first king of Egypt. Greek Sun God Apollo The pharaoh, said to be the son of Ra, was the sun-god's representative on earth. In later Egyptian religion, under the rule of Ikhnaton, the sun-god Aton gained complete supremacy in what was Egypt's only monotheistic or `single God' period. In Greece there were two sun deities, Apollo and Helios, although there was no institutionalized form of sun worship. The influence of the sun in religious belief also appears in Zoroastrianism, Mithraism, Roman religion, Hinduism, Buddhism, and among the Druids of England, the Aztecs of Mexico and the Incas of Peru.

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Poster design by Jonathan Simms

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THE SUN

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THE RADIATIVE ZONE From the core out to about 55 percent of the sun's radius is the Radiate Zone. This area is where the energy created at the core is transferred outward (radiated) by photons. Here is the roundabout method of energy release; each photon only travels 1 micron, which is one millionth of a metre, before it is absorbed by a gas molecule. This photon is then released in a random direction and then reabsorbed. By the time that photon makes its way out of the Sun as much as 50 million years may have passed.

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