The Nature of the Sun

From core to corona.
Artist's cutaway drawing of regions and structures of the Sun includes both the observed phenomena and layers of the exterior and the hypothesized energy-generating and transmitting zones of the interior. Many surface features are shaped by magnetic field patterns. Interior regions are calculated from conditions necessary for the observed solar energy to be produced by nuclear fusion of hydrogen deep inside the Sun.

What is the sun?

Human beings have always looked upon the Sun as the most important celestial body. Even primitive people realized that the light and heat from the Sun sustain all life on Earth, and they knew that any disturbance to the Sun's daily motion through the sky would have serious effects. Over the years, they devised many rituals to ensure the reliability of the Sun. Later, Greek philosophers declared that the Sun is a flawless sphere, its motions governed by perfect perpetual clock work. Sunspots glimpsed dimly by the ancients when thin cloud or thick mist made it possible to stare at the Sun were dismissed as unrelated objects passing in front of the glowing sphere.

This idea was accepted throughout the Western world until after the invention of the telescope, when Galileo proved that sunspots were true markings or structures (hence, "blemishes") on the surface of the Sun. Not only was the Sun imperfect, but it was changeable; the spots came and went over days and weeks. Later, astronomers discovered that the number of sun spots varies in a cycle, showing a maximum and a minimum about every eleven years.

More recently, the combination of nuclear energy theory, laboratory ex periments, and better solar observations has enabled scientists to obtain a good picture of the overall structure of the Sun. It is a ball of gas, composed of about 90 percent hydrogen, 9 percent helium, and only 1 percent of all the other elements such as carbon, nitrogen, oxygen, silicon, and iron. The diameter of the Sun is about 1,390,000 kilometers (865,000 miles), or about 109 times that of the Earth, and the Sun is 300,000 times as massive as the Earth.

At the Sun's center, theory predicts that the temperature reaches an incredible 15 million° C. This is the temperature of an exploding hydrogen bomb; it is hot enough to sustain the thermonuclear reactions that convert hydrogen atoms into helium, thus powering the Sun. In this way the Sun consumes about 5 billion kilograms (5 million tons) of its nuclear hydrogen fuel every second. Yet the Sun is so large that it has been burning hydrogen at this rate ever since it formed some 5 billion years ago, and it will continue to burn steadily for at least another 4 billion years.

The energy released by nuclear fusion in the heart of the Sun is eventually radiated away in all directions into space. A tiny fraction reaches the Earth, powering every process necessary for life. Even this tiny fraction is enormous. The solar energy striking the Earth is equal to 800 billion megawatts of power, an amount vastly more than the entire capacity of all of our power plants. Someday, we will learn to harness it effectively.

The solar interior may burn hydrogen at a steady rate, but a close look at the surface and outer layers shows that the Sun is not stable at all. Examined with modern techniques - especially space instruments - the solar atmosphere is seen with constant motion and violent activity. Solar disturbances of diverse kinds occur on time scales ranging from years down to thousandths of a second, involving regions ranging in size from the entire solar atmosphere to the smallest detail visible in our most powerful telescopes.

Space observations allow us to see the Sun in many wavelengths, or "colors," of light that are totally absorbed by the Earth's atmosphere and cannot be seen from the ground. High-energy gamma rays and X-rays, ultraviolet light, and much of the infrared and radio regions of the spectrum can be observed only from above most of our atmosphere. Each of these newly explored spectral regions, or "windows", yields unique information about physical processes and phenomena on the Sun which are inaccessible with ground-based telescopes.

Through these new windows that open for telescopes above the Earth's atmosphere, we can see radiation emitted from many different parts of the Sun. Each solar region has its own temperature, density, and other characteristic physical conditions and emits its own kind of light.

We have measured temperatures on the Sun ranging all the way from a mere 4200° C in the coolest regions to over 50 million° C in the hottest solar flares.

Rays from the Sun.
Solar features at different temperatures emit characteristic radiation of different forms, ranging from radio and infrared waves to visible light, ultraviolet, X-rays, and gamma rays. Drawing shows typical temperatures, while legend at the left indicates corresponding radiations. Infrared and visible light are suitable for study of coolest layers, while much information on the hot corona can only be gleaned from X-rays. Energetic atomic particles from solar flare explosions produce gamma rays.

