BOTTOM

Sun, star that, by the gravitational effects of its mass, dominates the planetary system that includes the earth. By the radiation of its electromagnetic energy, the sun furnishes directly or indirectly all of the energy supporting life on earth, because all foods and fuels are derived ultimately from plants using the energy of sunlight.

Because of its proximity to the earth, and because it is such a typical star, the sun is a unique resource for the study of stellar phenomena. No other star can be studied in such detail. The star closest to the sun is 4.3 light-years (4 × 1013 km/2.5 × 1013 mi) away. To observe features on its surface of comparable size to those that can be seen routinely on the sun would require a telescope almost 30 km (about 18.6 mi) in diameter. Such a telescope, moreover, would have to be put into space to avoid distortions caused by the earth's atmosphere.

History of Scientific Observation

In 1611 Galileo, using the recently invented telescope, discovered dark spots on the sun. (Chinese astronomers also reported sunspots as early as 200 BC.) Galileo's discovery marked the beginning of a new philosophical approach to studying the sun. The sun was finally viewed as a dynamic, evolving body, and its properties and variations were thus able to be understood scientifically.

The next major breakthrough in the study of the sun came in 1814 as the direct result of the use of the spectroscope by the German physicist Joseph von Fraunhofer. A spectroscope breaks up light into its component wavelengths, or colors. Although the spectrum of the sun had been observed as early as 1666 by the English mathematician and scientist Sir Isaac Newton, the accuracy and detail of Fraunhofer's work laid the foundation for the first attempts at a detailed theoretical explanation of the solar atmosphere.
Some of the radiation from the visible surface of the sun (called the photosphere) is absorbed by slightly cooler gas just above it. Only particular wavelengths of radiation are absorbed, however, depending on the atomic species present in the solar atmosphere. In 1859, the German physicist Gustav Kirchhoff first showed that the lack of radiation at certain wavelengths in the Fraunhofer spectrum of the sun was due to absorption of radiation by atoms of some of the same elements present on the earth. Not only did this show that the sun was composed of ordinary matter, but it also demonstrated the possibility of deriving detailed information about celestial objects by studying the light the objects emitted. This was the beginning of astrophysics.

Progress in understanding the sun has continued to be guided by scientists' ability to make new or improved observations. Among the advances in observational instruments that have significantly influenced solar physics are the spectroheliograph, which measures the spectrum of individual solar features; the coronagraph, which permits study of the solar corona without an eclipse; and the magnetograph, invented by the American astronomer Horace W. Babcock in 1948, which measures magnetic-field strength over the solar surface. The development of rockets and satellites has enabled scientists to observe radiation at wavelengths not transmitted through the earth's atmosphere. Among the instruments developed for use in space are coronagraphs as well as telescopes and spectrographs sensitive to extreme ultraviolet radiation and to X rays . Space instruments have revolutionized the study of the outer atmosphere of the sun.

The total amount of energy emitted by the sun in the form of radiation is remarkably constant, varying by no more than a few tenths of 1 percent over several days. This energy output is generated deep within the sun. Like most stars, the sun is made up primarily of hydrogen (specifically, 71 percent hydrogen, 27 percent helium, and 2 percent other, heavier elements). Near the center of the sun the temperature is almost 16,000,000 K (about 29,000,000° F) and the density 150 times that of water. Under these conditions the nuclei of individual hydrogen atoms interact, undergoing nuclear fusion. In this process two hydrogen nuclei combine to make one helium nucleus, and energy is released in the form of gamma radiation. This energy is equivalent to that which would be released from the explosion of 100 billion one-megaton hydrogen bombs per second. The nuclear "burning" of hydrogen in the core of the sun extends out to about 25 percent of the sun's radius.

The energy thus produced is transported most of the way to the solar surface by radiation. Nearer the surface, however, in the convection zone, covering approximately the last third of the sun's radius, energy is transported by the turbulent mixing of the gases. The photosphere is the top surface of the convection zone. Evidence of the turbulence of the convection zone can be seen by observing the photosphere and the atmosphere directly above it.

Turbulent cells in the photosphere give it an irregular, mottled appearance. This pattern, known as the solar granulation, is caused by turbulence in the upper levels of the convection zone. Each granule is about 2000 km (about 1240 mi) across. Although the pattern of granulation is always present, individual granules remain for only about 10 minutes. A much larger convection pattern is also present, caused by the turbulence that extends deep into the convection zone. This supergranulation pattern contains cells that last for about a day and average 30,000 km (about 18,600 mi) across.

The American astronomer George E. Hale discovered in 1908 that sunspots contain strong magnetic fields. A typical sunspot has a magnetic-field strength of 2500 gauss. For comparison, the earth's magnetic field has a strength of less than 1 gauss. Sunspots tend to occur in pairs, with the two spots having magnetic fields that point in opposite directions, one into and one out of the sun. The sunspot cycle, in which the number of sunspots varies from low to high and then low again over 11 years, has been known since at least the early 18th century. The intricate magnetic pattern associated with the solar cycle, however, was found only after the discovery of the sun's magnetic field.

