Sun

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.

If the sun were the size of a skyscraper, the earth would be the size of a person. The moon would be the size of a cocker spaniel standing next to the person. Jupiter, the largest planet, would be the size of a small building. The nearest star also would be about the size of a skyscraper. But it would be about 7 million miles (11 million kilometers) away.

The sun is nearer the earth than is any other star. For this reason, scientists study it to learn about stars 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.

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.

The Egyptians, Greeks, and many other ancient peoples thought the sun was a god. They worshiped the sun, made offerings to it, and built temples to honor it. Many early beliefs about the sun began when people tried to explain the sun's movement across the sky.

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--depends 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.

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.

Compared with other stars, the sun is only medium-sized. In fact, it is one of many stars that astronomers call yellow dwarfs. Some stars have a diameter 10 times as small as that of the sun. Other stars have a diameter as large as 1,000 times that of the sun. Astronomers call these huge stars supergiants. One supergiant, Betelgeuse, becomes larger and smaller in diameter, sometimes reaching almost 600 times the diameter of the sun. If the sun grew to the size of Betelgeuse, it would swallow up Mercury, Venus, Earth, and Mars.

From the earth, the sun looks like a circle. Astronomers often use the term disk for the part of the sun that can be seen from the earth. Some astronomers have measured the disk and found that it is slightly flattened in some places. But other astronomers are not certain how correct these measurements are.

Distance to the sun. The earth's distance from the sun varies from about 91,400,000 to 94,500,000 miles (147,100,000 to 152,100,000 kilometers). This distance varies because the earth travels around the sun in an orbit that has an elliptical (oval) shape. The average distance between the earth and the sun is about 93 million miles (150 million kilometers).

Suppose the orbit of the earth were the same as the orbit of Venus. The earth would be so close to the sun it would be too hot to support life as we know it. Now suppose the orbit of the earth were the same as the orbit of Mars. The earth would then be so far away from the sun it would probably be too cold to support anything but the sturdiest and simplest life forms.

Light travels at a speed of 186,282 miles (299,792 kilometers) per second. At this speed, light from the sun takes about 8 minutes and 20 seconds to reach the earth. When a spacecraft is escaping from the pull of the earth's gravity, it must travel at a speed of 25,000 miles (40,200 kilometers) per hour. If the spacecraft could maintain this speed on a journey to the sun--and not burn up--the trip would take 154 days, or slightly longer than five months.

The sun's brightness. The light and heat of the sun come from its surface. The amount of light and heat stays fairly constant, so that the actual brightness of the sun changes little. The changes in brightness that seem to take place result from weather conditions in the earth's atmosphere. These conditions affect the amount of sunlight that reaches any particular place on the earth. Sometimes a small increase in brightness may result from eruptions of gases on the sun's surface. Most of these eruptions, called flares, last from 10 minutes to an hour. But any changes in the total brightness of the sun caused by flares are not visible to the naked eye.

Sunlight contains all the colors of the rainbow. These colors blend to form white light, and so sunlight is white (see Color). But at times, some of the colors become scattered. We see only the remaining colors, and the sunlight appears colored. For example, when the sun appears high in the sky, some of the blue light rays are scattered in the earth's atmosphere. At such times, the sky looks blue and the sun appears to be yellow. At sunrise or sunset, the sun is near the horizon and the light must follow a longer path through the earth's atmosphere. As a result, more of the blue and green rays are scattered in the atmosphere, and the sun looks red. On rare occasions, the sun may look bright green for a moment when only an edge is visible above the horizon. This green flash occurs because the red rays of light are hidden below the horizon and the blue rays are scattered in the atmosphere.

The sun's heat. Of course, astronomers cannot measure the sun's temperature directly. They have determined it from indirect measurements on sunlight and from mathematical equations that are based on known physical laws. Astronomers estimate that the temperature at the center of the sun reaches about 27,000,000 ºF (15,000,000 ºC).

The sun's energy is produced at its center. This energy gradually flows to the surface. Midway between the sun's interior and its surface, the sun's temperature is approximately 4,500,000 ºF (2,500,000 ºC). The temperature decreases to about 10,000 ºF (5500 ºC) at the surface of the sun.

When the energy produced at the sun's center reaches the surface, it is sent out into space as radiant energy in the form of heat and light. People once thought this heat and light came from something that was burning. Today, scientists know that the sun's light and heat come from thermonuclear reactions in the center of the sun. Such reactions occur when lightweight atoms join and form heavier atoms. For more information about the thermonuclear reactions of the sun, see the section of this article called How the sun produces energy.

