Introduction to Sun

Sun, the star at the center of the solar system. Like other stars, the sun is a huge, hot, gaseous sphere. It is held together by its own massive gravity and is heated by nuclear reactions in its core. Radiation from the sun provides the energy that drives virtually all the natural processes that occur on the earth and in the earth's atmosphere.

No object in the universe is more important to the earth than the sun. Although only one two-billionth of the sun's radiation reaches the earth, that tiny portion is enough to make life possible. Without the heat of the sun, the earth would be frozen and lifeless. By a process called photosynthesis, most plants and algae and some bacteria use the energy of sunlight to produce their own food. Virtually all other life depends on these organisms for nourishment. Sunlight absorbed by them millions of years ago is available as energy today from fuels such as coal, oil, and natural gas. These fossil fuels were formed from decayed plant and animal matter.

From the sun's Latin name, sol, comes the term solar system for the sun together with the group of planets and other bodies whose motions are governed by the sun's gravitational attraction. The vast region dominated by the sun's attraction extends far beyond the orbits of Neptune and Pluto, the most remote planets. Within this region, the motions of all bodies are influenced by the pull of the sun's enormous mass, which is estimated to contain more than 99 per cent of all the matter in the solar system.

As a star, the sun is of only average size and temperature. Astronomers classify it within the main-sequence group of stars, along with millions of other similar stars in the Milky Way galaxy. Although it appears billions of times brighter than any other star seen from earth, the sun differs in appearance from other stars only because of its nearness to earth. If the sun were as distant as Proxima Centauri, the next nearest star to earth, it would appear as a moderately bright, yellow star.

The distance between the sun and the earth varies during the year because of the earth's elliptical orbit around the sun. The average distance is just under 93,000,000 miles (150,000,000 km). Light leaving the surface of the sun requires about eight minutes to reach the earth and about four hours to reach Neptune.

Structure of the Sun

Compared to other members of the solar system, the sun is almost unbelievably vast. Its diameter of some 865,000 miles (1,392,000 km) is more than 100 times that of the earth and nearly 10 times the diameter of Jupiter, the largest of the nine planets. More than one million earths could fit inside the space occupied by the sun. Although it may appear to be a solid body, the sun is entirely gaseous, since liquids and solids cannot exist at the temperature of even the coolest part of the sun.

The sun is made up of four general regions: the interior, the photosphere, the chromosphere, and the corona. Relatively little is known of the sun's interior, although calculations of its temperature, pressure, and density have been made, based on the sun's mass and diameter. These calculations indicate that the gas deep within the sun's core has a density more than eight times that of lead and a temperature greater than 30,000,000 °F. (16,667,000 °C.).

The photosphere is the yellowish, luminous surface and outermost layer of the sun's sphere. It is about 200 miles (320 km) thick and has an average temperature of about 10,000 °F. (5,540 °C.). Seen through a telescope, the photosphere has a distinctive granular or mottled appearance.

The lower portion of the sun's atmosphere, the chromosphere, is a constantly changing, turbulent region that reaches some 6,000 miles (9,660 km) above the photosphere. Temperatures in the lower part of the chromosphere are similar to those of the photosphere. With increasing distance from the surface, however, the temperature rises rapidly, reaching more than 1,000,000 °F. (556,000 °C.).

Even higher temperatures exist in the corona, the outer atmosphere. Like the chromosphere, the corona is of such low density that it is transparent and can be seen from earth only during a total eclipse or with the aid of a special instrument called a coronagraph. The corona's shape and extent vary from time to time, but normally the corona reaches out more than 1,000,000 miles (1,600,000 km) from the sun's surface.

By careful study of sunlight using spectroscopes and other instruments, scientists have found in the sun more than 70 of the natural chemical elements known on earth. Of these, hydrogen is by far the most abundant. Helium makes up most of the remainder, followed by smaller amounts of oxygen, nitrogen, and carbon. Trace amounts of heavier elements such as copper, gold, iron, and lead are also present, particularly in the cooler parts of the chromosphere.

