Chapter 15--Our Sun - Geological Sciences

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15 Our Star LEARNING GOALS 15.1 Why Does the Sun Shine? • What process creates energy in the Sun? • Why does the Sun’s size remain stable? • How did the Sun become hot enough for fusion in the first place? 15.2 Plunging to the Center of the Sun: An Imaginary Journey • What are the major layers of the Sun, from the center out? • What do we mean by the “surface” of the Sun? • What is the Sun made of? 15.3 The Cosmic Crucible • Why does fusion occur in the Sun’s core? • Why is energy produced in the Sun at such a steady rate? • Why was the Sun dimmer in the distant past? • How do we know what is happening inside the Sun? • What is the solar neutrino problem? Is it solved ? 15.4 From Core to Corona • How long ago did fusion generate the energy we now receive as sunlight? • How are sunspots, prominences, and flares related to magnetic fields? • What is surprising about the temperature of the chromosphere and corona, and how do we explain it? 15.5 Solar Weather and Climate • What is the sunspot cycle? • What effect does solar activity have on Earth and its inhabitants? 496
I say Live, Live, because of the Sun, The dream, the excitable gift. Anne Sexton (1928–1974) Astronomy today involves the study of the entire universe, but the root of the word astronomy comes from the Greek word for “star.” Although we have learned a lot about the universe up to this point in the book, only now do we turn our attention to the study of the stars, the namesakes of astronomy. When we think of stars, we usually think of the beautiful points of light visible on a clear night. The nearest and most easily studied star is visible only in the daytime—our Sun. Of course, the Sun is important to us in many more ways than as an object for astronomical study. The Sun is the source of virtually all light, heat, and energy reaching Earth, and life on Earth’s surface could not survive without it. In this chapter, we will study the Sun in some depth. We will learn how the Sun makes life possible on Earth. Equally important, we will study our Sun as a star so that in subsequent chapters we can more easily understand stars throughout the universe. 15.1 Why Does the Sun Shine? Ancient peoples recognized the vital role of the Sun in their lives. Some worshiped the Sun as a god, and others created elaborate mythologies to explain its daily rise and set. Only recently, however, have we learned how the Sun provides us with light and heat. Most ancient thinkers viewed the Sun as some type of fire, perhaps a lump of burning coal or wood. The Greek philosopher Anaxagoras (c. 500–428 B.C.) imagined the Sun to be a very hot, glowing rock about the size of the Greek peninsula of Peloponnesus (comparable in size to Massachusetts). Thus, he was one of the first people in history to believe that the heavens and Earth are made from the same types of materials. By the mid-1800s, the size and distance of the Sun were reasonably well known, and scientists seriously began to address the question of how the Sun shines. Two early ideas held either that the Sun was a cooling ember that had once been much hotter or that the Sun generated energy from some type of chemical burning similar to the burning of coal or wood. Simple calculations showed that a cooling or chemically burning Sun could shine for a few thousand years—an age that squared well with biblically based estimates of Earth’s age that were popular at the time. However, these ideas suffered from fatal flaws. If the Sun were a cooling ember, it would have been much hotter just a few hundred years earlier, making it too hot for civilization to have existed. Chemical burning was ruled out because it cannot generate enough energy to account for the rate of radiation observed from the Sun’s surface. A more plausible hypothesis of the late 1800s suggested that the Sun generates energy by contracting in size, a process called gravitational contraction.Ifthe Sun were shrinking, it would constantly be converting gravitational potential energy into thermal energy, thereby keeping the Sun hot. Because of its large mass, the Sun would need to contract only very slightly each year to maintain its temperature—so slightly that the contraction would be unnoticeable. Calculations showed that the Sun could shine for up to about 25 million years generating energy by gravitational contraction. However, geologists of the late 1800s had already established the age of Earth to be far older than 25 million years, leaving astronomers in an embarrassing position. Only after Einstein published his special theory of relativity, which included his discovery of E mc 2 ,did the true energy-generation mechanism of the Sun become clear. We now know that the Sun generates energy by nuclear fusion, a source so efficient that the Sun can shine for about 10 billion years. Because the Sun is only 4.6 billion years old today [Section 9.5],we expect it to keep shining for some 5 billion more years. According to our current model of solar-energy generation by nuclear fusion, the Sun maintains its size through a balance between two competing forces: gravity pulling inward and pressure pushing outward. This balance is called gravitational equilibrium (or hydrostatic equilibrium). It means that, at any point within the Sun, the weight of overlying material is supported by the underlying pressure. A stack of acrobats provides a simple example of this balance (Figure 15.1). The bottom person supports the weight of everybody above him, so the pressure on his body is very great. At each higher level, the overlying weight is less, so the pressure decreases. Gravitational equilibrium in the Sun means that the pressure increases with depth, making the Sun extremely hot and dense in its central core (Figure 15.2). THINK ABOUT IT Earth’s atmosphere is also in gravitational equilibrium, with the weight of upper layers supported by the pressure in lower layers. Use this idea to explain why the air gets thinner at higher altitudes. chapter 15 • Our Star 497
Page 1: PART V STELLAR ALCHEMY
Page 5 and 6: Table 15.1 Basic Properties of the
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Page 13 and 14: Figure 15.11 This tank of dry-clean
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Page 19 and 20: Figure 15.20 An X-ray image of the
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Chapter 15--Our Sun - Geological Sciences

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