The sun's mass makes up 99.8 percent of the mass of the entire solar system. The mass of the sun is about 1,047 times that of Jupiter, the largest planet in the solar system. The sun's mass is about 333,000 times that of the earth.

Because the sun is so massive, the force of gravity at its surface is much greater than the force of gravity at the surface of any of the planets. As a result, objects would weigh more on the sun than they would on any planet. A person who weighs 100 pounds (45 kilograms) on the earth would weigh about 2,800 pounds (1,270 kilograms) on the sun.

Through the force of gravity, the sun controls the orbits of the planets. The force of gravity also pulls the sun's gases toward the center of the sun. If there were nothing to balance the force of gravity on the sun, the sun would collapse. But it does not collapse because its gases are extremely hot. Hot gases have high pressure and try to expand. The pressure of the gases balances the force of gravity. As a result, the sun keeps its size and shape.

What the sun is made of. 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 a Greek word meaning sun.

Of the 112 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 percent of the mass of the sun. Scientists were able to identify the elements on the sun by studying the spectrum (pattern of colored lines) of light from the sun (see Light).

How the sun moves. Like the earth, the sun spins like a top. And, just as the earth revolves around the sun, the sun revolves around the center of the Milky Way galaxy.

The earth takes a day to rotate once on its axis, an imaginary line through the North and South poles. But the sun takes about a month to spin around once on the axis through its poles. The regions near the poles rotate about once a month, but the regions near the equator take a few days less than a month to spin around once. The difference in the rates of rotation is made possible by the sun's being a ball of gases. If the sun were a solid body, it could not rotate at different rates in different parts.

The earth takes a year to revolve around the sun, but the sun takes about 200 million years to make one revolution around the center of the Milky Way. During this period, the sun travels about 10 billion times as far as the distance between it and the earth.

The earth's atmosphere helps trap the heat of the sun. The atmosphere lets sunlight through to the surface of the earth. The light warms the earth, but the heat it creates cannot easily pass through the atmosphere into space. As a result, the earth is warmed by the sun. This behavior of the atmosphere is called the greenhouse effect because it resembles the action of a greenhouse. A greenhouse lets sunlight in to heat the plants, but the heat passes back through the roof and walls very slowly.

Life also depends on the sun for food. All living things--both plants and animals--are part of a process called the food chain. The food chain starts with green plants. These plants make their own food through the process of photosynthesis. During photosynthesis, plants combine energy from sunlight with carbon dioxide from the air and water from the soil to make food. In the process, the plants give off oxygen. Some plants are eaten by animals, which in turn are eaten by larger animals. People eat both animals and plants. Human beings and animals breathe the oxygen that the plants release during photosynthesis. They exhale the carbon dioxide that, in turn, is used by plants. See Photosynthesis.

Sunlight can also be harmful. Too much strong sunlight can burn the skin. The sun can seriously injure the eyes if a person looks at it directly.

Weather. Sunlight has a great influence on the earth's weather. For example, it evaporates water from rivers, lakes, and oceans, and this water later falls as rain or snow. When the water is suspended in the atmosphere, clouds appear. They reflect sunlight back into space. Sunlight also comes to the earth at various angles during different seasons. Clouds, and the angle at which sunlight reaches the earth, result in uneven heating of the earth's atmosphere. This uneven heating causes differences in air pressure. Air moves from high pressure areas to low pressure areas, causing wind and changes in weather.

The sun as an energy source. Until human beings learned to develop nuclear energy, sunlight supplied their energy needs. Plants used sunlight for photosynthesis. Animals ate the plants, and people used both plants and animals for food, clothing, and shelter.

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 coal, and by refining oil and natural gas, energy is released from the sun that was stored in the fossils millions of years ago.

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. See Solar Energy.

Many early beliefs about the sun were attempts to explain the sun's movement across the sky from east to west. The Greeks believed that the sun god Helios drove a chariot through the sky. The Egyptians believed that the sun god Re sailed a boat across the sky.

Other peoples who tried to explain the sun's motion included the Eskimos, and the Maoris of New Zealand. The Eskimos thought the sun took a boat trip at night beyond the northern horizon and was responsible for the aurora borealis, or northern lights (see Aurora). The Maoris believed that one of their heroes had fought the sun and crippled it, so that it limped across the sky.

Telling time and directions. Since ancient times, the sun has played an important part in people's efforts to keep track of time. The length of a day depends on the time the sun takes to return to a particular place in the sky as the earth rotates.

