Universe Eighth Edition Universe Roger A. Freedman William J. Kaufmann III CHAPTER 16 Our Star, the Sun CHAPTER 16 Our Star, the Sun

Stars in different stages of their evolution may generate energy using different nuclear reactions. These reactions can occur in the core or in a layer around the core. At the present, the energy of the Sun is generated A.in its central core by fission of heavy nuclei. B.from gravitational energy as the Sun slowly shrinks. C.in its core by radioactive decay of uranium. D.in the central core by fusion of helium nuclei and in an outer shell by fusion of hydrogen nuclei. E.in its central core by fusion of hydrogen nuclei. Q16.1

Stars in different stages of their evolution may generate energy using different nuclear reactions. These reactions can occur in the core or in a layer around the core. At the present, the energy of the Sun is generated A.in its central core by fission of heavy nuclei. B.from gravitational energy as the Sun slowly shrinks. C.in its core by radioactive decay of uranium. D.in the central core by fusion of helium nuclei and in an outer shell by fusion of hydrogen nuclei. E.in its central core by fusion of hydrogen nuclei. A16.1

The surface layers of the Sun are very massive. What stops the Sun from collapsing under its own weight? A.The strong nuclear repulsion between the atoms of these layers. B.Neutrinos exert a strong outward pressure, holding the layers up. C.The magnetic field exerts a strong force. D.The pressure of the very high-temperature gas within the Sun supports the outer layers. E.The interior of the Sun is under such high pressure that it is solid. Q16.7

The surface layers of the Sun are very massive. What stops the Sun from collapsing under its own weight? A.The strong nuclear repulsion between the atoms of these layers. B.Neutrinos exert a strong outward pressure, holding the layers up. C.The magnetic field exerts a strong force. D.The pressure of the very high-temperature gas within the Sun supports the outer layers. E.The interior of the Sun is under such high pressure that it is solid. A16.7

This photo shows solar granulation. The darker areas are regions where the gas is A.hotter. B.cooler. C.Doppler shifted. D.moving laterally. E.less dense. Q16.9

This photo shows solar granulation. The darker areas are regions where the gas is A.hotter. B.cooler. C.Doppler shifted. D.moving laterally. E.less dense. A16.9

The dark regions on this photo of the Sun are A.the corona. B.solar granules. C.Zeeman effects. D.sunspots. E.prominences. Q16.11

The dark regions on this photo of the Sun are A.the corona. B.solar granules. C.Zeeman effects. D.sunspots. E.prominences. A16.11

Stars in different stages of their evolution may generate energy using different nuclear reactions. These reactions can occur in the core or in a layer around the core. At the present, the energy of the Sun is generated A.in its central core by fission of heavy nuclei. B.from gravitational energy as the Sun slowly shrinks. C.in its core by radioactive decay of uranium. D.in the central core by fusion of helium nuclei and in an outer shell by fusion of hydrogen nuclei. E.in its central core by fusion of hydrogen nuclei. A16.1

The “fuel” of the Sun is ______, and the main products of the nuclear reactions include ______. A.hydrogen / helium, neutrinos, and gamma rays B.helium / only neutrinos and gamma rays C.hydrogen / neutrinos and microwaves D.helium / neutrinos and microwaves E.hydrogen / only neutrinos. Q16.2

Key Ideas Hydrogen Fusion in the Sun’s Core: The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus. Hydrogen Fusion in the Sun’s Core: The Sun’s energy is produced by hydrogen fusion, a sequence of thermonuclear reactions in which four hydrogen nuclei combine to produce a single helium nucleus. The energy released in a nuclear reaction corresponds to a slight reduction of mass according to Einstein’s equation E = mc 2. The energy released in a nuclear reaction corresponds to a slight reduction of mass according to Einstein’s equation E = mc 2. Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 10 7 K. In the Sun, fusion occurs only in the dense, hot core. Thermonuclear fusion occurs only at very high temperatures; for example, hydrogen fusion occurs only at temperatures in excess of about 10 7 K. In the Sun, fusion occurs only in the dense, hot core.

Key Ideas Models of the Sun’s Interior: A theoretical description of a star’s interior can be calculated using the laws of physics. Models of the Sun’s Interior: A theoretical description of a star’s interior can be calculated using the laws of physics. The standard model of the Sun suggests that hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius. The standard model of the Sun suggests that hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius. The core is surrounded by a radiative zone extending to about 0.71 solar radius. In this zone, energy travels outward through radiative diffusion. The core is surrounded by a radiative zone extending to about 0.71 solar radius. In this zone, energy travels outward through radiative diffusion. The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure. In this zone, energy travels outward primarily through convection. The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure. In this zone, energy travels outward primarily through convection.

Key Ideas Solar Neutrinos and Helioseismology: Conditions in the solar interior can be inferred from measurements of solar neutrinos and of solar vibrations. Solar Neutrinos and Helioseismology: Conditions in the solar interior can be inferred from measurements of solar neutrinos and of solar vibrations. Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected. Recent neutrino experiments explain why this is so. Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected. Recent neutrino experiments explain why this is so. Helioseismology is the study of how the Sun vibrates. These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun. Helioseismology is the study of how the Sun vibrates. These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun.

Key Ideas The Sun’s Atmosphere: The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona. Everything below the solar atmosphere is called the solar interior. The Sun’s Atmosphere: The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona. Everything below the solar atmosphere is called the solar interior. The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere. Its spectrum is similar to that of a blackbody at a temperature of 5800 K. Convection in the photosphere produces granules. The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere. Its spectrum is similar to that of a blackbody at a temperature of 5800 K. Convection in the photosphere produces granules.

Key Ideas Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere. Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules. Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere. Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules. The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density. The outermost layer of the solar atmosphere, the corona, is made of very high-temperature gases at extremely low density. Activity in the corona includes coronal mass ejections and coronal holes. The solar corona blends into the solar wind at great distances from the Sun. Activity in the corona includes coronal mass ejections and coronal holes. The solar corona blends into the solar wind at great distances from the Sun.

Key Ideas The Active Sun: The Sun’s surface features vary in an 11-year cycle. This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity. The Active Sun: The Sun’s surface features vary in an 11-year cycle. This is related to a 22-year cycle in which the surface magnetic field increases, decreases, and then increases again with the opposite polarity. Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field. The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next. Two such cycles make up the 22-year solar cycle. Sunspots are relatively cool regions produced by local concentrations of the Sun’s magnetic field. The average number of sunspots increases and decreases in a regular cycle of approximately 11 years, with reversed magnetic polarities from one 11-year cycle to the next. Two such cycles make up the 22-year solar cycle.

Key Ideas The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field. These changes are caused by convection and the Sun’s differential rotation. The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field. These changes are caused by convection and the Sun’s differential rotation. A solar flare is a brief eruption of hot, ionized gases from a sunspot group. A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona. A solar flare is a brief eruption of hot, ionized gases from a sunspot group. A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona.