The Sun

As the sun rushes through space at a speed of 150 miles per second, it takes many smaller bodies along with it. The sun and its smaller companions together are known as the solar system Together, these bodies are making a revolution around the Milky Way that takes 225 million years. These other members of the solar system range in size from the giant planet Jupiter to microscopic particles called micrometeorites and even smaller particles -- atoms and molecules of the interplanetary gar Earth is one of the largest bodies of the solar system, although it is quite small when compared with the sun or Jupiter.

Astronomers do not know exactly how far out the solar system extends. When it is at its farthest point from the sun (aphelion), some 4 and half billion miles, Pluto is the most distant known planet. Many comets, however, have orbits that take them even farther out, up to several hundred times the distance of Flute Even at that distance the sun's gravitational force dominates and can bring the comet back Some hundred billion comets form a tenuous halo in the outer parts of the solar system. Each is like a giant snowball, 1,000 to 10,000 feet in diameter.

The Sun is the central member of the solar system. Its gravitational force holds the other members in orbit and governs their motions. It far outweighs all other components of the solar system combined. In fact the run contains more than 99 percent of the mass of the entire solar system.

The sun is, however, only an average-sized star. If it were as far away from Earth as most stars are, it would look no larger or brighter than its neighbors. But since it is by far the nearest star and the only star whose surface details may be observed, it is also one of the major sources of information that scientists have about how stars behave.

The sun provides nearly all the heat and light and other forms of energy necessary for life on our planet In fact, the sun provider virtually all the energy of the solar system. Its gravitational attraction governs the motions (or kinetic energy) of the planets and other bodies. Radiation from its surface bathes the planets in all the electromagnetic radiation they receive, with some minor exceptions. These exceptions include the faint light from stars, the disintegration of radioactive materials on the planets, emissions of long-wave radiation by the planet Jupiter, and the radio waves and X rays from remote space.

Although the sun is a rather ordinary star, it is very important to the inhabitants of the Earth. The sun is the source of virtually all of the Earth's energy. Yet the Earth receives only half a billionth of the energy that leaves the sun. Because the sun's energy is so intense, there are some real dangers in studying it. The intense heat of the sun's rays can destroy the retinal cells, causing blindness. For this reason, the sun should never be viewed directly. Furthermore, there is no safe way to view the sun through an ordinary telescope. Smoked glass and dark glasses give no protection from the great concentration of heat and light. The only safe way to study the sun is to protect its image through a pinhole or a telescope onto a white screen.

The average distance of the sun from the Earth, arbitrarily called one astronomical unit by astronomers, is 149,497,875 kilometers (92,958,350 miles). The sun's radius is about 432,500 miles, or 109.3 times the volume of the earth. It has been calculated the sun's mass, or quantity of matter, is some 333,400 times as great as the Earth's mass.

Starts vary greatly in size and in color. They range from giant starts, which are much larger than the sun, to dwarf stars, which can be much smaller. In color they range from whitish-blue starts with very high surface temperatures (over 30,000 degree Kelvin, or 53,500 degree F) to relatively cooler stars (about 2,500 degree K, or 4,000 degree F). The sun is a yellow dwarf star, a kind that is common in Milky Way with a surface temperature of about 6,000 degree K (10,300 degree F). (The Kelvin temperature scale uses degrees of the same size as Celsius, or centigrade, degrees, but it is numbered from absolute zero, which is -273.15 degree C)

The sun looks like a burning sphere. In fact it is often pictured as a circle with flames surrounding it. But the sun is actually too hot for an Earth-type chemical reaction like burning to occur on its surface. Besides, if burning produced its energy, it would have run out of fuel many millions of years ago.

Various theories have been advanced to explain the sun's tremendous energy output. One said that all the bits of matter in the sun were exerting gravitational attraction on each other and causing the sun to shrink and become more tightly packed. This process, called gravitational contraction, does occur in some stars and can release a great deal of energy. However, gravitational contraction could produce energy for only 50 million years at most, while the sun's age must be at least as great as the Earth's age of 4 and half billion years.

Atomic theory finally provided an explanation. Scientists now agree that thermonuclear reactions are the source of solar energy. Albert Einstein's theoretical calculations showed that a small amount of mass could be converted to a great amount of energy. The vast amount of matter in the sun could provide fuel for billions of years of atomic reactions.

The sun's core is believed to be a superhot, extremely dense mass of atomic nuclei and electrons. Its temperature is calculated to be about 15,000,000 K (27,000,000¢XF). Under these conditions, nuclei can collide and fuse into new and heavier nuclei. This is a type of theomonuclear reaction called a fusion reaction. During such a reaction some of the mass of the nuclei changes to energy. Two specific processes, the carbon cycle and the proton-proton- reaction, occur most often.

1: the core 2: where the energy maintains 3: the convection zone 4: photosphere 5: solar surface

The Photosphere and Sunspots.