Ancient peoples used several kinds of devices to tell time. Sundials, for example, show the direction of the sun's shadow. The direction changes as the sun moves. Ancient calendars were based on the phases of the moon. The phases occur because sunlight reflected by the moon is seen from different angles as the moon circles the earth. Many ancient peoples built complex structures to learn about the sun's motion from north to south and back again as the seasons changed. Such monuments as Stonehenge in England probably tracked the motions of the moon and the sun.

Today, the sun has an important role in navigating and surveying. Navigators and surveyors carefully measure the position of the sun to find their own position--and various other points--on the earth.

Art, literature, and music. Many artists, authors, and composers have put the beauty and warmth of the sun in their work. The Dutch painter Vincent van Gogh created landscapes that expressed his joy with bright sunshine. The American poet Emily Dickinson wrote a poem called "The Sun," in which she described the rising and setting of the sun. The Russian composer Nikolai Rimsky-Korsakov included a beautiful song, "Hymn to the Sun," in his opera The Golden Cockerel.

Circular designs with extending spokes probably represent the sun and its rays. Varieties of this design include the cross, a symbol that appeared even before the time of Jesus Christ; and the swastika, a form of a cross.

Astronomers estimate that the Milky Way was formed between 10 billion and 15 billion years ago. The sun's age of about 4,600,000,000 years makes it one of the fairly young stars in the Galaxy. Some stars are much younger than the sun. They were formed during the last few million years.

How the sun was born. Throughout the Milky Way, and in space between the galaxies, are huge clouds of gases and dust. New stars are formed when portions of the gases and dust join together and begin to contract under the force of gravity. The contraction produces heat. As a mass of gases and dust shrinks, some of the heat increases the temperature at the center of the mass. Finally, the temperature at the center becomes so high that thermonuclear reactions begin to occur. These reactions produce energy and cause the mass of gases and dust to shine as a star.

Astronomers believe that the sun was formed from a rotating mass of gases and dust. They think the planets were formed from knots of gases and dust that collected at various distances from the center of the rotating mass. Scientists do not know many details of the birth of the solar system, but study and exploration of space, the moon, and the other planets are helping to increase their knowledge. Many astronomers believe that planets may also have formed near other stars when those stars came into being.

How long will the sun shine? The sun gets energy from thermonuclear reactions near its center. These reactions change hydrogen into helium. They release so much energy that the sun could shine for 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.

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 the 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.

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. The sun may throw off huge amounts of gases in violent eruptions called nova explosions 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.

After billions of years as a white dwarf, the sun will have used up all its energy and lost all its heat. Such stars are called black dwarfs. After the sun has become a black dwarf, the planets will be dark and cold. If the earth still has an atmosphere, the gases of the atmosphere will have frozen onto the earth's surface.

Beneath the sun's photosphere (surface) are the violently churning convection zone, the radiative zone, and the core, where the sun's energy is produced. This energy flows from the core to the photosphere and then out into space as radiant heat and light. Temperature, about 2,000,000 degrees F. (1,100,000 degrees C). Density, about 1/10 that of water. Temperature, about 4,500,000 degrees F. (2,500,000 degrees C). Density, about equal to that of water. Temperature, about 27,000,000 degrees F. (15,000,000 degrees C). Density, about 100 times that of water. Temperature, about 10,000 degrees F. (5500 degrees C). Density, between 1/1,000,000 and 1/10,000,000 that of water. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram by Herb Herrick.

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 ºF. (2,500,000 ºC), 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.

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 ºF. (1,100,000 ºC), and 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 convection 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.

The sun's surface, or photosphere, is about 340 miles (547 kilometers) thick, and its temperature is about 10,000 ºF. (5500 ºC). 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.

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 the violent churning of the gases in the convection zone.

The photosphere also has dark spots called sunspots. Sunspots are discussed in detail in the section of this article called The sun's stormy activity.

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. These colors are not all equally bright. 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. Where light has been absorbed, dark lines appear on the spectrum. These lines are called Fraunhofer lines, after Joseph von Fraunhofer, a German physicist who studied them during the early 1800's. Each element has its own characteristic pattern of Fraunhofer lines. Astronomers learned what elements are on the sun by comparing the Fraunhofer lines of the sun's spectrum with the lines that various elements show in laboratory experiments.

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 photosphere can be seen. The deeper gases are hotter than those near the surface, and the hotter gases give off brighter light.