Like most other bodies in space, the sun rotates about its axis. However, because the sun is entirely gaseous, some parts of it rotate faster than others. Regions near the sun's equator make a complete rotation in about 25 days, but toward the poles the period of rotation is as long as 34 days.

All of the sun's major characteristics—size, brightness, density, and temperature distribution—depend on two opposing forces. These are gravitation, which tends to compress the sun's mass; and pressure within the interior, which tends to expand the sun. Each force balances the other, keeping the sun in a state of equilibrium.

Why the Sun Shines

If the sun generated heat and light by ordinary burning, it would have long since consumed itself and become dark. Nor can the sun (or any other star) keep shining simply because it is extremely hot. Scientists know that since the sun has been shining for about 5 billion (5,000,000,000) years and will continue for at least that many more years, only a very large and long-lasting source of energy could power the sun. That source is the same as the one used in a hydrogen bomb—the transformation of matter into energy during nuclear fusion.

Albert Einstein in 1905 first defined the relationship of mass to energy with the formula Emc2. In a reaction following this formula, a very large amount of energy is released from a relatively small mass.

Deep within the interior of the sun, several types of fusion reactions take place. In the principal process, groups of four hydrogen nuclei are constantly combining to form single helium nuclei. The mass of each new helium nucleus is slightly less than the total mass of the four original hydrogen nuclei, the lost mass having been converted into energy. This energy eventually reaches the sun's surface, where it is released as light, heat, and other forms of electromagnetic radiation. Although the mass loss from each individual reaction is extremely small, the total loss from the entire sun is some 4,000,000 tons (3,600,000 metric tons) every second.

Solar Activity

The constant release of tremendous amounts of energy keeps the sun's surface in a state of seething, churning activity. Telescopic observations show that the surface is covered with fine granules, or cells, which are the tops of rising columns of hot gas from the interior. The granules are usually about 500 to 1,000 miles (800 to 1,600 km) in diameter and have a lifespan of only a few minutes. Spikelike spicules of gas, apparently associated with the surface granules, form constantly. They typically shoot upward to a height of 3,000 to 4,500 miles (4,800 to 7,200 km) and fall back within several minutes.

From time to time, irregular and seemingly dark features known as sunspots appear on the sun's surface. These are the most conspicuous and best-known of the sun's markings. The dark appearance of sunspots is due to their lower temperature—as much as 3,000 °F. (1,670 °C.) cooler than the surrounding surface. However, the darkness is only relative, since sunspots are actually very bright.

Sunspots appear both individually and in groups, and may range in diameter from 3,000 miles (4,800 km) to more than 100,000 miles (160,000 km). Their occurrence varies in a cycle averaging approximately 11 years between periods of maximum activity. At the peak period, hundreds of sunspots may be observed at one time. Although their precise causes and mechanisms are not clearly understood, sunspots apparently are associated with local magnetic disturbances beneath the surface.

Several other kinds of solar activity are related to sunspots and the sun's magnetic field. Appearing frequently near sunspot groups are bright, irregular spots called faculae. Resembling luminous clouds, faculae seem to float just above the surface and often occur shortly before the appearance of a new sunspot group.

Occasionally, a sunspot group shows a sudden and rapid increase in activity that results in a solar flare. Such an occurrence is one of the most dramatic of all solar events; within a few seconds, a tongue of atomic particles and glowing gas is violently ejected from the sun at speeds high enough to escape the sun's gravitational pull. Small flares are relatively common occurrences, but large ones are not; even during a year of peak sunspot activity only a few are seen.

Streamer-like prominences often arc through the sun's atmosphere, sometimes extending 100,000 miles (160,000 km) or more above the surface. When seen at the edge of the sun's disk, prominences may appear as long plumes or arches. Observed on the face of the sun, a prominence usually appears as a dark thread or filament. Prominences may last days or weeks before sinking back beneath the lower atmosphere.

