Chapter 15--Our Sun - Geological Sciences

More documents

Recommendations

Info

solar wind photosphere corona chromosphere convection zone core radiation zone solar wind Figure 15.4 The basic structure of the Sun. Nuclear fusion in the solar core generates the Sun’s energy. Photons of light carry that energy through the radiation zone to the bottom of the convection zone. Rising plumes of hot gas then transport the energy through the convection zone to the photosphere, where it is radiated into space. The photosphere, at a temperature of roughly 6,000 K, is relatively cool compared to the layers that lie above it. The temperature of the chromosphere, which is directly above the photosphere, exceeds 10,000 K. The temperature of the corona, extending outward from the chromosphere, can reach 1 million degrees. Because the coronal gas is so hot, some of it escapes the Sun’s gravity, forming a solar wind that blows past Earth and out beyond Pluto. inside the Sun, and your spacecraft is tossed about by incredible turbulence. If you can hold steady long enough to see what is going on around you, you’ll notice spouts of hot gas rising upward, surrounded by cooler gas cascading down from above. You are in the convection zone,where energy generated in the solar core travels upward, transported by the rising of hot gas and falling of cool gas called convection [Section 10.2].With some quick thinking, you may realize that the photosphere above you is the top of the convection zone and that convection is the cause of the Sun’s seething, churning appearance. As you descend through the convection zone, the surrounding density and pressure increase substantially, along with the temperature. Soon you reach depths at which the Sun is far denser than water. Nevertheless, it is still a gas (more specifically, a plasma of positively charged ions and free electrons) because each particle moves independently of its neighbors [Section 4.3]. About a third of the way down to the center, the turbulence of the convection zone gives way to the calmer plasma of the radiation zone,where energy is carried outward primarily by photons of light. The temperature rises to almost 10 million K, and your spacecraft is bathed in X rays trillions of times more intense than the visible light at the solar surface. 500 part V • Stellar Alchemy
COMMON MISCONCEPTIONS The Sun Is Not on Fire fission fusion We are accustomed to saying that the Sun is “burning,” a way of speaking that conjures up images of a giant bonfire in the sky. However, the Sun does not burn in the same sense as a fire burns on Earth. Fires on Earth generate light through chemical changes that consume oxygen and produce a flame. The glow of the Sun has more in common with the glowing embers left over after the flames have burned out. Much like hot embers, the Sun’s surface shines with the visible thermal radiation produced by any object that is sufficiently hot [Section 6.4]. However, hot embers quickly stop glowing as they cool, while the Sun keeps shining because its surface is kept hot by the energy rising from the Sun’s core. Because this energy is generated by nuclear fusion, we sometimes say that it is the result of “nuclear burning”— a term that suggests nuclear changes in much the same way that “chemical burning” suggests chemical changes. Nevertheless, while it is reasonable to say that the Sun undergoes nuclear burning in its core, it is not accurate to speak of any kind of burning on the Sun’s surface, where light is produced primarily by thermal radiation. No real spacecraft could survive, but your imaginary one keeps plunging straight down to the solar core.There you finally find the source of the Sun’s energy: nuclear fusion transforming hydrogen into helium. At the Sun’s center, the temperature is about 15 million K, the density is more than 100 times that of water, and the pressure is 200 billion times that on the surface of Earth. The energy produced in the core today will take about a million years to reach the surface. With your journey complete, it’s time to turn around and head back home. We’ll continue this chapter by studying fusion in the solar core and then tracing the flow of the energy generated by fusion as it moves outward through the Sun. 15.3 The Cosmic Crucible The prospect of turning common metals like lead into gold enthralled those who pursued the medieval practice of alchemy. Sometimes they tried primitive scientific approaches, such as melting various ores together in a vessel called a crucible. Other times they tried magic. Their getrich-quick schemes never managed to work. Today we know that there is no easy way to turn other elements into gold, but it is possible to transmute one element or isotope into another. If a nucleus gains or loses protons, its atomic number changes and it becomes a different element. If it gains or Figure 15.5 Nuclear fission splits a nucleus into smaller nuclei (not usually of equal size), while nuclear fusion combines smaller nuclei into a larger nucleus. loses neutrons, its atomic mass changes and it becomes a different isotope [Section 4.3].The process of splitting a nucleus into two smaller nuclei is called nuclear fission.The process of combining nuclei to make a nucleus with a greater number of protons or neutrons is called nuclear fusion (Figure 15.5). Human-built nuclear power plants rely on nuclear fission of uranium or plutonium. The nuclear power plant at the center of the Sun relies on nuclear fusion, turning hydrogen into helium. Nuclear Fusion The 15 million K plasma in the solar core is like a “soup” of hot gas, with bare, positively charged atomic nuclei (and negatively charged electrons) whizzing about at extremely high speeds. At any one time, some of these nuclei are on high-speed collision courses with each other. In most cases, electromagnetic forces deflect the nuclei, preventing actual collisions, because positive charges repel one another. If nuclei collide with sufficient energy, however, they can stick together to form a heavier nucleus (Figure 15.6). Sticking positively charged nuclei together is not easy. The strong force,which binds protons and neutrons together in atomic nuclei, is the only force in nature that can At low speeds, electromagnetic repulsion prevents the collision of nuclei. At high speeds, nuclei come close enough for the strong force to bind them together. Figure 15.6 Positively charged nuclei can fuse only if a highspeed collision brings them close enough for the strong force to come into play. chapter 15 • Our Star 501
Page 1 and 2: PART V STELLAR ALCHEMY
Page 3 and 4: I say Live, Live, because of the Su
Page 5: Table 15.1 Basic Properties of the
Page 9 and 10: in the creation of two gamma-ray ph
Page 11 and 12: Figure 15.9 Vibrations on the surfa
Page 13 and 14: Figure 15.11 This tank of dry-clean
Page 15 and 16: a This schematic diagram shows how
Page 17 and 18: weaker magnetic field stronger magn
Page 19 and 20: Figure 15.20 An X-ray image of the
Page 21 and 22: e quator N N N magnetic field line
Page 23 and 24: THE BIG PICTURE Putting Chapter 15
Page 25 and 26: Problems 9. Gravitational Contracti

Chapter 15--Our Sun - Geological Sciences

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?