Above the surface. About 100 miles (160 kilometers) above the photosphere, the temperature is about 7200 ºF. (4000 ºC). Above this point, the temperature rises again. In the chromosphere (the middle region of the sun's atmosphere), the temperature reaches about 50,000 ºF. (27,800 ºC).

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). A spicule lasts up to 15 minutes.

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 ºF. (2,200,000 ºC). The atoms of the corona are so far apart that the gases of the corona 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.

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.

The temperatures 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 temperatures 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.

What makes the sun shine? How can the sun continue to shine if it gives off so much energy every second? People have asked these questions for thousands of years. But only about 1900 did people begin to learn the answers.

Solar energy theories. Before people knew the age of the earth and the sun, they tried to explain how the sun produces light and heat. Some believed that the sun was a giant ball of burning coal. Others thought that meteors falling into the sun released the sun's energy. During the 1800's, two physicists, Hermann von Helmholtz of Germany and Lord Kelvin of Great Britain, supported the theory that the sun's energy came from the slow shrinking of the sun. All these theories proved wrong.

Scientists believe that the sun is about 4,600,000,000 years old. Only one source of energy--nuclear energy--could have kept it shining that long. In the early 1900's, a number of scientists formulated theories about nuclear energy.

Sir Arthur Eddington, a British astronomer, showed that the center of the sun has a temperature of many millions of degrees. At this temperature, the nuclei of atoms join together in the process of thermonuclear fusion. Two physicists, Hans Bethe of the United States and Carl F. von Weizsacker of Germany, described this process in the 1930's. They showed that thermonuclear fusion releases a sufficient amount of the sun's energy to keep the sun shining for billions of years.

The thermonuclear furnace. The changing of hydrogen into helium in the sun results in the release of the sun's energy in the form of heat and light. Helium is produced during a set of nuclear reactions. Scientists sometimes speak of these thermonuclear reactions as the "burning" of hydrogen. But such reactions are not "burning" as we think of the chemical process involving, for example, paper or wood.

The most important set of nuclear reactions in the sun is the proton-proton chain. These reactions involve protons and neutrons, the two major kinds of particles in the nucleus of an atom. The simplest of these proton-proton reactions has three steps. In the first step, two protons from two hydrogen nuclei fuse together. One of the protons immediately becomes a neutron by a process called beta decay. Now the proton and neutron make up the nucleus of a form of hydrogen called deuterium. In the second step of the proton-proton reaction, the deuterium nucleus captures another proton and becomes a light form of helium. In the third step, two light helium nuclei fuse, forming an ordinary helium nucleus. When they fuse, they give off two protons. The resulting helium nucleus has two protons and two neutrons. Thus, the proton-proton reaction converts four protons into one helium nucleus. However, the helium nucleus contains slightly less matter than did the four separate protons. Some of the matter that made up these protons has become the energy that the sun radiates.

Another set of nuclear reactions produces less energy in the sun than do the proton-proton chains. These reactions form the carbon-nitrogen-oxygen cycle. In this cycle, protons are added successively to the nuclei of carbon, nitrogen, and oxygen. Carbon becomes nitrogen, and nitrogen sometimes becomes oxygen but more often becomes carbon. Some of the nuclei that have been formed undergo the process of beta decay. After four protons have been added, a helium nucleus is given off.

Hydrogen, the most plentiful element in the universe, makes up about three-fourths of the sun's mass. There is enough hydrogen in the sun to keep it shining for billions of years.

Solar magnetism. Astronomers have found that sunspots, flares, prominences, 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.

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 a 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 field, and stormy activity takes place.

Sunspots. 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 form a sunspot group. Still later, the magnetic loops spread out and cover a wider area, and their sunspots fade away.

A typical sunspot may have a diameter of about 20,000 miles (32,000 kilometers). Most sunspots have two parts. The inner part, called the umbra, may have a diameter of about 8,000 miles (13,000 kilometers)--approximately the size of the earth's diameter. The outer part, called the penumbra, may have a diameter of about 12,000 miles (19,300 kilometers). The penumbra is hotter--and thus brighter--than the umbra. Some sunspots have no penumbra because of their small size.

The temperature of the gas that occurs in the upper photosphere and in the chromosphere above a sunspot group often rises about 1500 ºF. (815 ºC) above its normal temperature. As a result, it radiates more light than do surrounding gases. This light appears in patches called faculae, flocculi, or plages. These patches are more easily seen near the edge of the disk than near the center of the sun.

The number of visible sunspots varies from about 5 to approximately 100. It takes about 11 years for the number to increase from the minimum to the maximum. This 11-year period is called the sunspot cycle.

