SUN, in Astronomy

intensely hot, self-luminous body of gases at the center of the solar system. Its gravitational attraction maintains the planets, comets, and other bodies of the solar system in their orbits.

General Characteristics of the Sun

The sun is actually a star of about medium size; it appears larger than the other stars because of its relative nearness to the earth. The earth's distance from the sun varies from 91,377,000 mi (147,053,000 km) at perihelion to 94,537,000 mi (152,138,000 km) at aphelion (see apsis). The mean distance is c.92,960,000 mi (149,591,000 km); this is taken as the astronomical unit (AU) of distance used for measuring distances within the solar system. The sun is approximately 865,400 mi (1,392,000 km) in diameter, and its volume is about 1,300,000 times that of the earth. Its mass is almost 700 times the total mass of all the bodies in the solar system and 332,000 times that of the earth. The sun's surface gravity is almost 28 times that of the earth; i.e., a body on the surface of the sun would weigh about 28 times its weight on earth. The density of the material composing the sun is about one fourth that of the earth; compared with water, the sun's average density is 1.41. At its center, the sun has a density of over 100 times that of water, a temperature of 10 to 20 million degrees Celsius, and a pressure of over 1 billion atmospheres.

Observations of sunspots and studies of the solar spectrum indicate that the sun rotates on its axis from east to west; because of its gaseous nature its rate of rotation varies somewhat with latitude, the speed being greatest (a period of almost 25 days) in the equatorial region and least at the poles (a period of about 35 days). The axis of the sun is inclined at an angle of about 7° to the plane of the ecliptic.

The bright surface of the sun is called the photosphere. Its temperature is about 6,000 degrees Celsius. The photosphere appears darker near the edge (limb) of the sun's disk because of greater absorption of light by the sun's atmosphere in this area; this phenomenon is called limb darkening. During an eclipse of the sun the chromosphere and the corona (the outer layers of the sun's atmosphere) are observed. Also of interest is the high-speed, tenuous extension of the corona known as the solar wind.

Production of Solar Energy

The vast and continual production of solar energy cannot be attributed merely to combustion, to the gradual cooling of a hot body, to the fall of meteorites into the sun, or to gradual shrinkage with transformation of potential energy into heat (a theory proposed by Helmholtz). The theory of relativity with its implication of the equivalence of mass and energy led to the assumption that energy stored in the atoms constituting the sun's gases is constantly being released by conversion of some of the masses of the atom's nuclei during nuclear transmutations (see nuclear energy). H. A. Bethe proposed a cycle of nuclear reactions known as the carbon cycle, or CNO bi-cycle, to account for the nuclear changes. In this cycle carbon acts much as a catalyst, while hydrogen is transformed by a series of reactions into helium and large amounts of high-energy gamma radiation are released. It is now thought that the so-called proton-proton process is a more important energy source; this process begins with the collision of two protons and ends with the production of helium, while gamma radiation is released throughout.

See nucleosynthesis; stellar evolution.

The Study of the Sun

By means of the spectroscope much has been learned about the composition of the sun. There are numerous dark lines of varying widths in the solar spectrum. These were first intensively studied by Joseph Fraunhofer and are commonly known by his name. From a study of the lines the chemical composition of the sun is determined on the basis of the discovery by Kirchhoff that the dark lines correspond in position to the bright lines characteristic of the spectra produced by elements in the laboratory. The darkness of the lines in the sun's spectrum is attributed to the presence of a slightly cooler layer of gases above the photosphere, known as the reversing layer, which absorbs selectively the light of the photosphere and thus causes dark lines instead of bright ones to be observed through the spectroscope. By comparison of the sun's spectrum with laboratory spectra of incandescent elements, most of the elements known on earth have been identified in the sun's atmosphere.

Beyond the red portion of the visible solar spectrum is the infrared spectrum; for the study of these heat rays S. P. Langley invented the bolometer, a highly sensitive electrical device for measuring temperature. Solar heat and energy are measured by an instrument called the pyrheliometer. Other instruments devised especially for the study of the sun are the coronagraph and the spectroheliograph. These instruments have revealed a number of interesting phenomena occurring during the periods of solar activity associated with sunspots, e.g., faculae, plages (flocculi), prominences, and flares.

Importance to Terrestrial Life

Without the heat and light of the sun, life as we know it could not exist on the earth. Since solar energy is used by green plants in the process of photosynthesis, the sun is the ultimate source of the energy stored both in food and fossil fuels. Solar heating sets up convection currents, and thus is the source of the energy of moving air. Falling rain also owes its energy to the sun because of the relation of solar radiation to the water cycle.

Bibliography

See K. Hufbauer, Exploring the Sun: Solar Science since Galileo (1993); R. Krippenhahn, Discovering the Secrets of the Sun (1994); K. J. H. Phillips, Guide to the Sun (1995); P. O. Taylor, Beginners Guide to the Sun (1996); S. T. Suess and B. T. Tsurutani, ed., From the Sun: Auroras, Magnetic Storms, Solar Flares, Cosmic Rays (1998).

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