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SUN
Although the sun is a rather ordinary star, it is very important to the inhabitants of the Earth. The sun is the source of virtually all of the Earth's energy. Yet the Earth receives only half a billionth of the energy that leaves the sun. Because the sun's energy is so intense, there are some real dangers in studying it. The intense heat of the sun's rays can destroy the retinal cells, causing blindness. For this reason, the sun should never be viewed directly. Furthermore, there is no safe way to view the sun through an ordinary telescope. Smoked glass and dark glasses give no protection from the great concentration of heat and light. The only safe way to study the sun is to project its image through a pinhole or a telescope onto a white screen.
The Sun's Position in the Universe
The average distance of the sun from the Earth, arbitrarily called one astronomical unit by astronomers, is 149,597,870 kilometers (92,958,350 miles). The sun's radius is about 432,500 miles, or 109.3 times the radius of the Earth, giving the sun a volume of about 1,306,000 times the volume of the Earth. It has been calculated that the sun's mass, or quantity of matter, is some 333,400 times as great as the Earth's mass.
A ray of light traveling from the sun at about 186,282 miles per second takes about 8 minutes 19 seconds to reach the Earth. Light from those other suns, the stars, takes much longer to reach the Earth. Light from the next nearest star, Alpha Centauri, takes more than four years to arrive, and light from the center of our galaxy, the Milky Way, takes many thousands of years. Because the sun is so near, it seems much larger than the other stars. They are visible on Earth only as bright points, even when viewed with the most powerful telescopes.
Stars vary greatly in size and in color. They range from giant stars, which are much larger than the sun, to dwarf stars, which can be much smaller than the sun. In color they range from whitish blue stars with very high surface temperatures (more than 30,000 Kelvin, or 53,500 F) to relatively cool red stars (less than 3,500 K, or 5,840 F). The sun is a yellow dwarf star, a kind that is common in the Milky Way, and has a surface temperature of about 5,800 K (10,000 F). (The Kelvin temperature scale uses degrees of the same size as Celsius, or centigrade, degrees, but it is numbered from absolute zero, which is -273.15 C.)
Studying the Sun
The telescope has been used in solar studies since 1610. With the telescope, scientists could describe the sun's appearance, watch the movement of sunspots, and measure the sun's rotation. The solar tower telescope, a special vertical telescope, was invented for use in solar studies. Its long focal length can give very large images of the sun (to more than 30 inches in diameter). The coronagraph, another special telescope, is used to examine the sun's atmosphere. The instrument blocks the direct light from the sun's disk and allows its dim outer atmosphere, called the corona, to be viewed.
When a ray of the sun's light, which appears white, is passed through a prism or a diffraction grating, it spreads out into a series of colors called a spectrum. Scientists analyze this spectrum to determine what chemicals make up the sun as well as their abundance, location, and physical states.
In 1814 Joseph von Fraunhofer began a thorough study of the sun's spectrum. He found that it was crossed by many dark lines, now called absorption lines or Fraunhofer lines. Meanwhile, other scientists had been studying the light emitted and absorbed by elements in the gaseous state when they were heated in the laboratory. They discovered that each element always produced a set of bright emission lines associated with that element alone. The dark solar line that Fraunhofer had called D was shown to have the same position in the spectrum as the brilliant line that sodium gave off when it was heated in the laboratory.
Scientists now believe that the dark bands represent elements in the sun's atmosphere. The lines are dark because the elements in the sun's atmosphere absorb the bright lines given off by the same element on the sun's disk.
Linking the lines of the spectrum with the elements that emit or absorb them provided a way to study the composition of the sun's surface. Almost all elements known on Earth have been shown to exist on the sun. Studies of the solar spectrum have revealed that hydrogen makes up some 92 percent of the sun's atmosphere and helium about 8 percent. Carbon, nitrogen, oxygen, sodium, and other elements are also present.
The spectroheliograph and the birefringent filter are also used to study the sun's atmosphere. Both of these instruments can limit the light that passes through them to a very small range of wavelengths, such as the red light emitted by hydrogen or the violet light of calcium.
In 1942 it was discovered that, besides the known kinds of solar radiation such as light and X rays, the sun also emitted radio waves. One cause of these radio waves is the thermal motion of atoms in the sun's atmosphere. Studies with radio telescopes have shown that radio waves are emitted by a sphere larger than the visible atmosphere of the sun, evidence that its atmosphere extends farther than can be seen.
On the ground, the effectiveness of any telescope is limited because the Earth's atmosphere absorbs much of the sun's radiation. With advances in space science, rockets and artificial satellites were launched above the terrestrial atmosphere. They were equipped with instruments that began to fill in the gaps in the solar spectrum.
The Sun's Violent Core
The sun looks like a burning sphere. In fact, it is often pictured as a circle with flames surrounding it. But the sun is actually too hot for an Earth-type chemical reaction like burning to occur on its surface. Besides, if burning produced its energy, it would have run out of fuel many millions of years ago.
Various theories have been advanced to explain the sun's tremendous energy output. One said that all the bits of matter in the sun were exerting gravitational attraction on each other and causing the sun to shrink and become more tightly packed. This process, called gravitational contraction, does occur in some stars and can release a great deal of energy. However, gravitational contraction could produce energy for only 50 million years at most, while the sun's age must be at least as great as the Earth's age of 4 1/2 billion years.
Atomic theory finally provided an explanation. Scientists now agree that thermonuclear reactions are the source of solar energy. Albert Einstein's theoretical calculations showed that a small amount of mass could be converted to a great amount of energy. The vast amount of matter in the sun could provide fuel for billions of years of atomic reactions. The sun's core is believed to be a superhot, extremely dense mass of atomic nuclei and electrons. Its temperature is calculated to be about 15,000,000 K (27,000,000 F). Under these conditions, nuclei can collide and fuse into new and heavier nuclei. This is a type of thermonuclear reaction called a fusion reaction. During such a reaction some of the mass of the nuclei changes to energy. Two specific processes, the carbon cycle and the proton-proton reaction, occur most often.
