21st CENTURY ASTRONOMY FOURTH EDITION Laura Kay, Stacy Palen, Brad Smith, and George Blumenthal FOURTH EDITION 21ST CENTURY ASTRONOMY F O U RT H E D ITIO N 21ST CENTURY ASTRONOMY LAURA KAY t Barnard College STACY PALEN t Weber State University BRAD SMITH t Santa Fe, New Mexico GEORGE BLUMENTHAL t University of California—Santa Cruz n W W NORTON & COMPANY ˨̑˨ W W Norton & Company has been independent since its founding in 1923, when William Warder Norton and Mary D Herter Norton first published lectures delivered at the People’s Institute, the adult education division of New York City’s Cooper Union The firm soon expanded its program beyond the Institute, publishing books by celebrated academics from America and abroad By midcentury, the two major pillars of Norton’s publishing program—trade books and college texts—were firmly established In the 1950s, the Norton family transferred control of the company to its employees, and today—with a staff of 400 and a comparable number of trade, college, and professional titles published each year—W W Norton & Company stands as the largest and oldest publishing house owned wholly by its employees Copyright © 2013 by Laura Kay, Stacy Palen, Bradford Smith, and George Blumenthal Copyright © 2010 by Jeff Hester, Bradford Smith, George Blumenthal, Laura Kay, and Howard G Voss Copyright © 2007 by Jeff Hester, David Burstein, George Blumenthal, Ronald Greeley, Bradford Smith, and Howard G Voss Copyright © 2002 by Jeff Hester, David Burstein, George Blumenthal, Ronald Greeley, Bradford Smith, Howard G Voss, and Gary Wegner Since this page cannot accommodate all the copyright notices, the Credits pages, starting on p C1, constitute an extension of the copyright page All rights reserved Printed in the United States of America Fourth Edition Editor: Erik Fahlgren Marketing Manager: Stacy Loyal Editorial Assistant: Renee Cotton Managing Editor, College: Marian Johnson Associate Managing Editor, College: Kim Yi Copy Editor: Stephanie Hiebert Developmental Editor: Erin Mulligan Science Media Editor: Rob Bellinger Assistant Editor, Supplements: Jennifer Barnhardt Production Manager: Eric Pier-Hocking Art Director and Designer: Rubina Yeh Photo Researchers: Stephanie Romeo, Rona Tuccillo Permissions Manager: Megan Jackson, Bethany Salminen Compositor: Precision Graphics Manufacturing: Transcontinental ISBN: 978-0-393-91878-6 (pbk.) W W Norton & Company, Inc., 500 Fifth Avenue, New York, N.Y 10110 www.wwnorton.com W W Norton & Company Ltd., Castle House, 75/76 Wells Street, London W1T 3QT 1 2 3 4 5 6 7 8 9 0 Laura Kay thanks her partner, M.P.M She dedicates this book to her late uncle, Lee Jacobi, for an early introduction to physics, and to her late colleagues at Barnard College, Tally Kampen and Sally Chapman Stacy Palen thanks the wonderful colleagues in her department and the crowd at Bellwether Farm, all of whom made room for this project in their lives, even though it wasn’t their project Brad Smith dedicates this book to his patient and understanding wife, Diane McGregor George Blumenthal gratefully thanks his wife, Kelly Weisberg, and his children, Aaron and Sarah Blumenthal, for their support during this project He also wants to thank Professor Robert Greenler for stimulating his interest in all things related to physics Brief Contents Part I Introduction to Astronomy Chapter 1 Why Learn Astronomy? Chapter 2 Patterns in the Sky—Motions of Earth Chapter 3 Motion of Astronomical Bodies Chapter 4 Gravity and Orbits Chapter 5 Light 25 63 89 117 Chapter 6 The Tools of the Astronomer 151 Part II The Solar System Chapter 7 The Birth and Evolution of Planetary Systems 185 Chapter 8 The Terrestrial Planets and Earth’s Moon 215 Chapter 9 Atmospheres of the Terrestrial Planets 257 Chapter 10 Worlds of Gas and Liquid—The Giant Planets 291 Chapter 11 Planetary Adornments—Moons and Rings 323 Chapter 12 Dwarf Planets and Small Solar System Bodies 359 Part III Stars and Stellar Evolution Chapter 13 Taking the Measure of Stars 395 Chapter 14 Our Star—The Sun 427 Chapter 15 Star Formation and the Interstellar Medium Chapter 16 Evolution of Low-Mass Stars 491 Chapter 17 Evolution of High-Mass Stars 521 Chapter 18 Relativity and Black Holes 551 Part IV Galaxies, the Universe, and Cosmology Chapter 19 The Expanding Universe Chapter 20 Galaxies 581 611 Chapter 21 The Milky Way—A Normal Spiral Galaxy Chapter 22 Modern Cosmology 641 667 Chapter 23 Large-Scale Structure in the Universe 695 Chapter 24 Life 723 vi 459 434 ˃ ˆ ˥ Our Star—The Sun Hotter regions are more crowded with photons than cooler regions… Hotter region Cooler region making it more likely that photons will move from hotter to cooler regions than in the reverse direction As a result, radiation Net energy transfer carries energy outward from the hot, energy-generating core of the Sun ʹʼ ˰ ʽ Higher-temperature regions deep within the Sun produce more radiation than lowertemperature regions farther out Although radiation flows in both directions, more radiation flows from the hotter regions to the cooler regions than from the cool regions to the hot regions Therefore, radiation carries energy outward from the inner parts of the Sun move by chance from the hotter (more crowded) region to the cooler (less crowded) region than in the reverse direction There is a net transfer of photons and photon Radiation carries energy energy from the hotter from hotter regions to region to the cooler region, cooler regions and in this way radiative transfer carries energy outward from the Sun’s core If temperature varies by a large amount over a short distance, then the concentration of photons varies sharply as well This difference favors rapid radiative energy transfer The transfer of energy from one point to another by radiation also depends on how freely radiation can move Opacity impedes the from one point to another outward flow of radiation within a star The degree to which matter impedes the flow of photons is called opacity The opacity of a material depends on many things, including the density of the material, its composition, its temperature, and the wavelength of the photons moving through it Radiative transfer is most efficient in regions where opacity is low In the inner part of the Sun, where temperatures are high and atoms are ionized, opacity comes mostly from the interaction between photons and free electrons (electrons not attached to any atom) Here opacity is relatively low, and radiation readily carries the energy produced in the core outward through the star The region in which radiative transfer is responsible for energy transport extends 71 percent of the radius toward the surface of the Sun from the core This region is the Sun’s radiative zone (Figure 14.6) Even though the opacity of the radiative zone is relatively low, photons travel only a short distance before being absorbed, emitted, or deflected by matter, much like a beach ball being batted …is carried outward Convective zone first by radiation… Energy produced in the Sun’s core… Core Radiative zone …then by convection… …then away from the Sun by radiation Energy radiated from the Sun’s surface ʹʼ ˰ ʾ The interior structure of the Sun is divided into zones based on where energy is produced and how it is transported outward ˃ˆ˰˄˥The Sun Is Powered by Nuclear Fusion ˆ˅ˇ about by a crowd of people (Figure 14.7) Each interaction sends the photon in an unpredictable direction—not necessarily toward the surface of the star The path that a photon follows is so convoluted that, on average, it takes the energy of a gamma-ray photon produced in the interior of the Sun about 100,000 years to find its way to the outer layers of the Sun Opacity serves as a blanket, holding energy in the interior of the Sun and letting it seep away only slowly From a peak of 15 million K at the center of the Sun, the temperature falls to about 100,000 K at the outer margin of the radiative zone At this temperature, atoms are no longer completely ionized, so there are fewer free electrons and the opacity is therefore higher As the opacity increases, radiation becomes less efficient at carrying energy from one place to another The energy that is flowing outward through the Sun “piles up.” The physical sign that energy is accumulating is that the temperature gradient —how rapidly the temperature drops with increasing distance from the center of the Sun—becomes very steep Radiative transfer carries energy from hotter regions to cooler regions, smoothing out temperature differences between them As the opacity increases, radiation becomes less effective in smoothing out temperature differences, so temperature differences between one region and another become greater Nearer the surface of the Sun, radiative transfer becomes so inefficient (and the temperature gradient so steep) that a different way of transporting energy takes over: convection As in a hot-air balloon, cells (or packets) of hot gas become buoyant and rise up through the lowertemperature gas above them, carrying energy with them Just as convection carries energy from the interior In the outer part of the of a planet to its surface, Sun, energy is carried by or from the Sun-heated convection surface of Earth upward through Earth’s atmosphere, it also plays an important role in the transport of energy outward from the interiors of many stars, including the Sun The solar convective zone (see Figure 14.6) extends from the outer boundary of the radiative zone outward to just below the visible surface of the Sun In the outermost layers of stars, radiation again takes over as the primary mode of energy transport Energy from the outermost layers of a star is transported into space through radiation Even so, the effects of convection can be seen as a perpetual roiling of the visible surface of the Sun What If the Sun Were Different? As noted earlier, a key point to take into account when calculating a model of the interior of the Sun is balance (a) (b) VISUAL ANALOGY ʹʼ ˰ʿ (a) When a crowd of people plays with a beach ball, the ball never travels very far before someone hits it, turning it in another direction The ball moves randomly, sometimes toward the front of the crowd, sometimes toward the back It often takes a ball a long time to make its way from one edge of the crowd to the other (b) Similarly, when a photon travels through the Sun, it never travels very far before it interacts with an atom The photon