The Sun, the central body of the \Tsolar system\t and the closest \Tstar\t, is an immense sphere of glowing gas 1.39 million km (860,000 mi) in diameter at an average distance from the Earth of 149,591,000 km (92,960,000 mi). It is composed mainly of hydrogen, with about 5 percent by number of helium and heavier elements. Its mass of 1.99 X (10 to the power of 33) is sufficient for the mutual gravitational attraction of the molecules to prevent the hot solar gases from expanding rapidly into the relative vacuum of interstellar space. The Sun generates energy at the rate of 3.9 X (10 to the power of 33) ergs/sec by burning hydrogen to helium through nuclear \Tfusion\t reactions in its interior. This energy is radiated into space mostly in visible and infrared light and is largely responsible for the continuation of life on our planet. Compared with the largest known stars, with diameters a thousand times larger and masses several hundred times greater, the Sun merits its astronomical designation as a dwarf star. Its mass and radius are close to the average mass and size of all stars in the Galaxy, however, because many stars are even smaller and less massive than ours. The Sun's spectrum, surface temperature, and color lead to its further classification as a G2 dwarf in the scheme of spectral types used by astronomers. The spectral intensity of light radiated by its surface gases is a maximum at wavelengths near 5000 angstroms, thus giving sunlight its characteristic yellow color. Modern study of the Sun began in 1611 with Galileo's observations of sunspots and his discovery of solar rotation from their motions. The first approximately correct determination of the Sun's size and distance from the Earth was made in 1684, from data obtained by the French Academy from triangulation observations of Mars during its close approach to the Earth in 1672. The discovery of the solar absorption-line spectrum by Joseph von Fraunhofer in 1814, and its physical interpretation by Gustav Kirchhoff in 1859, opened the era of solar astrophysics, during which the effective study of the physical state and chemical composition of the solar material became possible. The strong magnetic fields of sunspots were detected by George Ellery Hale in 1908, and the role of nuclear fusion in producing solar energy was elucidated by Hans Bethe in 1939. Modern developments continue to change scientists' perception of the Sun. The solar wind was not detected directly until 1962, and the sources of its high-speed recurrent streams awaited the observations of coronal holes in 1969. STRUCTURE OF THE SUN From its innermost core to its corona, and to the solar wind that extends even to the Earth, the Sun has a structure typical of most stars of its kind. Inner Core The weight of the Sun's outer layers compresses the gas of the innermost region to a density about 100 times that of water and raises the central temperature to about 15 million K (27,000,000 deg F). Throughout the Sun's interior, atoms collide frequently and with enough energy to ionize the gas, which is then referred to as a plasma (see \Tplasma physics\t). In the inner third of the Sun the collisions among ions are energetic enough to cause nuclear reactions at a rate sufficient to liberate the energy required to give the Sun's observed luminosity. The specific set of reactions thought to be most effective in generating energy in the Sun involves the burning of hydrogen to helium, following the specific sequence of reactions known as the \Tproton-proton reaction\t. Present evidence suggests that the plasma of the central nuclear burning region of the Sun is not mixed with the material in the outer shells. Thus the proton-proton reaction will continue only until the hydrogen of the central region, some 10 percent of the Sun's mass, becomes transformed into helium after about 10 billion years. The Sun's age is estimated to be about 5 billion years (see \Tstellar evolution\t). The gamma rays and X rays emitted by the nuclear reactions travel outward with little absorption through the solar interior, because the electrons that allow an atom to absorb light have mostly been stripped from the nuclei by interatomic collisions. The Convection Zone and the Photosphere Nearer the Sun's visible surface, as the weight of overlying gas diminishes, the gas pressure and thus the density and temperature required to support this layer in hydrostatic equilibrium decrease rapidly. At a distance of about two-thirds the solar radius from the center, where the temperature has dropped to about 1 million K, the hydrogen and helium are no longer completely ionized. The neutral atoms absorb radiation moving outward from the central nuclear burning regions. In this region the heating and consequent expansion of parcels of the fluid cause them to rise because of their lower densities, and transport their heat upward. The net upward flux of heat carried by the resulting pattern of up- and down-flowing convection is the dominant mode of energy transport in the outer third of the Sun. Convection continues to be efficient in transporting heat until layers are reached where the density is so low that radiation from the hot up-flowing gas can escape directly