A sunspot cycle begins when sunspots appear at high solar latitudes. As the cycle continues, more sunspots appear closer to the sun's equator. During a cycle, the north and south magnetic poles of each pair of 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.

Astronomers do not know why sunspot cycles take place. But they know that the cycles are closely connected with other kinds of solar activity. All types of solar activity become most intense during the maximum phase of a sunspot cycle.

Flares. After a sunspot group has existed for a long time, the magnetic lines of force usually become jumbled. As a result of the jumbling, magnetic energy is stored in the corona. The energy may be released in a spectacular discharge called a flare. In a flare, the magnetic lines of force become reconnected into a simpler pattern. The energy is released in the form of light, heat, and fast-moving atomic nuclei and electrons called solar cosmic rays.

A flare may be as small as a sunspot or as large as a sunspot group. The temperature in a flare is about twice as high as the temperature at the sun's surface. Flares radiate much light into space, and astronomers can photograph them against the background light of the sun. Small flares may last about 10 minutes. The largest ones last about an hour.

Large flares produce so many solar cosmic rays that important consequences occur on the earth. For example, the rays disrupt radio communications. They endanger astronauts in space, where the earth's magnetic field is not present to protect them from such large amounts of radiation. After astronomers learn to predict the occurrence of large flares, they will be able to make space travel safer.

Prominences are one of the most interesting features of the sun. Each of these bright arches of gas outlines a long, strong bundle of magnetic lines of force. Prominences shine brightly because their gases have a higher density and radiate light more efficiently than do the gases in the chromosphere and the corona.

A typical prominence may reach 20,000 miles (32,000 kilometers) above the sun's surface. Its total length may be 120,000 miles (190,000 kilometers), and the gases may be 3,000 miles (4,800 kilometers) thick.

There are two kinds of prominences--quiescent and active. A quiescent prominence changes little in appearance during its two- or three-month existence. An active prominence changes rapidly during a period of only several hours. Some active prominences erupt and fling their gases rapidly into space.

Solar radiation. The sun gives off many kinds of radiation besides visible light and heat. These radiations include radio waves, ultraviolet rays, and X rays.

Astronomers use radio telescopes to study the radio waves given off by the sun. These telescopes allow scientists to learn about solar storms. Strong bursts of radio waves occur during violent solar activity. These bursts originate in the sun's atmosphere above sunspot regions, particularly when flares occur. The bursts last from a few minutes to a few days.

Ultraviolet rays consist of waves of light that are shorter than the waves of violet light on the visible spectrum. They are invisible to the human eye. Ultraviolet rays cause sunburn, and too much exposure to them may cause skin cancer. The earth's atmosphere absorbs much of this radiation. The sun gives off more ultraviolet rays and X rays during periods of violent activity than during calm periods. Flares greatly increase the amount of radiation from the sun.

X rays are another form of solar radiation. The sun's X rays can injure or destroy the tissues of living creatures. The earth's atmosphere shields human beings from most of this radiation.

The solar wind. The corona is so hot that its gases continually expand away from the sun. This flow of gases continues into space until the gases mix with those near the outer planets of the solar system. The flow of gases is called the solar wind. When the solar wind reaches the earth's orbit, it is traveling at 1 million to 2 million miles (1.6 million to 3.2 million kilometers) per hour. The solar wind confines the earth's magnetic field into a specific volume of space called the magnetosphere. The boundary of the magnetosphere is about 40,000 miles (64,000 kilometers) from the earth.

Most of the solar wind comes from regions of the corona that have relatively low temperature and density. These regions are called coronal holes. In coronal holes, the pattern of the sun's magnetic field differs from the pattern found where sunspots and other forms of solar activity occur. In regions of solar activity, the magnetic lines of force loop back to the surface of the sun. In coronal holes, however, the lines of force extend outward into space. Large amounts of solar wind flow away from the sun along these lines of force. Much of the solar wind flows outward from large coronal holes at the sun's poles.

Flares are another source of solar wind. Flares inject high-speed particles into the solar wind. As a result, the solar wind presses harder on the earth's magnetosphere and causes magnetic storms on the earth. These magnetic storms interfere with radio communications, and they can make compass needles swing wildly.

About A.D. 150, the astronomer Ptolemy of Alexandria declared that the earth was a stationary body in the center of the universe. He believed that the sun, moon, planets, and stars all circled the earth.

The true relationship between the earth and the sun became known in the early 1500's. In 1543, the Polish astronomer Nicolaus Copernicus stated that the sun was at the center of the solar system. He said that the earth and the other planets revolved around the sun.

