As far as life on earth is concerned, the sun is the most important star in the sky. For centuries, astronomers have studied its appearance and behavior in an attempt to discover how it functions. But it is not only the sun’s physical properties that have been the subject of study: the determination of the earth-sun distance has also long been a focus of attention. The currently accepted value of 92,960,000 miles (149,600,000 kilometers) for this distance is known as an astronomical unit. Such accuracy is essential for astronomers to determine the scale of the solar system and for space scientists to guide spacecraft to other planets.
The astronomical unit
The Creek astronomer Aristarchus of Samos was the first to calculate the distance from the earth to the sun. His value of 2,983,000 miles (4,800,000 kilometers), although far short of the true value, was reached through direct measurement. The theory behind his geometric method was sound, but instruments available at the time did not enable angles to be measured accurately.
The first fairly accurate calculation was made by the Italian astronomer Giovanni Cassini in 1672. His value of 86,103,000 miles (138,570,000 kilometers) for the earth-sun distance was only a little short of the currently accepted figure. Since then, calculations of the astronomical unit have steadily improved.
Sunspots and flares
Observation of the sun reveals many features both on and above its surface. (Such observations are made using special techniques and instruments; looking at the sun directly, with or without a telescope, is extremely hazardous and can instantly produce blindness.) Sunspots mar the bright outer photosphere, and flares and prominences can also be seen. These violent releases of energy within the solar atmosphere result from nuclear processes that take place at the center of the sun.
Sunspots were first studied telescopically in 1610. In that year, the German astronomer Johannes Fabricius published his observations of sunspots, although at about the same time Galileo used their apparent motion across the solar disk to measure the rotational period of the sun. He arrived at a value of just less than one month. In 1863, Richard Carrington, a British astronomer, discovered the sun’s differential rotation—that the period at the equator is shorter than that at the poles. Modern estimates of the rotational periods are 26 days at the equator and 37 days at the poles.
A typical sunspot has two regions: a dark umbra and a surrounding penumbra. The um-bral temperature is about 4,000K, whereas the penumbra is even hotter at between 5,000 and 6,000K. Sunspots appear dark because even at such high temperatures they are cooler than the surrounding photosphere. (If they were viewed independently of their background, they would shine brighter than an electric arc lamp.)
Sunspots are depressions on the solar surface. In 1769, Alexander Wilson observed a spot on the limb of the sun, which revealed that the penumbra on the far side of the spot was broader than on the foreshortened near side. This effect (the Wilson effect) was explained by assuming that the spots are hollows.
Sunspot activity recurs in a regular cycle, the idea of which was first put forward by the Danish astronomer Horrebow in 1775-1776. During a cycle, spots appear between 30° and 40° north or south of the sun’s equator. The region of formation gradually progresses toward the equator until spots form at latitudes of 7° or 8°; maximum sunspot activity occurs at about 15°. This regular change in the latitude of spot formation is known as Sporer’s law, after the German astronomer Gustav Spdrer, who investigated the phenomenon in 1861. The average length of the sunspot cycle is slightly more than 11 years, although it can vary considerably—from as little as 7.5 years to as long as 17 years.
Associated with sunspots are faculae, discovered by Christoph Scheiner in 1611. These features are luminous clouds, composed mainly of hydrogen, which lie above the solar surface. They usually appear in the region where a sunspot group is about to form, and have an average duration of about 15 days.
Flares are outbursts of energy that also have their origins in sunspot regions. Discovered by Carrington in 1859, these too are fairly short-lived features. Radio disturbances and auroral displays on earth are the direct result of flare activity on the sun.
Prominence’s are yet another type of eruptive feature in the solar atmosphere. Visible with the naked eye only during a total solar eclipse, they can be studied by spectroscopic observation of hydrogen emissions. There are two basic kinds of prominence’s: quiescent prominence’s, which are relatively stable and long-lived, and eruptive prominence’s, which can display rapid motion and typically attain heights of more than 20,000 miles (32,000 kilometers) above the surface.
The neutrino puzzle
Nuclear reactions at the center of the sun produce nearly all the sun’s energy. During these processes, neutrinos are released. They are tiny particles with no mass or electrical charge. Neutrinos are emitted from the sun at the speed of light and pass straight through the earth or any other solid body they encounter. As a result, they are difficult to detect and measure. They do, however, react with chlorine to produce argon. To prove the existence of neutrinos, an experiment has been set up involving a tank that contained nearly 130,000 gallons (492,000 liters) of chlorine compound, placed at the bottom of a disused mine in South Dakota. But the experiment has not collected the number of neutrinos that had been anticipated. This may mean that the particles disintegrate on their journey to earth or that the physics of the solar interior is not yet fully understood.