moves randomly, sometimes toward the center of the Sun, sometimes toward the outer edge It takes a long time for a photon to make its way out of the Sun The temperature and density at each point within the model Sun must be just right so that transport of energy away from the core by radiation and convection just balances the amount of energy produced by fusion in the core The density, temperature, and pressure of the model Sun must vary from point to point in such a way that the outward push of pressure is everywhere balanced by the inward pull of gravity Finally, the whole model must depend on only two things: the total mass of gas from which the star is made, and the chemical composition of that gas What if a hypothetical star had the same mass, surface temperature, and composition as the Sun, but was some- 436 ˃ ˆ ˥ Our Star—The Sun how larger than the Sun? What would happen to the balance between the amount of energy generated within this hypothetical star and the amount of energy that it radiates away into space? Follow along in Figure 14.8 as we consider what would happen if the Sun were “too large.” Because this hypothetical star would have more surface area than the Sun, it would be able to more effectively radiate its energy into space For a 1-solar-mass star to have a larger size than the Sun, it would have to be more luminous than the Sun Now let’s consider what is going on in the interior of this hypothetical star Because the star is larger than the Sun but contains the same amount of mass as the Sun, the force of gravity at any point within the hypothetical star would be less than the force of gravity at the corresponding location within the Sun (This difference is a result of the inverse square law of gravitation: if the radius R is larger in the hypothetical star, then 1/R must be smaller.) With weaker gravity, the weight of matter pushing down on the interior of the hypothetical star would be less than in the Sun Because hydrostatic equilibrium means that the pressure at any point within a star is equal to the weight of overlying matter, the pressure at any point in the interior of this hypothetical star would be less than the pressure at the corresponding point in the Sun This reduction in pressure would affect the amount of energy the star produced The proton-proton chain runs faster at higher temperatures and densities, so the lower pressure in the interior of the hypothetical star means that less energy would be generated there than in the core of the Sun This hypothetical star would have to be more luminous than the Sun, but at the same time it would be producing less energy in its interior than the Sun does This discrepancy violates the balance that must exist in any stable star between the amount of energy generated within the star and the amount of energy radiated into space The hypothetical star cannot exist! Stated another way, even if the Sun were pumped up to a size larger than it actually is, it would not remain that way Less energy would be generated in its core, while more energy would be radiated away at its surface The Sun would be out of balance As a result, the Sun would lose energy, the pressure in the interior of the Sun would decline, and the Sun would shrink back toward its original (true) size The same thought experiment could be done the other way around, asking what would happen if the Sun were smaller than it actually is With less surface area, it would ʹʼ ˰ ˀ A star like the Sun can have only the structure that the Sun has Here we imagine the fate of a Sun with too large a radius Imagine a hypothetical Sun that has a mass of M but is larger than the Sun £ True Hypothetical Sun “larger” Sun With more surface area to radiate, the larger Sun would be more luminous than the true Sun… Fgrav Fgrav F = GMm r2 P P Hydrostatic equilibrium …but gravity would be weaker inside the larger Sun Weaker gravity means that the pressure in the interior of the larger Sun would be lower Lower pressure means lower temperature and less energy generated in the larger Sun than in the actual Sun The larger Sun would not produce enough energy to replace the energy that was radiated away As the out-of-balance hypothetical Sun lost energy, it would shrink until the energy produced again balanced the energy radiated away ˃ˆ˰˅˥The Interior of the Sunˆ˅ˉ radiate less energy At the same time the Sun’s mass would be compacted into a smaller volume, driving up the strength of gravity and therefore the pressure in its interior Higher pressure implies higher density and temperature, which in turn would cause the proton-proton chain to run faster, increasing the rate of energy generation Again there would be a contradiction—an imbalance This time, with more energy being generated in the interior than was being radiated away from the surface, pressure in the Sun would build up, causing it to expand toward its original (true) size 14.3 The Interior of the Sun The standard model of the Sun correctly matches observed global properties of the Sun such as its size, temperature, and luminosity This is a remarkable feat, but the model predicts much more than these properties In particular, the standard model of the Sun predicts exactly