The Sun Essay
The sun is a huge, glowing ball of gases at the center of the solar system. The earth and the other seven planets travel around it. The sun is only one of billions of stars in the universe. As a star, there is nothing unusual about it. But the sun is more important to people than any other star. Without the heat and light of the sun, there could be no life on earth (Stix 49).
The diameter (distance through the center) of the sun is about 865, 000 miles (1,392,000 kilometers), about 109 times the diameter of the earth. Because the sun is about 93 million miles (150 million kilometers) from the earth, it does not appear larger than the moon. But the sun’s diameter is 400 times as large as that of the moon. The sun is also almost 400 times farther from the earth than is the moon (Stix 49). The sun is nearer the earth than is any other star. For this reason, scientists study it to learn about stars as much farther away. The visible surface of the sun consists of hot gases that give off light and heat. Only about one two – billionth of the sun’s light and heat reaches the earth. The rest is lost in space (Stix 51).
The temperature of any place on the earth depends on the position of the sun in the sky. The temperature greatly affects the weather of a region. Tropical regions near the equator have a hot climate because the sun shines almost directly overhead at noon. Regions near the North Pole and the South Pole have a cold climate because the sun never rises far above the horizon (Stix 54).
Today, we know we must have the sun as a source of heat, light and other kinds of energy. All life on the earth – people, animals and plants – depend on this energy from the sun. Plants use sunlight to make their own food and in the process give off oxygen. People and animals eat the plants and breathe in the oxygen. In turn, people and animals breathe out carbon dioxide, which plants combine with energy from sunlight and water from the soil to produce more food (Stix 54).
Scientists estimate that the sun and the rest of the objects in the solar system are about 4,600,000,000 years old. They believe that the sun will continue to be a source of energy for at least another 5 billion years (Stix 56). About three – fourths of the mass of the sun consists of hydrogen, the lightest known element. Almost a fourth of the sun’s mass consists of helium. Scientists discovered this gas on the sun before they found it on the earth. The word helium comes from the Greek word meaning sun (Stix 56).
Of the 109 known elements, 91 occur naturally in or on the earth. The other elements are artificially created. At least 70 of the earth’s natural elements have been found on the sun. But all these elements – except hydrogen and helium – make up only between 1 and 2 per cent of the mass of the sun. Scientists were able to indentify the elements on the sun by studying the spectrum (pattern of colored lines) of light from the sun (Stix 59).
Until human beings learned to develop nuclear energy, sunlight supplied their energy needs. Plants used sunlight from photosynthesis. Animals ate the plants, and people used both plants and animals for food, clothing and shelter (Stix 59). People also use the energy in fossil fuels – coal, oil and natural gas. These fuels come from plants and animals that lived millions of years ago. After the plants and animals died, they were buried by soil in swamplands or on the sea floor. By burning cola, and by refining oil and natural gas, energy is released from the sun that was stored in fossils millions of years ago (Stix 59).
In addition, people use sunlight for power in other ways. For example, the effects of sunlight cause wind, which some people use to power windmills. Sunlight also evaporates water, which falls as rain. The rain forms rivers. Hydroelectric power plants on the rivers use the power of moving water to generate electricity. Solar furnaces use mirrors to focus sunlight to heat water in boilers. Solar energy cells provide power for artificial satellites and spacecraft (Stix 62).
The sun gets energy from the thermonuclear reactions near its center. These reactions change hydrogen into helium. They release so much energy that the sun could not shine fro about 10 billion years with little change in its size or brightness. The sun is about 4,600,000,000 years old, and it probably will shine for at least another 5,000,000,000 years (Golub & Pasachoff 114).
By studying other stars, astronomers can predict what the rest of the sun’s life will probably be like. About 5,000,000,000 years from now, they believe, the center of the sun will shrink and become hotter. The surface temperature will fall slightly. The higher temperature of center will increase the rate at which hydrogen changes into helium, and the amount of energy given off by the sun will also increase. The outer regions of the sun will expand about 30 to 40 million miles (48 to 64 million kilometers) – about the distance to Mercury, the planet nearest the sun. The sun will then be a red giant star. When the sun is a red giant, the earth’s temperature will become too high for life to exist there (Golub & Pasachoff 114).