The production of sunshine
The sun may be the central and most important body in the solar system, but grouped with the rest of the stars, it is classed merely as a yellow dwarf. Virtually all our light and much of our heat comes from the sun, and without its energy, life on earth would cease to exist. The sun is associated with various astronomical phenomena, including the beautiful corona that surrounds the totally eclipsed solar disk, and the ghostly auroras that are produced in the earth’s atmosphere as a result of the interaction of energized solar particles with the outer atmospheric layers.
The sun has been observed in many ways for centuries. Records of solar eclipses go back to early times, and sunspots have been studied since the advent of the telescope in the early part of the seventeenth century. Even ancient civilizations recognized the importance of the sun and worshiped it as a god. In more modern times, the examination of complex features of the sun, such as the corona and prominences, has taken place only during the last 200 years or so. Solar flares have been scrutinized only since the mid-nineteenth century. Space exploration has made possible more detailed observations of the sun. X-ray and ultraviolet emissions could be recorded once measuring instruments were lifted by rocket above the earth’s atmosphere, which absorbs electromagnetic radiation at these (and some other) wavelengths.
The source of solar energy
As with any other star, the sun produces its energy by nuclear reactions in its central core. The temperature and pressure in the core is so intense that hydrogen, initially the most abundant gas in the sun, is converted by thermonuclear fusion reactions into helium. This conversion takes place when four hydrogen nuclei are fused together to make one helium nucleus. The reaction initiates a release of energy that travels by convection through the main body of the sun to emerge as visible radiation at the surface. It is this continuous nuclear process that keeps the sun, and all other stars, shining as they do.
The structure of the sun
When we observe the sun in visible light, the surface, known as the photosphere, can be seen. It is a turbulent region 340 miles (547 kilometers) deep. Closer examination of the photosphere reveals a curious granular structure, each granule measuring 620 miles (1,000 kilometers) across. The granules are fairly shortlived and are produced by turbulent upcur-rents of energy from within the sun.
Energy released from the photosphere passes through the almost transparent chromosphere, which is several thousand miles thick. The temperature of matter within this region rises from about 4,500 K at the bottom to about 1,000,000 K in the outer reaches. The gases of the chromosphere are extremely tenuous. Ultraviolet observations show, however, that it is a very active, dynamic region of extremely high temperatures, through which energy released from the sun passes to the corona and then on to interplanetary space beyond.
The sun’s outer atmosphere
The corona is the outermost layer of the sun’s atmosphere. The boundary between the chromosphere and the corona is a thin transition zone within which temperatures rise dramatically to 2,000,000 K. This boundary region is only a few miles thick. Since satellites have been able to carry X-ray telescopes above the dense layers of the earth’s atmosphere, studies at these wavelengths have revealed a number of surprises.
The corona is made up of an inner and an outer region. The lower, or inner, corona consists of streams of atomic particles that tend to follow the lines of magnetic regions on the solar surface, forming arches or loops. In places, there are no coronal loops, especially at times of solar minima, when there are fewer active regions on the solar disk, such as sunspots and their associated magnetic fields.
Further examination of the corona reveals large “holes” through which energized particles escape directly into space. Coronal holes are regions with weak magnetic fields where the field lines are open and not looped. The release of particles sometimes dramatically alters the density of solar particle fields in the neighboring interplanetary environment.
The outer corona is more tenuous in nature than its inner counterpart. The temperature is still high, at about 1,000,000 K, although the component particles are much more widely spaced than in the lower corona. The whole corona has no effective outer boundary and eventually it thins out to become the solar wind. The outer region of the corona can be observed during a total solar eclipse, its misty form reaching out far beyond the solar limb. It marks the last region of the solar atmosphere through which streams of energized particles must pass before they flow out through the solar system as the solar wind.
Light and heat are not the only forms of energy from the sun that affect the earth. The solar wind also produces its own effects. Aurorae, too, are among the many results of interactions between atomic particles from the sun and the outer layers of the earth’s atmosphere. Another is airglow, the faint radiance that ensures that the night sky is never totally dark.