2396_AWL_Bennett_Ch15/pt5 6/25/03 3:40 PM Page 495 PART V STELLAR ALCHEMY 2396_AWL_Bennett_Ch15/pt5 6/25/03 3:40 PM Page 496 15 Our Star LEARNING GOALS 15.1 Why Does the Sun Shine? • Why was the Sun dimmer in the distant past? • What process creates energy in the Sun? • How do we know what is happening inside the Sun? • Why does the Sun’s size remain stable? • What is the solar neutrino problem? Is it solved ? • How did the Sun become hot enough for fusion 15.4 From Core to Corona in the first place? • How long ago did fusion generate the energy we 15.2 Plunging to the Center of the Sun: now receive as sunlight? An Imaginary Journey • How are sunspots, prominences, and flares related • What are the major layers of the Sun, from the to magnetic fields? center out? • What is surprising about the temperature of the • What do we mean by the “surface” of the Sun? chromosphere and corona, and how do we explain it? • What is the Sun made of? 15.5 Solar Weather and Climate 15.3 The Cosmic Crucible • What is the sunspot cycle? • Why does fusion occur in the Sun’s core? • What effect does solar activity have on Earth and • Why is energy produced in the Sun at such its inhabitants? a steady rate? 496 2396_AWL_Bennett_Ch15/pt5 6/25/03 3:40 PM Page 497 I say Live, Live, because of the Sun, from some type of chemical burning similar to the burning The dream, the excitable gift. of coal or wood. Simple calculations showed that a cooling or chemically burning Sun could shine for a few thousand Anne Sexton (1928–1974) years—an age that squared well with biblically based esti- mates of Earth’s age that were popular at the time. However, these ideas suffered from fatal flaws. If the Sun were a cool- stronomy today involves the study of the ing ember, it would have been much hotter just a few hun- entire universe, but the root of the word dred years earlier, making it too hot for civilization to have existed. Chemical burning was ruled out because it cannot astronomy comes from the Greek word A generate enough energy to account for the rate of radiation for “star.” Although we have learned a lot about the observed from the Sun’s surface. universe up to this point in the book, only now do A more plausible hypothesis of the late 1800s sug- gested that the Sun generates energy by contracting in size, we turn our attention to the study of the stars, the a process called gravitational contraction.Ifthe Sun were namesakes of astronomy. shrinking, it would constantly be converting gravitational When we think of stars, we usually think of the potential energy into thermal energy, thereby keeping the Sun hot. Because of its large mass, the Sun would need to beautiful points of light visible on a clear night. The contract only very slightly each year to maintain its tem- nearest and most easily studied star is visible only perature—so slightly that the contraction would be un- in the daytime—our Sun. Of course, the Sun is im- noticeable. Calculations showed that the Sun could shine for up to about 25 million years generating energy by grav- portant to us in many more ways than as an object itational contraction. However, geologists of the late 1800s for astronomical study. The Sun is the source of virtu- had already established the age of Earth to be far older than ally all light, heat, and energy reaching Earth, and life 25 million years, leaving astronomers in an embarrassing position. on Earth’s surface could not survive without it. Only after Einstein published his special theory of In this chapter, we will study the Sun in some relativity, which included his discovery of E ϭ mc2,did depth. We will learn how the Sun makes life possible the true energy-generation mechanism of the Sun become clear. We now know that the Sun generates energy by nu- on Earth. Equally important, we will study our Sun as clear fusion, a source so efficient that the Sun can shine for a star so that in subsequent chapters we can more about 10 billion years. Because the Sun is only 4.6 billion easily understand stars throughout the universe. 