After the sun has used up its thermonuclear energy as a red giant, astronomers believe it will begin to shrink. After the sun shrinks to about the size of the earth, it will become a white dwarf (Golub & Pasachoff 117). The sun may throw off huge amounts of gases in violent eruptions called nova explosion as it changes from a red giant to a white dwarf. A star that becomes a white dwarf has entered a final stage of its existence (Golub & Pasachoff 118).
After billions of years as a white dwarf, the sun will have used up all its energy and lose all its heat. Such stars are called black dwarfs. After the sun has become a black dwarf, the planets will be dark and cold. It the earth still has an atmosphere, the gases of the atmosphere will have frozen onto the earth’s surface (Golub & Pasachoff 118). The inner third of the interior of the sun is called the sun’s core. The temperature in the core is about 27,000,000 F (15,000,000 C). The material that makes up the core is more than 100 times as dense as water, but it still consists of gases. Thermonuclear reactions, which produce the sun’s light and heat, occur in the core (Bracher 4).
Beyond the core is the radiative zone, which extends through about the middle third of the sun’s interior. In the radiative zone, the average temperature is about 4,500,000 F (2,500,000 C), and the gases are about as dense as water. The parts of the radiative zone that are nearer the sun’s surface are cooler than those that are closer to the sun’s core. Because radiant heat normally flows from a hot place to a cooler one, the energy produced in the sun’s core flows through the radiative zone, toward the surface of the sun. This outward flow of heat is called radiation (Bracher 4).
The convection zone begins about two – thirds of the way from the center of the sun and ends about 137 miles (220 kilometers) below the sun’s surface. The temperature in this zone is about 2,000,000 F (1,100,000 C), and the gases are about a tenth as dense as water. The gases are about a tenth as dense as water. The gases are so cloudy that energy from the sun’s core cannot travel through the convention zone by radiation. Instead, the energy causes the gases to undergo violent churning motions called convection and turbulence. These motions carry most of the sun’s energy to the surface (Bracher 7).
The sun’s surface, or photosphere, is about 340 miles (547 kilometers) thick, and its temperature, is about 10,000 F (5500 C). The photosphere is actually the innermost layer of the sun’s atmosphere. It is from one – millionth to 1 ten – millionth as dense as water (Bracher 9).
The photosphere contains many small patches of gas called granules. A typical granule lasts only 5 to 10 minutes, and then it fades away. As old granules fade away, the sun’s surface becomes marked with new ones. Scientists believe the granules are produced by violent churning of the gases in the convention zone (Bracher 9). The photosphere gives off the sun’s energy in the form of heat and light. The sunlight given off by the photosphere is made up of many colors. Various elements in the photosphere absorb some of the colors and prevent the sun from giving off those colors. Scientists can see what colors are absorbed by passing sunlight through a glass prism to form a spectrum.
These lines are called Fraunhofer lines, after Joseph von Fraunhoter, a German physicist who studied them during the early 1800’s. Each element has its own characteristics, pattern of Fraunhofer lines. Astronomers learned what elements are on the sun by comparing the Fraunhofer lines of sun’s spectrum with the lines that various elements show in laboratory experiments (Bracher 11).
In photographs of the sun, the region near the edge of the disk does not appear so bright as the central region. This effect is called limb darkening. It occurs because light from the central region follows a more direct path to the earth than does light from the edge of the disk. As a result, less of the central light is absorbed by the sun’s gases, and more light from deep within the atmosphere can be seen. The deeper gases are hotter then those near the surface and the hotter gases give of brighter light (Bracher 11).
About 100 miles (160 kilometers) above the photosphere, the temperature is about 7200 F (4000 C). Above this point, the temperature rises again. In the chromospheres (middle region of the sun’s atmosphere), the temperature reaches about 50,000 F (27,800 C) (Bracher 11).
The chromosphere consists of hot gas in violent motion. Some of the gas forms streams called spicules that measure as much as 500 miles (800 kilometers) thick and shoot up as high as 10,000 miles (16,000 kilometers) (Bracher 14).