which Neutrinos escape freely nuclear reactions should from the core of the Sun be occurring in the core of the Sun, and at what rate The nuclear reactions that make up the proton-proton chain produce a vast number of neutrinos Since neutrinos barely interact with other ordinary matter, almost all of the neutrinos produced in the heart of the Sun travel freely through the outer parts of the Sun and on into space as if the outer layers of the Sun were not there The core of the Sun lies buried beneath 700,000 km of dense, hot matter, yet the Sun is transparent to neutrinos It takes thermal energy produced in the heart of the Sun 100,000 years to find its way to the Sun’s surface, but the solar neutrinos streaming through you as you read these words were produced by nuclear reactions in the heart of the Sun only 81/3 minutes ago (This is how far away the Sun is in light-minutes Neutrinos travel very nearly at the speed of light.) In principle, neutrinos offer a direct window into the very heart of the Sun’s nuclear furnace Astronomers Use Neutrinos to Observe the Heart of the Sun Transforming the promise of neutrino astronomy into reality is a formidable technical challenge The same property of neutrinos that makes them so exciting to astronomers— the fact that their interaction with matter is so feeble that they can escape unscathed from the interior of the Sun— also makes them notoriously difficult to observe Suppose astronomers wanted to build a neutrino detector capable of stopping half of the neutrinos falling on it This hypotheti- cal detector would need the stopping power of a piece of lead a light-year thick Yet despite the difficulties, neutrinos offer a unique window into the Sun so powerful that they are worth going to great lengths to try to detect Fortunately, the Sun produces an enormous number of neutrinos As you lie in bed at night, about 400 trillion solar neutrinos pass through your body each second, having already passed through Earth Because there are so many neutrinos about, a neutrino detector does not have to be very efficient to be useful Several methods have been devised to measure neutrinos from the Sun and from other astronomical sources, and a number of such experiments are under way These experiments have successfully detected neutrinos from the Sun, and in so doing they have provided crucial confirmation that nuclear fusion reactions are responsible for powering the Sun As with many good experiments, however, measurements of solar neutrinos raised new questions while answering others After their initial joy at confirming that the Sun really is a nuclear furnace, astronomers became troubled that there seemed to be only about a third to a half as many solar neutrinos as predicted by solar models The difference between the predicted and measured flux of solar neutrinos was referred to as the solar neutrino problem (Process of Science Figure) One possible explanation of the solar neutrino problem was that the working model of the structure of the Sun was somehow wrong This possibility seemed unlikely, however, because of the many other successes of the solar model A second possibility was that an understanding of the neutrino itself was incomplete The neutrino was long thought to have zero mass (like photons) and to travel at the speed of light But if neutrinos actually have a tiny amount of mass, then theories from particle physics predict that solar neutrinos should oscillate (alternate back and forth) among three different kinds, or “flavors”—the electron, muon, and tau neutrinos (Figure 14.9) Only one of these types, the electron neutrino, could interact with the atoms in the earlier neutrino detectors (described in Connections 14.2 on page 440), so neutrino oscillations provided a convenient explanation for why only about a third of the expected number of neutrinos were detected And, as seen in Figure 14.9b, electron neutrinos should also change flavor as they interact with solar material during their escape from the Sun After several decades of work on the solar neutrino problem, this last idea won out Work currently under way at high-energy physics labs, nuclear reactors, and neutrino telescopes around the world is showing that neutrinos have a nonzero mass This work has also uncovered evidence of neutrino oscillations Solving the solar neutrino problem is a good example of how science works—how a better model of the neutrino 438 ˃ ˆ ˥ Our Star—The Sun Neutrino oscillation can change an electron neutrino νe into a muon neutrino νμ in two ways (a) νe An electron neutrino νμ νe It begins to change is emitted in space νe νμ into a muon neutrino (b) νe νμ It is now a A measurement here has a The muon neutrino changes 50-50 chance of finding an electron or muon neutrino back to an electron neutrino muon neutrino An electron neutrino is emitted in the Sun’s core At a particular matter density, it converts to a muon neutrino and stays in that form νe νμ νμ νμ ʹʼ ˰ ˁ If neutrinos have mass, they should oscillate among the three types: electron, muon, and tau (a) A neutrino oscillates here between electron and muon types (b) Changes from one type to another can also take place in the presence of matter at a certain