into space. This layer is the visible surface of the Sun, known as the photosphere. Direct evidence for the size scales, velocities, and shapes of solar convective scales can be deduced from observations of convectionlike cellular motions at the photosphere. Small-scale cells called granules are about 1,000 km (620 mi) in diameter and are formed by hot up-flowing gases, surrounded by cooler down-flowing gases, moving about 1 km/sec (22,000 mph). Supergranules form a larger set of polygonal cells, of diameter roughly 30,000 km (18,600 mi), detected by their horizontal velocities of about 0.5 km/sec (1,100 mph). In addition to transporting heat, convective motions of the Sun's gases are also thought to have important consequences for solar rotation, solar magnetism, and for the structure of the Sun's outer layers above the photosphere. Convection may help to explain the observation that the gases of the solar photosphere do not rotate rigidly--the angular rate at the equator is some 50 percent higher than that at latitudes of plus or minus 75 degrees. Although a satisfactory theory of this basic solar property does not yet exist, models of the fluid mechanics of rotating, convecting shells indicate that such velocity differences might result from the forces exerted upon rising and falling convecting gases as the Sun rotates about its axis at the observed sidereal rate of about 25 days at the solar equator. The angular rotation rate also appears to increase inward, at least immediately below the photosphere, at a rate of 5 percent in the first 15,000 km (9,300 mi). The Sun's magnetic field, observed at the photosphere, does not have the basic north-south dipole symmetry observed in the terrestrial magnetic field at the Earth's surface. The solar field lines seem to be wound around the Sun's rotation axis and roughly follow lines of constant latitude, rather than longitude. This property is inferred from the observed alternation of magnetic polarity in bipolar sunspot groups. The magnetic dipole axes of such groups tend to be oriented east-west, and within a given hemisphere (above or below the solar equator) the western half of all dipoles is generally of the same magnetic polarity. The polarity of dipoles in the northern and southern hemispheres is opposite. This law of alternation of polarities is called the Hale-Nicholson law. The plasma of the solar convection zone is about as good a conductor as copper wire under room-temperature conditions. When a large volume of this material moves through a magnetic field, as in solar convection, it induces a large electric current that deforms the original field so as to displace it along with the motion. The mutual influence of magnetic fields and moving plasmas is known as \Tmagnetohydrodynamics\t (MHD). MHD studies show that the Sun's differential rotation will tend to stretch and pull out magnetic-field lines into the observed toroidal geometry. Near the photosphere the known temperature, the mean molecular weight, and the acceleration of solar gravity indicate that the density decreases hydrostatically at the rapid rate of a factor of ten roughly every 1,000 km (620 mi) radially outward. This rapid decrease explains the sharp edge or limb of the Sun, even when seen with telescopes, because the shell in which the gas passes from being opaque to transparent is less than 1,000 km (620 mi) thick, and subtends less than 1 arc second as viewed from the Earth. When looking at the center of the Sun's disk, it is possible to see deeper into the absorbing solar atmosphere than when looking toward the limb, where the line of sight is more nearly tangent to the photosphere. Because the temperature increases inward below the photosphere, the line of sight toward the center of the disk sees hotter, and thus brighter, layers. This phenomenon explains the prominent limb darkening seen in pictures of the photosphere. A spectrogram of the solar light shows a bright background continuum traversed by many dark absorption lines. The continuum radiation that is visible to the eye, roughly between 4000 A and 7000 A, is emitted when electrons released from the relatively easily ionized heavy elements are captured by neutral hydrogen atoms. The dark Fraunhofer lines, such as the H and K lines of ionized calcium, are formed when light of certain discrete wavelengths is preferentially scattered by the particular species of neutral atoms or ions that are abundant at the density and temperature of the photosphere. The light emerging through the photosphere at these wavelengths is changed in frequency by multiple scattering of the photons from atoms and rapidly moving electrons, and is emitted instead in the continuum. The Chromosphere Above the photosphere, the temperature drops to a minimum of about 4,500 K, and then, remarkably enough, begins to rise. During a few seconds around totality during a solar eclipse, a thin ring (annulus) about 10,000 km (6,200 mi) thick around the limb is seen shining with a reddish glow, leading to its designation as the chromosphere ("color sphere"). Upon examination with a telescope and spectrograph at high resolution, most of the chromospheric emission is seen to come from very fine jets of outward-moving