Sun

I

Introduction

Sun, the star that, by the gravitational effects of its mass, dominates the solar system—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 except for that supported by deep-ocean hydrothermal vents, because all foods and fuels except for these are derived ultimately from plants using the energy of sunlight. See Photosynthesis; Solar Energy.

Because of its proximity to the Earth (average distance 149,597,870 km (92,960,116 mi), known as an astronomical unit, (AU), 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. Lying at very great distances from Earth, the stars in the night sky appear as unresolved point sources. Spectroscopic studies of distant stars of solar type allow astronomers to infer that these show similar patterns of behaviour to the Sun, including magnetic activity cycles and flares. It is believed that other stars have spots similar to sunspots.

II

History of Scientific Observation

For most of the time that human beings have been on the Earth, the Sun has been regarded as an object of special significance. Many ancient cultures worshipped the Sun, and many more recognized its significance in the cycle of life. Aside from its calendrical or positional importance in marking, for example, solstices, equinoxes, and eclipses (see Archaeoastronomy), the quantitative study of the Sun dates from the discovery of sunspots, while the study of its physical properties was not initiated until much later.

Chinese astronomers occasionally observed sunspots with the naked eye as early as 200 bc. But around 1611 Galileo and others, including the German Jesuit astronomer Christoph Scheiner (1575-1650), used the recently invented telescope to observe them systematically. This work marked the beginning of a new approach to studying the Sun. The Sun came to be viewed as a dynamic, evolving body, and its properties and variations could thus 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 colours. Although the spectrum of the Sun had been observed as early as 1666 by the English mathematician and scientist 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 dark, so-called Fraunhofer lines at certain wavelengths in the spectrum of the Sun were due to absorption of radiation by atoms of some of the same elements as are 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 they emitted. This was the beginning of astrophysics. The occurrence of a fairly regular cycle of sunspot activity was recognized around 1844 by the German amateur astronomer Heinrich Schwabe.

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, invented by George Ellery Hale, which allows observations to be made at isolated wavelengths such as those emitted by ionized hydrogen or ionized calcium; the Lyot coronagraph, which permits study of the solar corona by producing an artificial, instrumental “eclipse”; and the magnetograph, invented by the American astronomer Horace W. Babcock in 1948, which measures magnetic-field strength over the solar surface.

Early rocket experiments in the late 1940s demonstrated the advantages of lifting instruments such as coronagraphs above the Earth’s distorting atmosphere. The most effective observations in short ultraviolet and X-ray wavelengths, which cannot penetrate the atmosphere, have been made from satellites in orbit above the Earth. For example, NASA launched a series of Orbiting Solar Observatories between 1962 and 1975.

Great progress in observing and understanding violent solar phenomena at short wavelengths came with the manned Skylab mission in 1973-1974, which was equipped with a dedicated solar telescope. The Solar Maximum Mission satellite (Solar Max) launched in 1980 was used to make some very useful observations prior to instrument failure; following its recovery and repair by astronauts aboard the space shuttle Challenger in 1984, the satellite was used to follow activity around the 1986 solar minimum. The Japanese Yohkoh (“Sunbeam”) satellite launched in August 1991 extended the series of solar observations at short electromagnetic wavelengths, revealing a great deal about the dynamic nature of the corona during a three-year period of operation which coincided with very high activity. As part of the International Solar Terrestrial Physics programme, the SOHO (Solar and Heliospheric Observatory) satellite, launched in 1995, is stationed at a stable orbital point 1.5 million km (937,500 mi) sunwards of the Earth, to provide continuous monitoring.

Instruments aboard probes in interplanetary space have also been important in examining processes in the solar wind. Magnetometers and other equipment aboard the Pioneer and Voyager spacecraft have been invaluable in measuring the Sun’s sphere of influence. The Ulysses spacecraft, launched in 1990, was the first to take measurements of the solar wind at high latitudes. In August 2001, NASA launched the Genesis probe, which took up a high Earth orbit, outside the planet's magnetosphere, in order to collect samples of ions from the solar wind for return to Earth for analysis. The mission was intended to provide detailed information about the composition and properties of the solar wind, and an insight into the nature of the solar nebula from which the solar system formed. In September 2004 the sample return capsule from Genesis crashed in the Utah desert, causing much data to be lost. Two years later, in September 2006, the Japanese Hinode (“Sunrise”) satellite was launched to observe, in the optical, ultraviolet, and X-ray wavelengths, changes in the magnetic field at the Sun's surface and how these can produce solar flares. The satellite began returning data the following January. See Space Exploration.

III

Composition and Structure

The Sun has a diameter of 1,390,000 km (870,000 mi).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 per cent 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 per cent hydrogen, 27 per cent helium, and 2 per cent other, heavier elements). Near the centre of the Sun the temperature is almost 16 million K (about 29 million degrees F) and the density is 150 times that of water. Under these conditions the nuclei of individual hydrogen atoms interact, undergoing nuclear fusion (see Nuclear Energy). The net result of a series of such processes is that four hydrogen nuclei combine to make one helium nucleus, and energy is released in the form of gamma radiation. Vast numbers of nuclei react every second, generating energy 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 per cent of the Sun’s radius.

The energy thus produced is transported most of the way to the solar surface by radiation. Photons of light may take as long as 100,000 years to emerge from the core, undergoing a “random walk” outwards through the Sun’s dense interior. Nearer the surface, in the convection zone, occupying approximately the last third of the Sun’s radius, energy is transported by the turbulent mixing of the gases.

A

The Photosphere

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 convection cells in the photosphere give it an irregular, mottled appearance. This pattern is known as the solar granulation. Each granule is about 2,000 km (1,240 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 (18,600 mi) across. The photosphere has a temperature of almost 5770 K (9930° F).