The Sun's Radiation

The end product of the sun's violent thermonuclear reactions is energy emitted over the complete spectrum of electromagnetic radiation. The electromagnetic spectrum consists of radiation of all known wavelengths, including those of visible light as well as those too long or too short to be seen by the human eye.

In order of increasingly longer wavelengths, the sun's radiation includes gamma rays, X rays, and ultraviolet light; next are visible light and infrared light, followed by a wide range of radio waves.

The sun also gives off radiation in another form. Because the corona, or outer atmosphere, is unstable, streams of charged particles are constantly pushed out by the sun in what is called the solar wind. The higher-energy particles in the solar wind are sometimes known as solar cosmic rays.

The earth's atmosphere acts as a shield to block out certain wavelengths of radiation harmful to life, particularly cosmic rays, X rays, and nearly all ultraviolet light. Of the radiation that penetrates the atmosphere, most is visible light, or sunlight, and infrared light, which produces heat in objects it strikes. The small amount of ultraviolet light that passes through the upper atmosphere aids the human body in producing vitamin D and is also responsible for causing suntan and sunburn.

Certain short radio waves emitted by the sun also pass through the atmosphere, but longer radio waves are reflected back into space. Radiation and particles emitted from large solar flares and those carried by the solar wind sometimes create strong disturbances in the earth's magnetic field. The auroras commonly seen in polar latitudes result when charged solar particles react with atoms of the upper atmosphere.

Studying the Sun

To many ancient astronomers, the sun was a ball of pure fire that revolved around the earth. This general belief was held until the early 1600's, when telescopes first came into use and the sun was recognized to be at the center of the solar system.

Galileo and other 17th-century astronomers saw numerous sunspots through their telescopes, but had no idea what they were seeing. In 1785, the British astronomer William Herschel determined that the sun was a star within the Milky Way galaxy. Herschel also suggested, incorrectly, that sunspots revealed a cool, dark crust beneath the fiery visible surface. He further proposed that this relatively cool interior might contain intelligent life. Speculation such as Herschel's continued for many years as astronomers struggled with inadequate tools to learn more about the sun.

With the development in the mid-1800's of such important astronomical aids as photography and spectroscopy, the true nature of the sun began to be discovered. More than 60 chemical elements were soon found to exist on the sun's surface, including one that was unknown on earth. The new element, named helium after the Greek word for sun, helios, was eventually found to occur on earth as well.

The mystery of the sun's energy source was not solved until extensive nuclear research began in the 1930's. Using the newly discovered principles of nuclear physics, the German-American physicist Hans Bethe and others were able to explain the processes by which the sun and other stars obtain their energy.

Much of the equipment and many of the techniques used by solar astronomers are similar to those used by other astronomers. Because of the size and brilliance of the sun's image, however, large-diameter telescopes are not needed. Some solar telescopes resemble small conventional refractor or reflector telescopes. Others, such as the McMath solar telescope on Kitt Peak in Arizona, use a series of large mirrors to reflect the sun's image and relay it to an observing room for spectroscopic study.

An important tool used with some solar telescopes is the coronagraph, an instrument that artificially eclipses the sun so that the corona and chromosphere may be observed. The spectroheliograph produces a photograph of the sun by recording light of a single wavelength—such as that emitted by atoms of hot hydrogen gases in the chromosphere—to reveal otherwise invisible details in the sun's atmosphere.

Intensive studies are also made of the sun's invisible radiation, particularly X rays, infrared light, and radio waves. Since X rays and some infrared light do not penetrate the earth's atmosphere, their observation is usually done with the help of artificial satellites or high-altitude balloons. Detailed investigations of the entire solar spectrum have been made with the aid of such satellites as those of the Orbiting Solar Observatory (OSO) series. Because there are millions of stars similar to the sun in the Milky Way galaxy alone, astronomers frequently use information from solar observation as an aid in understanding more about other stars.