gas called spicules, at a temperature of about 15,000 K and a density of some (10 to the power of 11) particles/cu cm. A spicule lasts some 5 to 10 minutes and is typically 6,000 km (3,700 mi) high and perhaps one-tenth as thick. The gases are moving outward at speeds of about 10 km/sec (22,000 mph). The Corona During a total solar eclipse, or with a \Tcoronagraph\t, the Sun's atmosphere can be seen extending to several solar radii beyond the photospheric limb as a faint glow, about one million times less bright than the disk. The height of the corona was for some time puzzling to scientists, because it seemed that the density should drop off so rapidly that no corona would be visible at distances of even a small fraction of solar radius above the limb. The explanation for this discrepancy came in 1940 when certain unidentified lines seen in the spectrum of the corona were demonstrated as arising from transitions in iron ionized up to 13 times, implying temperatures of several million K. Because a hot gas is expected to be compressed relatively less by the weight of overlying layers than a cool one, the high coronal temperature explained why the corona remains visible much farther above the limb than might be expected. The specific mechanism that heats the corona to such a high temperature is still unclear, and this question is the focus of much of modern solar research from satellites. The coronal gas close to the Sun is visible with the naked eye during eclipses because it scatters photospheric light from electrons in the plasma. The hot coronal plasma also emits its own ultraviolet and X-ray light when rapidly moving electrons collide with ions of the heavier elements. For instance, the lines of 9-times-ionized magnesium and 11-times-ionized silicon are prominent in the ultraviolet spectrum. The heating of the corona is not a matter of simple heat flow from the cooler photosphere, by either conduction, convection, or radiation, because such a heat flow would violate the second law of thermodynamics. Most likely, acoustic or other forms of waves generated by gas motions at the photosphere may carry energy into the coronal medium and dissipate it into heat, balancing the corona's losses. Another alternative is dissipation of electric currents in the highly conducting coronal plasma, in much the same way that joule heating raises the temperature of a common resistor. The Solar Wind Because the outward gradient of gas pressure in the hot corona is too high to be balanced by solar gravity, this outermost layer of the atmosphere expands into space at a steady rate. At the Earth's orbit, the outward velocity of this \Tsolar wind\t reaches between 300 and 700 km/sec (185 and 435 mi/sec). The density there, however, is only between 1 and 10 particles/cu cm, so that the mass flux is only about (10 to the power of -13) solar masses per year. Nevertheless the solar wind has observable effects on the upper atmosphere of the Earth; for instance, it is thought to be responsible for most of the auroras seen at high terrestrial latitudes. SOLAR ACTIVITY The intense magnetic fields produced in the solar interior influence the physical structure of the photosphere, chromosphere, and corona in a complex and time-varying way described collectively as solar activity. Sunspots, Faculae, and Flares The magnetic fields emerge at the visible layers as toroidal loops of magnetic flux up to 100,000 km (62,000 mi) in diameter. Their most obvious effect at the photosphere is to produce the dark \Tsunspots\t and bright faculae that constitute an active region. If, as believed, the intense radially directed fields inhibit convection, and thus reduce the efficiency of the dominant heat-transport process to the photosphere, the low temperature and relative darkness of sunspots would be explained. How intense fields can also produce a net facular brightening under similar circumstances is still unclear. An active region grows in horizontal extent as the loop emerges, from less than 5,000 km (3,100 mi) across, to more than 100,000 km (62,100 mi) within 10 days. During this period of rapid growth the probability of a spectacular eruption, called a \Tsolar flare\t, is highest. A large flare is marked by a rapid brightening within a few minutes of a considerable area of the active region by a factor of 5 to 10, as seen in chromospheric radiations such as the H alpha line of hydrogen. Only the very largest flares can be detected in integrated white light against the bright photosphere. The most violent and spectacular effects of the eruption, however, take place in the corona above. Here, a set of the magnetic loops above the spots and faculae may increase their brightness in X-ray and ultraviolet light by a factor of 100 or more, even more rapidly than the change seen in the chromosphere. Charged particles are accelerated to relativistic energies, and strong centimeter-wave emission is generally detected. Some flares also produce powerful meter-wave radio bursts, and large volumes of hot plasma, called sprays, are often ejected into space at speeds exceeding the escape velocity of 617 km/sec (380 mi/sec) from the solar gravitational field. The cataclysmic event decays more slowly, over