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 gener- ation by nuclear fusion, the Sun maintains its size through a balance between two competing forces: gravity pulling 15.1 Why Does the Sun Shine? inward and pressure pushing outward. This balance is called Ancient peoples recognized the vital role of the Sun in their gravitational equilibrium (or hydrostatic equilibrium). lives. Some worshiped the Sun as a god, and others created It means that, at any point within the Sun, the weight of elaborate mythologies to explain its daily rise and set. Only overlying material is supported by the underlying pressure. recently, however, have we learned how the Sun provides us A stack of acrobats provides a simple example of this bal- with light and heat. ance (Figure 15.1). The bottom person supports the weight Most ancient thinkers viewed the Sun as some type of of everybody above him, so the pressure on his body is fire, perhaps a lump of burning coal or wood. The Greek very great. At each higher level, the overlying weight is less, philosopher Anaxagoras (c. 500–428 B.C.) imagined the Sun so the pressure decreases. Gravitational equilibrium in the to be a very hot, glowing rock about the size of the Greek Sun means that the pressure increases with depth, making peninsula of Peloponnesus (comparable in size to Massa- the Sun extremely hot and dense in its central core (Fig- chusetts). Thus, he was one of the first people in history to ure 15.2). 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 THINK ABOUT IT were reasonably well known, and scientists seriously began Earth’s atmosphere is also in gravitational equilibrium, with the to address the question of how the Sun shines. Two early weight of upper layers supported by the pressure in lower ideas held either that the Sun was a cooling ember that had layers. Use this idea to explain why the air gets thinner at higher once been much hotter or that the Sun generated energy altitudes. chapter 15 • Our Star 497 2396_AWL_Bennett_Ch15/pt5 6/25/03 3:40 PM Page 498 pressure gravity Figure 15.2 Gravitational equilibrium in the Sun: At each point inside, the pressure pushing outward balances the weight of the overlying layers. Figure 15.1 An acrobat stack is in gravitational equilibrium: The contraction made the Sun hot enough to sustain nuclear lowest person supports the most weight and feels the greatest fusion in its core. Ever since, energy liberated by fusion has pressure, and the overlying weight and underlying pressure decrease maintained the Sun’s gravitational equilibrium and kept the for those higher up. Sun shining steadily, supplying the light and heat that sus- tain life on Earth. Although the Sun today maintains its gravitational equilibrium with energy generated by nuclear fusion, the energy-generation mechanism of gravitational contraction was important in the distant past and will be important 15.2 Plunging to the again in the distant future. Our Sun was born from a col- Center of the Sun: lapsing cloud of interstellar gas. The contraction of the cloud released gravitational potential energy, raising the An Imaginary Journey interior temperature higher and higher—but not high In the rest of this chapter, we will discuss in detail how enough to halt the contraction. The cloud continued to the Sun produces energy and how that energy travels to shrink because thermal radiation from the cloud’s surface Earth. First, to get a “big picture” view of the Sun, let’s carried away much of the energy released by contraction, imagine you have a spaceship that can somehow with- even while the interior temperature was rising. When the stand the immense heat and pressure of the solar interior central temperature and density eventually reached the and take an imaginary journey from Earth to the center values necessary to sustain nuclear fusion, energy genera- of the Sun. tion in the Sun’s interior matched the energy lost from the surface in the form of radiation. With the onset of fusion, Approaching the Surface the Sun entered a long-lasting state of gravitational equilib- rium that has persisted for the last 4.6 billion years. As you begin your voyage from Earth, the Sun appears as a About 5 billion years from now, when the Sun finally whitish ball of glowing gas. With spectroscopy [Section 7.3], exhausts its nuclear fuel, the internal pressure will drop, and you verify that the Sun’s mass is 70% hydrogen and 28% gravitational contraction will begin once again. As we will see helium. Heavier elements make up the remaining 2%. later, some of the most important and spectacular processes The total power output of the Sun, called its luminos- in astronomy hinge on this ongoing “battle” between the ity, is an incredible 3.8 ϫ 1026 watts. That is, every second, crush of gravity and a star’s internal sources of pressure. the Sun radiates a total of 3.8 ϫ 1026 joules of energy into In summary, the answer to the question “Why does the space (recall that 1 watt ϭ 1 joule/s).