The temperature of the sun’s atmosphere climbs rapidly above the chromosphere. A region above the chromosphere called the corona has an average temperature of about 4,000,000 F (2,200,000 C). The atoms of the corona are so far apart that gases of the coraona have little heat. If it were possible for an astronaut to be in the corona and shielded from the direct rays of the sun, the astronaut’s space suit would have to be heated (Golub & Pasachoff 118).
The temperature drops slowly from the corona outward into space. The corona has no well – defined boundary. Its gases expand constantly away from the sun. This expansion of its gases is called solar wind (Golub & Pasachoff 118).
The temperature of the chromosphere and the corona are a puzzle to astronomers. Heat flows from hot areas to cooler areas, and yet the photosphere is cooler than the outer regions of the sun’s atmosphere. Astronomers believe that the high temperature of the chromosphere and the corona result from the turbulence of gases in the convection zone, combined with the influence of magnetic fields produced in the sun’s interior (Golub & Pasachoff 120).
The sun radiates (gives off) energy into space in the form of light and heat. Every second, about 4 million short tons (3.6 million metric tons) of the sun’s mass change into energy. The earth gets only about 4 pounds (1.8 kilograms) or about one two – billionth, of the total energy radiated by the sun every second. But this amount is enough to make life possible on earth (Sukhatme & Sukhatme 97).
Astronomers have found that sun spots, flares, prominence, and other stormy activities on the sun occur because of changes in the patterns of magnetic fields on the sun. A magnetic field occupies the space around a magnet where magnetism exerts a force. Magnetic fields contain magnetic lines of force, or flux lines. In a bar magnet the lines of force form a simple pattern (Stenflo 85; Title 165).
The sun has a magnetic field that somewhat resembles the pattern of a bar magnet, especially near the sun’s poles. But near the sun’s equator, the magnetic pattern is always changing because the movement of gases there makes the magnetic field irregular. Atoms of those gases are ionized. An ion is an atom or group of atoms that has either gained or lost electrons. Many atoms of gas on the surface of the sun have lost electrons and form a type of gas called plasma. Particles trapped in a magnetic field usually follow the magnetic lines of force. But the motion of large quantities of plasma tends to change the direction of these lines. As a result, changes occur in the pattern of the sun’s magnetic fields, and stormy activity takes place (Stenflo 85; Title 168).
Sometimes a strong loop of magnetic lines of force extends through the sun’s surface. Where the lines cross through the surface, they lower the temperature of the gas. This gas does not shine so brightly as the surrounding gas, and it appears as a sunspot. Because a magnetic loop both leaves and reenters the surface, two sunspots are associated with the loop. After a few days, a magnetic loop may break up into several thinner loops. Each of these loops crosses the surface at a different place. The original sunspot breaks up into several sunspots that from a sunspot group. Still later, the magnetic loops spread out and over a wider area, and their sunspots fade away (Stenflo 87; Title 170).
A sunspot cycle begins when sunspots appear at high solar attitudes. As the cycle continues, more sunspots are reversed from those of the previous cycle. The north and south magnetic poles of the sun’s general magnetic field also become reversed. Thus, the sun takes two sunspot cycles, or 22 years, to go through a complete set of magnetic changes (Stenflo 89; Title 173).
Bracher, Katherine. “The Interior of the Sun.” Mercury 28 (1999): 4.
Golub, Leon and Jay M. Pasachoff. The Solar Corona. New York: Cambridge University Press, 1997.
Sukhatme, S.P., and K. Sukhatme. Solar Energy. India: Tata McGraw-Hill, 1996.
Stenflo, Jan Olof. Solar Magnetic Fields: Polarized Radiation Diagnostics. Springer, 1994.
Stix, Michael. The Sun: An Introduction. Springer, 2002.
Title, Alan. “Towards Understanding the Sun’s Magnetic Fields and their Effects.” AIP Conference Proceedings 703 (2004): 163 – 173.
University/College: University of Arkansas System
Type of paper: Thesis/Dissertation Chapter
Date: 20 March 2017
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