a few hours, after liberating up to (10 to the power of 32) ergs of energy, by a mechanism that is not well understood and is at the center of current research. Sunspots generally last a few weeks, with the most persistent large spots surviving for 2 to 3 months. The faculae continue to mark an active region for somewhat longer. Eventually, it appears that the random motions of convection near the photosphere disassemble the magnetic flux loop and disperse it into smaller magnetic elements distributed over the solar surface. Away from the active regions, less extended fields of comparable intensity (1,000 to 2,000 gauss) are measured, but they are confined to a polygonal network that coincides with the edges of the supergranular convective cells mentioned above. Loops, Prominences, and Coronal Holes Above the photosphere the magnetic fields over an active region can be seen by their effect on the distribution of temperature and density in the chromosphere and corona. Here, prominent loop-shaped structures seen in X rays and ultraviolet light show how the field lines extend to 100,000 km (62,000 mi) or more above a spot, and then connect back to the photosphere, generally within the same active region. In other regions of the corona immense sheets of relatively cool (10,000 K as opposed to the 1 to 3 million K of the corona) condensed plasma, called prominences, are supported at heights up to 200,000 km (124,000 mi). In certain large areas, called coronal holes, the coronal emission is significantly depressed, indicating a low density of the million-degree plasma. Studies indicate that in these regions the field lines continue radially outward and do not form closed structures, as in loops or prominences. Models show that the hot corona can then flow out into interplanetary space more easily, leaving a deficit of coronal material. Such holes are particularly common at the north and south solar poles, where no active regions with closed fields are observed. Solar Activity Cycle Solar activity exhibits a cycle over a period of about 22 years. The most easily observed feature of the cycle is the approximately 11-year variation in the number of sunspots on the disk. At the beginning of a new cycle the first groups emerge at relatively high latitudes, between 35 and 40 degrees. Their magnetic polarity (orientation of the dipole axis in solar coordinates) is opposite to that of the last groups of the preceding cycle in that hemisphere. Thus two consecutive 11-year cycles of spot number are required to return to a given level of spot number and also of spot group polarity. This 22-year solar magnetic cycle seems to have been quite regular in the past 100 years and more. Historical evidence, however, indicates that between approximately 1640 and 1710 hardly any spots were visible at all, suggesting that the present range of solar activity cannot be taken for granted. Such long-term irregularities in solar activity are of practical interest, because the solar fluxes of charged particles and ultraviolet radiation are directly controlled by the level of activity through active regions, flares, and coronal holes. The solar variation in these emissions is known to affect the upper atmosphere and may have important influences on climate as well. Solar-terrestrial effects are under close study. RECENT DEVELOPMENTS The Sun still holds many mysteries. For example, the proton-proton reaction thought to be the source of most solar energy should also produce a certain number of the particles called \Lneutrino\ls, yet studies thus far have detected significantly fewer neutrinos than theory predicts. One radical suggestion is that the Sun produces fewer neutrinos than expected because it has an iron-plasma core that amounts to about 0.5 percent of its total mass. Other physicists have theorized that weakly interacting massive particles (WIMPs)--predicted by \Tgrand unification theories\t and sometimes invoked to account for the "missing matter" in the universe--might exist deep within the Sun and lower its temperature enough to explain the lack of neutrinos. Yet another proposal is that electron-type neutrinos in the Sun's core change on the way out into muon-type neutrinos unobservable by the detectors now in use. In the early 1960s, radial oscillations of the photosphere were detected that have since been explained as the resonant trapping of acoustic waves between certain layers of the convection zone of the Sun; the WIMP theory helps to explain some details of the oscillations, as well. Close studies of these oscillations are enabling scientists to probe the density, temperature, and velocity patterns of the invisible subphotospheric layers. Scientists have also observed that the Sun's diameter fluctuates by about 0.01 percent of the average diameter over a nearly 80-year cycle, and longer-period pulsations may be possible. Peter Foukal Bibliography: Bartusiak, Marcia, "Seeing into the Sun," \Tmosaic\t, Spring 1990; Foukal, Peter, Solar Astrophysics (1990); Stix, Michael, The Sun (1989); Time-Life Books Editors, The Sun (1990); Wentzel, D.L., The Restless Sun (1989); Whitmire, Daniel, and Reynolds, Ray, "The Fiery Fate of the Solar System," Astronomy, April 1990.