The apparent surface of the Sun, the photosphere, has a temperature of about 6000° C, which decreases with height to a minimum value of 4200° C. The solar gas in this region shines mostly in visible and infrared light. Above the minimum temperature region, the gas gets hotter. The chromosphere (the next higher part of the Sun's atmosphere) has a temperature of about 10,000° C and glows brightly in ultraviolet light. The million-degree corona, farther out around the Sun, is best seen by the X-rays it emits. The very highest energy radiation given off by the Sun is not due to a hot gas but is actually produced by erupting streams of very-high speed electrons and protons which strike the ordinary atoms of the Sun's atmosphere with sufficient force to generate X-rays and gamma rays. On Earth we use a very similar process on a vastly smaller scale to generate X-rays for medical examinations.

In addition to the X-rays, ultraviolet, and other forms of light from the Sun, satellite-borne instruments have observed pulses and streams of atomic particles that are emitted from the Sun and travel outward to the Earth and beyond. These particles provide actual samples of solar material. Their composition tells what the Sun is made of and how matter is ejected from its atmosphere. From space we can even detect the solar magnetic field, which stretches out into the far limits of the solar system.

The Sun as a star

We sometimes forget that there is one star that is easily visible in the day time: our Sun. The Sun is the only star close enough to be studied in detail, but we are confident that all the processes in the Sun must also occur in billions of distant stars throughout the universe. To understand the nature and behavior of other stars, we must first understand our own. At the same time, observations of other kinds of stars help put the Sun in perspective.

The Sun is a relatively typical star among the approximately 100 billion stars in our Milky Way galaxy. The masses of most other stars that we see range from approximately one-tenth the mass of the Sun to about 30 solar masses. The surface temperatures of most stars range from about 2000° C to 40,000° C. Although the Sun is somewhat on the cool side at about 6000° C, hot stars are rare, and most normal stars are cooler than the Sun. Compared to some of the explosive stars - novae and supernovae - which sometimes appear in the sky, the Sun is stable and ordinary.

This long-term stability of our Sun probably was crucial for the development of life on Earth. Biologists believe that a relatively stable average temperature had to prevail on Earth during the past 3 billion years, in order for life to evolve to its present state. The relative stability of the Sun is also important to astronomers trying to understand the basic nature of it and other stars. Violent activity in the Sun could mask the more subtle and long-enduring processes which are the basic energy transport mechanisms of our star. Fortunately, they are not hidden, and we have been able to map the trend in solar properties with height above the visible surface.

Above the minimum temperature region in the photosphere, we have measured how the gas gets hotter as it thins out with height. The chromosphere and corona, each hotter than the layer below, are warmed by the transfer of energy from below, by processes that still are not well understood.

Until space observations became possible we knew nothing about coronae in any other stars, and had only marginal information about the properties of stellar chromospheres. Now, space observations have shown us that a large fraction of the stars in the sky have chromospheres and coronae.

On several dozen stars, we have even detected activity which may be connected with sunspot (or "starspot") cycles like those of our own Sun. X-ray telescopes carried on satellites have recorded flares in other stars that are far more powerful than the already impressive flares of the Sun. By observing the strength and frequency of these events on stars with masses, ages, and rotation rates which differ from those of the Sun, we search for answers to such basic questions as: "How does the sunspot cycle period depend on the star's rotation rate?" or "What is the relation between the temperature of a star's corona and the strength of its magnetic field?" By deciphering the general pattern of stellar properties we can better under stand what makes things happen on the Sun.

The Sun presents us with a bewildering variety of surface features, atmospheric structures, and active phenomena. Sunspots come and go. The entire Sun shakes and oscillates in several different ways at the same time. Great eruptions called prominences hang high above the Sun's surface for weeks, suspended by magnetic force, and then sometimes shoot abruptly into space from the corona. The explosions called solar flares emit vast amounts of radiation and atomic particles in short periods of time, often with little or no warning.

Space observations have discovered many new aspects of solar events that were hidden from ground based observatories. The hottest spots on the Sun shine primarily in ultraviolet and X-rays, rather than in visible light. Thus, only from space can we map the true structure of high temperature solar flares and determine their physical conditions. Space observatories have shown us the higher, hotter layers of the Sun's atmosphere that normally are invisible from the ground. Instruments on satellites revealed that in flares and other violent disturbances the Sun acts like an atomic accelerator, driving electrons and protons to velocities approaching the speed of light. At such high speeds, the particles emit the high-energy X-rays and gamma rays measured by our satellites. Sometimes they even induce nuclear reactions on the surface of the Sun.

Two aspects of our improved knowledge of the Sun deserve special attention. One is the role of magnetic flelds in determining virtually all aspects of the structure and be havior of the Sun's upper atmosphere. The other is the discovery of the solar wind, a stream of atomic particles that constantly evaporate from the Sun's atmosphere and are accelerated to speeds of hundreds of kilometers per second, escaping into space in all directions.