Sun
To early humans the Sun was an object of warmth and security. The ancients attributed the Sun to their most important god and accorded it the highest honors. Nothing was more important than the Sun, except the Earth, which was thought to be at the center of the universe and the celestial body around which everything revolved.
This concept lasted for thousands of years. It was not until the sixteenth century, when Polish astronomer Nicholas Copernicus placed the Sun at the center of the universe, that concepts began to change. Italian philosopher Giordano Bruno (1548-1600) thought the universe was infinite with a countless number of Suns in it. Around 1610, Galileo was probably the first to study the Sun with his telescope. (NOTE: Never look directly at the Sun. Permanent eye damage can result.) He discovered spots on the Sun that shifted position from day to day and used them to determine the Sun's rotation rate.
While it is impossible to visit the gaseous Sun and bring back a sample of it to study, much can be learned by examining its light. In the 1660s Isaac Newton passed sunlight through a prism and saw that it was composed of all the colors of the spectrum. So began the science of spectroscopy. One hundred fifty years later, William Hyde Wollaston discovered dark lines in the spectrum where the light was missing. Joseph Fraunhofer (1787-1826), observing the spectrum in even greater detail, counted 600 such lines. Their nature was a mystery until 1859, when Gustav Kirchhoff explained that the dark lines were caused by elements in the Sun that absorb those wavelengths of light. By determining which elements corresponded to those wavelengths, it would be possible to ascertain the composition of the Sun. In 1864, William Huggins identified the first elements, and today more than 60 elements have been identified in the solar spectrum.
About 75 percent of the Sun is hydrogen; helium makes up about 23 percent. The remaining 2 percent includes carbon, nitrogen, oxygen, and neon, among others.
Though the sun's composition was now known, how it produced its energy remained a mystery until 1938, when Hans Bethe worked out the details of nuclear fusion. Deep in the Sun's core, where the temperature is aboout 10 million degrees Celsius, nuclear fusion takes place. Four atoms of hydrogen fuse into one atom of helium. The helium atom contains slightly less mass than the original four hydrogen atoms because some mass has been converted into energy. This energy streams outward from the Sun's core, causing it to shine and maintaining its immense bulk against its own gravity. Arthur Eddington suggested that the tendency of the Sun's matter to contract under its own gravity was held in check by the outward pressure of radiation, with equilibrium established at each layer within the Sun.
When astronomers look at the Sun, they observe its photosphere, which is about 200 miles (320 km) deep and about 9,935° F (5,500° C). The photosphere has a mottled appearance called granulation. Granules are columns of hot gas rising from beneath the photosphere: from the convection zone (one-third into the interior of the Sun, 1,980,000° F [1,100,000° C] and the radiative zone (the middle third of the Sun's interior, 4,500,000° F [2,500,000 ° C]).
Sunspots are not actual "spots" on the sun's surface; they are regions that are much cooler than the surrounding area--which is why they look black by comparison--and are areas of strong magnetic fields. Associated with spots are bright regions called plages that have been observed for centuries; Christopher Scheiner (1575-1650) referred to them as faculae. These are hot regions where more energy is being emitted than in the surrounding area.
Above the photosphere is the chromosphere. Until the coronagraph was invented, this region of the Sun was only visible during a total solar eclipse. The chromosphere is 1,250 to 1,850 mi. (2,000 to 3,000 km) thick, and the temperature climbs throughout it to 180,000° F (100,000 ° C) at the upper level, or corona, which extends millions of miles above the chromosphere and reaches temperature of several million degrees Celsius. Continually escaping from the Sun is a thin solar wind of charged particles that flows throughout the entire solar system.
Associated with the magnetic regions in the Suns' atmosphere are a number of so-called "active" features or structures. Among the most impressive of these are the prominences, which look like toungues of flame rising above the Sun. Some prominences remain stable for several days and are tens of thousands of miles high. It is believed that prominences form from material in the corona that cools and flows downward along magnetic fields.
Solar flares, which occur mainly during periods of sunspot activity, may result from a sudden release of energy from magnetic fields in the corona. They produce an incredible amount of energy in only five to ten minutes. The largest flares generate enough energy to supply the United States for 100,000 years. Flares are primarily responsible for producing the aurora borealis in the Earth's atmosphere.
The Sun is believed to have formed from a great nebula 4.5 billion years ago. This "nebular hypothesis" was first proposed over 200 years ago by Immanuel Kant (1724-1804) and, independently, by Pierre Laplace. Gravitational collapse produced the heat that raised the temperature high enough for nuclear fusion to begin. Having formed, the sun stabilized and became a "main sequence " star, as defined by Ejnar Hertzsprung and Henry Norris Russell. The Sun is classified as a type G2 star on the Hertzsprung-Russell diagram. It is currently at middle-age and should last another 5 billion years or more. When nuclear fusion ceases in the core, the Sun will expand and become a red giant star, engulfing the Earth. When the Sun's last reserves of fuel are expended, its core will collapse and its outer layers will be ejected, forming a planetary nebula. Left behind will be a small, dense white dwarf star.
The Sun has been under intense observational scrutiny over the past few decades. Long-term gound-based programs have been carried out by the National Solar Observatory, with stations at Kitt Peak, near Tucson, Arizona, and at Sacramento Peak, near Alamogordo, New Mexico. In the early 1980s, a series of satellites was launched to monitor the total energy output of the Sun over a period of years. The Solar Maximum Mission and Earth Radiation Budget satellites were two of these long-lived missions, and they have demonstrated that the Sun is a remarkably stable star, with its total energy output varying by only a tenth of a percent of an entire solar cycle. Further long-term satellite missions have been launched or are in development, to ensure that no breaks occur in our monitoring of the total solar energy output, as well as to expand our observing capability to progressively broader regions of the spectrum.
The Ulysses spacecraft was launched on October 6, 1990, with the goal of observing the Sun from a never-before seen vantage point: over its poles. To achieve this, the spacecraft was launched from the space shuttle toward Jupiter, eventually flying by the giant planet in February 1992. Using a so-called "gravitational slingshot" from Jupiter, Ulysses' orbit was altered into one that brought it back over the solar poles in 1994 and 1995. By 1998, the spacecraft was headed back out toward Jupiter, and it is planned to carry out several of these large, looping passes over the solar poles before 2007. This curious orbital configuration is necessary because the space shuttle orbits in a plane near the ecliptic (the plane of Earth's orbit around the Sun), which is approximately aligned with the solar equator. It is impossible to launch a spacecraft directly into a polar orbit in this way; an external source of gravity is needed to sling the spacecraft into the desired orbit. Jupiter provides this catalyst for Ulysses .
On December 2, 1995, the Solar Heliospheric Observatory (SOHO) was launched into a unique orbit--it sits in the so-called first Lagrange point of the Earth's orbit, where the Sun's gravity, Earth's gravity, and the centrifugal force acting on the satellite all cancel one another out, allowing the spacecraft to precede Earth around the Sun. This is more advantageous than simply launching the satellite into orbit around Earth, for in the latter case the Earth would block the satellite's view of the Sun for an interval during every orbit. SOHO's instruments are designed to study every aspect of the Sun, from its core to its outermost atmosphere.
This is the complete article, containing 1,406 words
(approx. 5 pages at 300 words per page).