The sun's surface, called the photosphere (sphere of light) is the lowest visible layer of the sun. The temperature of the photosphere ranges from 7,500 K (13,000¢XF) at the bast to 4,700 K (8,000¢XF) at the top. Its average temperature is 5,800 K (10,000¢XF). The photosphere has a definite texture. Many small, luminous grains are separated by dark areas that look like nets or canals. High altitude photographs of the photosphere's granulation show that the grains are hundreds of miles in diameter. They are continually forming and disappearing. According one hypothesis, the grains are the tops of gas columns that ascend and descend through the photosphere.

The uniformity of the granulation indicates that a relatively calm condition exists at the solar surface. This is periodically subject to violent disturbances. Generally, these disturbances appear as darker points, called pores, on the more luminous background of the photosphere. The pores usually grow rapidly in number and in size to form a large single sunspot or a group of sunspots.

Typical sunspots have a dark, circular center, called the umbra, surrounded by a lighter area, the penumbra. Rays issuing from the center of the umbra form the penumbra. Sunspots vary greatly in size but are always small compared to the size of the sun. When they appear in groups, they may extend over thousands of miles. The darkness of the umbra is a sign that the sunspots are cooler than the photosphere. The umbras appear to be some 2,000 K (3,100¢XF) cooler than the photosphere. Furthermore, when they approach the sun's edge, the umbras appear to be lower than the photosphere as well.

The regular observations from 1750 to the present reveal that the spots appear and disappear in a definite cycle, and that they are limited to the two zone of the sun contained between latitudes 40¢X and 5¢X of its northern and southern hemispheres. Their cycle lasts an average of 11 years. At the beginning of a cycle a few spots appear at around 35¢X latitudes, then they rapidly increase in number, reaching a maximum in the course of around five years. At the same time they move slowly toward the equator. During the next six years their number decreases while they continue to approach the equator. There the cycle ends and at the same time another cycle starts immediately.

The Chromosphere.

The layer above the photosphere is called the chromosphere (sphere of color) because of its reddish color, visible during total eclipses of the sun. The lower chromosphere absorbs some of the light that is emitted from the photosphere, causing the dark absorption lines of the sun's spectrum. This absorption occurs because the lower chromosphere is cooler than the photosphere. However, its temperature rises with height until at the upper boundary it has reached almost 1,000,000 K (1,800,000¢XF).

Much of the sun's "weather" takes place in the chromosphere. When the chromosphere is viewed under hydrogen light or under the violet light of calcium, bright areas called plages appear. The plages are usually located above or near sunspots and may be extensions of bright patches - faculae - that occur on the photosphere near sunspots.

A far more violent phenomenon is the solar flare, a sudden chromospheric eruption from a plage area. The solar flare may emit high energy radiation and highly energetic charged particles. Solar flares usually form very rapidly, reaching their maximum brilliance within minutes, after which they slowly die out. Very strong solar flares may emit X rays, radio waves, and swarms of charged particles. These enormous spurts of energy could be very dangerous for space travelers above the protection of the Earth's atmosphere because such fast-moving radiation can penetrate spaceship walls and damage body cells.

The solar corona

The chromosphere is surrounded by the solar corona, a faintly luminous outer atmosphere. As this atmosphere is thousands of times dimmer than the sun's disk, it is usually invisible. Before the invention of the coronagraph, it could be seen only when the sun was totally eclipsed by the moon. When the photosphere is blocked out, the corona appears as a silvery halo with long arcs and streamers. The arcs are usually visible above disturbed regions, especially where prominence are present. When sunspots are at a minimum, the corona has long streamers along the equator with shorter rays at the poles. Its shape then resembles that of force lines around a magnetic sphere. This shape changes when sunspots are at a maximum. The corona then appears almost circular, with streamers distributed uniformly around the disk.

Solar Wind

The solar wind is a continuous outward flow of ionized gas (plasma) from the corona of the sun, which extends beyond the earth's orbit and into interstellar space. It consists chiefly of a mixture of protons and electrons, plus the nuclei of some heavier elements in smaller numbers. The particles that make up the solar wind are formed when the coronal gases expand and evaporate. About a million tons of gas per second flow away from the sun by this process. The high temperatures of the corona accelerate the particles to speeds great enough to allow them to escape from the sun's gravitational field. As they leave, the particles carry part of the sun's magnetic field along with them. Because of the sun's rotation and the steady outflow of particles, the lines of the magnetic field carried by the solar wind trace curves in space. The solar wind is responsible for deflecting the tails of passing comets away from the sun. when the solar wind encounters the earth's magnetic field, a shock wave results. In the vicinity of the Earth, the solar wind produces magnetic storms, fading of radio transmissions, and the polar auroras.

The Properties of Sun.

Chemical Composition of the Sun

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