Of sunspot pairs in the sun's northern hemisphere, the spot that leads its partner in the direction of rotation has a magnetic-field direction opposite to that of a leading sunspot in the southern hemisphere. As a new 11-year cycle begins, the magnetic-field direction of leading sunspots in each hemisphere reverses. Thus, the full solar cycle, including the magnetic-field polarity, takes approximately 22 years. In addition, the sunspots on the sun at any given time tend to occur at the same latitude in each hemisphere. This latitude moves from about 45° to about 5° during the sunspot cycle.

Because each sunspot exists for, at most, only a few months, the 22-year solar cycle reflects deep-seated and long-lasting processes in the sun and not just the properties of individual sunspots. Although not fully understood, the phenomena of the solar cycle appear to result from the interactions of the sun's magnetic field with the convection zone in the outer layers of the sun. These interactions, furthermore, are affected by the rotation of the sun, which is not the same at all latitudes. The sun rotates once every 27 days near the equator but once every 31 days nearer the poles.

Much of the sun's magnetic field lies outside of sunspots. The pervasiveness of the sun's magnetic field adds complexity, diversity, and beauty to the outer atmosphere of the sun. For example, the larger scale turbulence in the convection zone pushes much of the magnetic field at and just above the photosphere to the edges of the supergranulation cells. Radiation from the layer just above the photosphere, called the chromosphere, clearly shows the pattern. Within the supergranule boundaries, jets of material shoot into the chromosphere to an altitude of 4000 km (about 2500 mi) in 10 minutes. These so-called spicules are caused by the combination of turbulence and magnetic fields at the edges of the supergranule cells.

Near the sunspots, however, the chromospheric radiation is more uniform. These sites are called active regions, and the surrounding areas, which have smoothly distributed chromospheric emission, are called plages, after the French word for "beach." Active regions are the location of solar flares, explosions caused by the very rapid release of energy stored in the magnetic field (although the exact mechanism is not known). Among the phenomena that accompany flares are rearrangements of the magnetic field, intense X-radiation, radio waves, and the ejection of very energetic particles that sometimes reach the earth, disrupting radio communications and causing auroral displays .

The outer solar atmosphere, which extends for several solar radii from the disk of the sun, is the corona. All the structural details in the corona are due to the magnetic field. Most of the corona consists of great arches of hot gas: smaller arches within active regions and larger arches between active regions. The arched and sometimes looplike shapes are caused by the magnetic field.

In the 1940s the corona was discovered to be much hotter than the photosphere. The photosphere, or visible surface, of the sun has a temperature of almost 6000 K (10,800° F). The chromosphere, which extends for several thousand kilometers above the photosphere, has a temperature near 30,000 K (about 54,000° F). But the corona, which extends from just above the chromosphere far out into interplanetary space, has a temperature of over 1,000,000 K (1,800,000° F). In order to maintain this temperature, a direct input of energy to the corona is necessary.

Finding the mechanism by which this energy reaches the corona is one of the classic problems of astrophysics. It is still unsolved, although many mechanisms have been proposed. Because recent observations from space have shown the colona to be a collection of magnetic loops, how these loops are heated has become a major focus of astrophysical research.

The magnetic field can also trap cooler material above the sun's surface, although the cooler material cannot remain stable there for more than a few days. These phenomena can be seen during an eclipse as small regions, which are called prominences, at the very edge of the sun, like jewels in a crown. Frequently they subside, but occasionally they erupt, blowing solar material into space.

Within one or two solar radii from the surface of the sun, the coronal magnetic field is strong enough to trap the hot, gaseous coronal material in large loops. Farther away from the sun the magnetic field is weaker, and the coronal gas can literally push the magnetic field out into space. When this happens, material flows along the magnetic field for great distances in the solar system. The constant flow of material pushing out from the corona is called the solar wind, and it tends to come from regions called coronal holes. The gas there is cooler and less dense than the rest of the corona, resulting in less radiation. The solar wind from large coronal holes (which can last for several months) is unusually strong. Because of the solar rotation, these regions of strong solar wind, called high-speed solar wind streams, tend to recur every 27 days as seen from the earth. The solar wind causes disturbances that can be detected in the earth's magnetic field.

The sun's past and future have been inferred from theoretical models of stellar structure. During its first 50 million years, the sun contracted to approximately its present size. Gravitational energy released by the collapsing gas heated the interior, and when the core was hot enough, the contraction ceased and the nuclear burning of hydrogen into helium began in the core. The sun has been in this stage of its life for about 4.5 billion years.

Enough hydrogen is left in the sun's core to last another 4.5 billion years. When that fuel is exhausted the sun will change: As the outer layers expand to the orbit of the earth or beyond, the sun will become a red giant star, slightly cooler at the surface than at present and 10,000 times brighter. It will remain a red giant, with helium-burning nuclear reactions in the core, for only about half a billion years. The sun is not massive enough to go through successive cycles of nuclear burning or a cataclysmic explosion, as some stars do. After the red giant stage it will shrink to a white dwarf star, about the size of the earth, and slowly cool for several billion years.

Contributed by:
Randolph H. Levine

TOP