Gradually, astronomers realized that the sun was a star and began to study it more scientifically. In 1904, the American astronomer George Ellery Hale set up the Mount Wilson Observatory near Pasadena, Calif. This observatory included instruments for the study of the sun. Hale believed that by studying the sun, scientists could learn much about other stars. He made popular the word astrophysics, meaning the study of astronomical bodies by using the methods practiced in physics.

Modern solar study. The earth receives far more light from the sun than it does from the other stars. To help astronomers study the light, solar telescopes spread it out as much as possible. A telescope operated by the National Solar Observatory on Kitt Peak, near Tucson, Ariz., produces an image of the sun that is about 30 inches (75 centimeters) in diameter. Even so, turbulence in the earth's atmosphere limits the detail that can be seen on the sun's surface. The smallest features that can be distinguished are about 500 miles (800 kilometers) across. Other solar telescopes include those at the Sacramento Peak Observatory near Alamogordo, N. Mex.; the Pic du Midi Observatory in the Pyrenees Mountains of France; and the University of Hawaii's Institute for Astronomy at Haleakala, on Maui Island.

Astronomers use solar spectrographs to analyze the sun's spectrum. These instruments spread out the colors of the spectrum to help in the study of sunlight.

One special instrument, the coronagraph, is used only for solar work. With it, astronomers can photograph the sun's corona without waiting for a total eclipse. The coronagraph is a tube with a disk in the middle to block out the light from the photosphere and chromosphere. It enables astronomers to create an "eclipse" whenever they want to study the corona.

Most solar radiation is best studied from space. During the 1960's, scientists first used rockets and then satellites to learn about the sun's ultraviolet radiation. The best pictures of the surface of the sun were taken from a high-flying balloon that was part of an experiment called Project Stratoscope. Skylab, a manned space laboratory launched in 1973, carried several telescopes that measured the sun's ultraviolet and X-ray radiation. These telescopes enabled astronomers to view coronal holes for the first time. The telescopes also showed that the corona is both heated and shaped by the magnetic fields that are produced in the sun's interior.

During the 1960's, astronomers used space probes (satellites sent into outer space) to study solar cosmic rays and the solar wind. The Pioneer spacecraft that were sent past the sun, and the Mariner flights to Mars and Venus, provided much information about these features of the sun. The astronauts of the Apollo 11 and Apollo 12 lunar flights conducted experiments to help scientists learn more about the solar wind.

During the 1970's, astronomers discovered that much of the motion in the sun's atmosphere occurs in the form of waves. Scientists believe these waves are produced by motions in the convection zone of the sun's interior. The study of the waves, known as helioseismology, has revealed variations of temperature, density, and chemical composition in the sun's interior.

In 1980, the United States launched a spacecraft called Solar Maximum Mission Satellite. It showed that sunspots reduce the amount of solar energy that reaches the top of the earth's atmosphere. In 1990, the European Space Agency and the United States launched the solar probe Ulysses from the U.S. space shuttle Discovery. Ulysses first traveled to Jupiter. The gravity of that planet altered the probe's orbit, sending Ulysses toward the sun. The new orbit carried Ulysses over the sun's polar regions. In 1994, Ulysses became the first spacecraft to observe the sun from a polar orbit.

Contributor: Robert W. Noyes, Ph.D., Prof. of Astronomy, Harvard Univ.; Physicist, Smithsonian Astrophysical Observatory.

Related articles include:

Eclipse; Light; Radiation; Rainbow; Solar Energy.

What is a red giant? A yellow dwarf?

Why is sunlight white? Why does it sometimes appear colored?

What is the diameter of the sun? How does the sun compare in size with other stars? How does it compare with the earth?

How does the coronagraph help astronomers study the sun?

In what ways do people use the sun as a source of energy?

What are sunspots, flares, and prominences?

How old is the sun?

About how far is the earth from the sun?

What is solar wind?

Davis, Don, and Levasseur-Regourd, A. C. Our Sun and the Inner Planets. Facts on File, 1989.

George, Michael. The Sun. Creative Education, 1991.

Friedman, Herbert. Sun and Earth. W. H. Freeman, 1986.

Hufbauer, Karl. Exploring the Sun. Johns Hopkins, 1991.

Noyes, Robert W. The Sun, Our Star. Harvard, 1982.

Phillips, Kenneth J. H. Guide to the Sun. Cambridge, 1992.

Wentzel, Donat G. The Restless Sun. Smithsonian Institution, 1989.

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