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T*re SmHar $sn*erEmr The interior of the sun cannot be observed directly. For that reason, all it is based on information acquired from the energy it radiates and from theoretical studies. The source of the sun's energy was not discovered until the late 1930s. we know about w&nline r-Go \*__. s%nffi For: t.inks on nuclear fusion in the sun Visitl www. SciLinks.org Nuclear Fusion Deep in its interior, the sun produces energy by a process known as nuclear fusion. This nuclear reaction converts four hydrogen nuclei into the nucleus of a helium atom. Tremendous energy is released. W During nuclear fusion, energy is released because solne matter is actually converted to energyr as shown in Figure 18. How does this process work? Consider that four hydrogen atoms have a combined atomic mass of 4.032 atomic mass units (4 X 1.008) whereas the atomic mass of helium is 4.003 atomic mass units, or 0.029 less than the combined mass of the hydrogen. The tiny missing mass is emitted as energy according to Einstein's equation: E: Web Code: cin-7243 mc2 E equals energy, m equals mass) and c equals the speed of light. Because the speed of light is very great (300,000 km/s), the amount of energy released from even a small amount of mass is enormous. The conversion ofjust one pinhead's worth of hydrogen to heliurn generates more energy than burning thousands of tons of coal. Most of this energy is in the form of high-energy photons that work their way toward the solar surface. The photons are absorbed and reemitted iil i. i, {' j: ii-l ll. :ll 5ij $.' tt: Ii illl -t{ ii i4l many times until they reach a layer just below the photosphere. Here, convection currents help transport this energy to the solar surface, where it radiates through the transparent chromosphere and corona. Only a small percentage of the hydrogen ln the nuclear reaction is actually converted to energy. Nevertheless, the sun is consuming an estimated 600 million tons of hydrogen each second; about 4 million tons are converted to energy. As liydrogen is consumed, the product of this reaction-helium-forms the solar core, which continually grows in size. Figure 18 Nuclear Fusion During nuclear fusion, four hydrogen nuclei combine to form one helium nucleus.some matter is converted to energy. Hca,Jinrg What hoppens during the *f p f;heckp,eoint process of nuclear fusion? Sturlying the Sun 689 lIl eharme&erHst&es off Stmr"s A great deal is known about the universe beyond our solar system. This knowledge hinges on the fact that stars, and even gases in the "empty" space between stars, radiate energy in all directions into space. The key to understanding the universe is to collect this radiation and unravel the secrets it holds. Astronomers have devised many ways to do just that. We will begin by examining some prop- IF. lf erties;of stars, such as color, temperature, and mass. Stan Color and Temrperature Study the stars in Figure 2 and note their color. ffi Color is a clue to a star's temperature. \r/ery hot stars with surface temperatures above 30,000 K emit most of their energy in the fcrrm of sh,rrt-wavelength light and therefbre appear blue. Red stars are much cooler, anci most of their energy is emitted as longer-wavelengtl-r recl light. Stars with temperatures between 5000 and 6000 K appear yeilolv,like the sun. Figure 2 Stars of Orion This time-lapse photograph shows stars as streaks across the night sky as Earth rotates. The streaks ciearly show different star colors. Binary Stars emd Ste8[mr Mas$ In the early nineteenth century, astronomers discovered that many stars orbit each other. These pairs of stars, pulled toward binary stars" More than 50 percent of the stars in the universe may occllr in pairs or multiples. ffiBinary stars are used to determine the star property most diffieult to calculate-its mass. The mass of a body can be calculated if it is attached try gravity to a partner. 'Ihis is the case for any binary star system. As shown in Figure 3, birrary stars orbit each other around a cornmon point called the center of mass. For stars of equal mass, the center of mass iies exactly halfviay between them. If one star is more massive than its pilrtner, their common center will be closer to the more massive one, If the sizes of their orbits are known, each other by gr:avity, are calied the stars'masses can be cletermined. F?e:;lrJirn94 C&reckg.roinlt Whot is a binory star systemT ' Figure 3 Connmon Center of Mass iq For stars of equal mass. the center of mass lies in the middle. B A star twice as massive as its partner is twice as close to the center of mass. lt therefore has a smaller orbit than its less massive partner. Eeyontl Our Solar System 7Ol Wer&sspr&s mg-ffinxsswffi ffi Hmg r&m Early in the twentieth century, Einar Hertzsprung and Henry Russell independently developed a graph used to study stars. It is now called a Hertzsprr.urg-Russell diagran-r (H-R ciiagram). ffiA HertzsprungRussell diagram shgws the relationship between the absolute magnitude and temperature of stars. By studying H-R diagrams) we learn a great deal about the sizes, colors, and temperatures of stars. In the H-R diagram shown in Figure 5, notice that the stars are not unifornrly distributed, About 90 percent are main-sequence stars that fall along a band that runs from the upper-left corner to the lowerright corner of the diagram. As you can see, the hottest main-sequence stars are the brightest, and the coolest main-sequence stars are the din-rmest. Figure.5 Hertzsprung-Russell Diagram ln this idealized chart, stars are plotted according to temperature and absolute magnitude. 7O4 Chapter 25 The brightness of the rnain-sequence stars is also related to their irlass. The hottest blue stars are about 50 times more massive than the sun, while the coolest red stars are only 1/10 as massive. Therefore, on the H-R diagram, the main-sequence stars appear in decreasing order, from hotter, rnore massive biue stars to cooler, less massive red stars. Above and to the right of the main sequence in the H-R diagram lies a group of very bright stars called red giants. The size of these giants can be estimated by comparing them with stars of known size that have the same surface temperature. Objects witir equal surface temperatlues radiate the same amount of energy per unit area. Therefore, any difference in the brightness of two stars having the same surface temperature is due to their relative sizes. Some stars are so large that they are called supergiants. Betelgeuse, a bright red supergiant in the constellation Orion, has a radius about 800 times that of the sun. Stars in the lower-central part of the H-R diagrarn are much fainter than mainsequence stars of the salne temperature. Some probably are no bigger than Earth. This group is called white dwarfs, although not all are white.