SUN

This time-lapse film shows the formation and dissolution of granules, updrafts of gas that form star around which the Earth and the other components of the solar system revolve. It is the dominant body of the system, constituting more than 99 percent of its entire mass. The Sun is a source of an enormous amount of energy, a portion of which provides the Earth with the light and heat necessary to support life. The Sun is a sphere of luminous gas 1,392,000 km (864,950 miles) in diameter. Its mass is 1.99 1033 grams, or about 330,000 times the mass of the Earth. The Sun generates energy by nuclear fusion reactions in its core at a rate of 3.86 1033 ergs per second. Although its core temperature is close to 15,000,000 K, the temperature of the surface of the Sun (the photosphere) is only about 5,800 K. This is average in terms of stellar temperatures, and the Sun is an average star in every respect. Just one of 100 billion stars in the Milky Way Galaxy, it is classified as a yellow dwarf of spectral class G2 and falls in the middle of the main sequence on the Hertzsprung-Russell diagram. The Sun's apparent magnitude is -26.5, but its absolute magnitudethe brightness it would appear to have at a standard distance of 10 parsecs (32.6 light-years)is a mere +4.6, which is near the limit of naked-eye visibility. The Sun appears extremely bright to the terrestrial observer only because it is the star nearest the Earth, lying at an average distance of 149,600,000 km (92,957,000 miles). The Sun is so massive that its constituent matter is strongly compressed by gravity. At the Sun's core, the compressed gas is at such a high temperature that nuclear-fusion reactions are triggered. The dominant energy-producing reaction at the core is the proton-proton chain. Under intense heat and pressure, protons (hydrogen nuclei) collide and combine one after the other to form stable helium nuclei. The helium nuclei are slightly less massive than the protons that combined to produce them, and this residual mass is released as energy. The Sun converts five million tons of matter into energy every second; this is a negligible proportion of its total mass. Energy is first released as gamma rays, but this form of electromagnetic radiation undergoes a considerable number of interactions with overlying material on its way to the photosphere. Several hundreds of thousands of years later, the degraded radiation emerges mainly as visible light and infrared radiation (heat). A by-product of the proton-proton reaction is neutrinos. Because these particles have neither mass nor electrical charge, they escape from the Sun at the speed of light. Recent experiments designed to detect solar neutrinos reaching the Earth, however, have found fewer than expected. This discrepancy may suggest that the Sun's core temperature is slightly cooler than 15 million K, that the exact mix of elements at the core is different from that inferred from the composition of its surface layers, or that the neutrinos interact with the solar mass and are converted into a different, undetectable form of neutrino. Solar radiation emerges at the photosphere. Detailed spectral studies reveal that the composition of this region is about 90 percent hydrogen, 9.9 percent helium, and a small admixture of heavy elements (e.g., iron, calcium, and sodium). This reflects the chemical makeup of the material from which the Sun was formed, though fusion reactions will have altered the mix in the interior. Observations of the photosphere show that the Sun rotates slowly. Because it is a gaseous body, however, the Sun spins at different rates at different latitudes, with the equatorial regions spinning fastest. One rotation takes 36 days at the poles, but only 25 days at the equator. On more detailed examination, the photosphere is found to be in constant motion, literally bubbling up and down as energy emerges in a network of 1,000-kilometre wide granulation cells. It has been recently established that the whole solar surface oscillates back and forth about 4 km every 2 hours 40 minutesnearly twice the predicted resonant period of vibration for the Sun. The appearance of the photosphere is continually changing, with an increase or decrease in the number of sunspots. These areas of the solar surface, which may measure as much as 50,000 km across, are places where strong local magnetic fields inhibit the normal convective motions of the photosphere. The gas inside a sunspot is about 1,500 K cooler than its surroundings, and so the spot appears dark against the Sun's disk. Magnetic activity extends into the Sun's atmosphere. Upward-moving jets of gas, called spicules, emerge from the chromosphere, the inner atmosphere that extends out to about 7,000 km above the solar surface. The spicules apparently follow the same loops of magnetic field that break through the photosphere to cause sunspots. In the corona, the outer atmosphere that constitutes the luminous envelope of the Sun, sudden changes in the local magnetic fields appear to result in the formation of prominences, especially eruptive onesflamelike protuberances of coronal matter in which constituent atoms and ions emit light upon capturing electrons. Another type of feature, called a loop prominence, is produced by matter ejected into the corona by solar flares, violent eruptions associated with rapidly evolving magnetic fields in photospheric sunspot regions. Such eruptions initially release streams of high-speed electrons and atomic nuclei, followed by a secondary emission of large amounts of ultraviolet, gamma, and X radiation. The flares increase the intensity of the solar wind (q.v.), a continuous outflow of charged particles from the corona. The solar wind moves through interplanetary space at speeds of 350 to 700 km per second, and it extends at least to the orbit of Neptune. The solar activity cycle, during which the number of sunspots, prominences, and flares increase from minimum to maximum before decreasing again, extends for a period of 11 years. Although this cycle has recurred regularly for many thousands of years, the Sun appears to have remained virtually unchanged. The Sun is not expected to undergo any dramatic changes for another 5 billion years, when it will expand into a red giant star as it approaches the later stages of its life (see star). star around which the Earth and the other components of the solar system revolve. It is the dominant body of the system, constituting more than 99 percent of its entire mass. The Sun is the source of an enormous amount of energy, a portion of which provides the Earth with the light and heat necessary to support life. The Sun is classified as a G2 V star, with G2 standing for the second hottest stars of the yellow G classof surface temperature about 5,800 kelvins (K)and the V representing a main sequence, or dwarf, star, the typical star for this temperature class. (G stars are so called because of the prominence of a band of atomic and molecular spectral lines that the German physicist Joseph von Fraunhofer designated G.) The Sun exists in the outer part of the Milky Way Galaxy and was formed from material that had been processed inside a supernova. The Sun is not, as is often said, a small star. Although it falls midway between the biggest and smallest stars of its type, there are so many dwarf stars that the Sun falls in the top 5 percent of stars in the neighbourhood that immediately surrounds it. Additional reading Popular works on the Sun include Herbert Friedman, Sun and Earth (1986); Ronald G. Giovanelli, Secrets of the Sun (1984); and Robert W. Noyes, The Sun, Our Star (1982). Karl Hufbauer, Exploring the Sun: Solar Science Since Galileo (1991), chronicles the history of developments in this field. Works of a more technical nature include Peter Foukal, Solar Astrophysics (1990); R.O. Pepin, J.A. Eddy, and R.B. Merrill, The Ancient Sun: Fossil Record in the Earth, Moon, and Meteorites (1980); Michael Stix, The Sun: An Introduction (1989), showing the many techniques and ideas utilized to study the Sun; Wasaburo Unno et al., Nonradial Oscillations of Stars (1979); Harold Zirin, Astrophysics of the Sun (1988); A.N. Cox, W.C. Livingston, and M.S. Matthews (eds.), Solar Interior and Atmosphere (1991); and papers from three Skylab Solar Workshops: Jack B. Zirker (ed.), Coronal Holes and High Speed Wind Streams (1977); Peter A. Sturrock, Solar Flares (1980); and Frank Q. Orrall, Solar Active Regions (1981). Harold Zirin