density found within the Sun An electron neutrino created in the Sun’s core is converted to a muon neutrino, which then arrives at Earth Early neutrino detectors would not have recognized this muon neutrino showed that the solar neutrino problem was real and not merely an experimental mistake, and how a single set of anomalous observations was later confirmed by other, more sophisticated experiments All of this effort has led to a better understanding of basic physics Helioseismology Can Be Used to Probe the Sun’s Interior Models of Earth’s interior predict how density and temperature change from place to place within the planet These density and temperature differences affect the way pressure waves travel through Earth, bending the paths of these waves Geologists test models of Earth’s interior by comparing measurements of seismic waves from earthquakes with model predictions of how seismic waves should travel through the planet The same basic idea has now been applied to the Sun Detailed observations of motions of material from place to place across the surface of the Sun show that the Sun vibrates or “rings,” something like a struck bell Compared to a well-tuned bell, however, the vibrations of the Sun are very complex, with many different frequencies of vibrations occurring simultaneously These motions are echoes of what lies below Just as geologists use seismic waves from earthquakes to probe the interior of Earth, solar physicists use the surface oscillations of the Sun to test their understanding of the solar interior This science is called helioseismology (Figure 14.11) To detect the disturbances of helioseismic waves on the surface of the Sun, astronomers must measure Doppler shifts of less than 0.1 meter per second (m/s) while detecting changes in brightness of only a few parts per million at any given location on the Sun Tens of millions of different wave motions are possible within the Sun Some waves travel around the circumference of the Sun, providing information about the density of the upper convection zone Other waves travel through the interior of the Sun, revealing the density structure of the Sun close to its core Still others travel inward toward the center of the Sun, until they are bent by the changing solar density and return to the surface All of these wave motions are going on at the same time The Global Oscillation Network Group (GONG), a network of six solar observation stations spread around the world, enables astronomers to observe the surface of the Sun approximately 90 percent of the time To interpret helioseismology data, scientists compare the strength, frequency, and wavelengths of the data against predicted vibrations calculated from models of the Process of Science LEARNING FROM FAILURE The first detections of neutrinos raised more questions than they answered The Hypothesis: The Sun’s energy comes from nuclear fusion, which produces neutrinos The Test: A specific number of neutrinos must be produced each day to account for the brightness of the Sun The experiment: Homestake detects one-third as many neutrinos as predicted The Conclusion: One of these things is true Scientists don’tunderstand nuclear fusion Scientists don’t understand neutrinos But thousands of experiments on Earth support our understanding! New Hypothesis: What if neutrinos come in three types and Homestake can detect only one type? Part of the “scientific attitude” is to find failure exciting When experiments not turn out as expected, good scientists get excited– there is something new to understand! 440 ˃ ˆ ˥ Our Star—The Sun Connections 14.2 Neutrino Astronomy A neutrino telescope hardly fits anyone’s expectation of what a telescope should look like The first apparatus designed to detect solar neutrinos consisted of a cylindrical tank filled with 100,000 gallons of dry-cleaning fluid—C2Cl4, or perchloroethylene—buried 1,500 meters deep within the Homestake Gold Mine in Lead, South Dakota A tiny fraction of neutrinos passing through this fluid interact with chlorine atoms, causing this reaction: 37 Cl + ν → 37Ar + e– The 37Ar formed in the reaction is a radioactive isotope of argon The tank must be buried deep within Earth to shield the detector from the many other types of radiation capable of producing argon atoms The argon is flushed out of the tank every few weeks and measured The Homestake detector (Figure 14.10a) operated from the late 1960s to the early 1990s Over the course of days, roughly 1022 (10 billion trillion) solar neutrinos passed through the Homestake detector Of these, on average only one neutrino interacted with a chlorine atom to form an atom of argon Even so, this interaction produced a measurable signal Since then, many other neutrino detectors have been built, each using different reactions to detect neutrinos of different energies In the 1990s, the Soviet-American Gallium Experiment (SAGE) and the European Gallium Experiment (GALLEX) and its successor the Gallium Neutrino Observatory used reactions involving the conversion of gallium atoms into germanium atoms ( 71Ga + ν → 71Ge + e-) to detect solar neutrinos Other detectors capable of detecting all three flavors of neutrinos include the Super-Kamiokande, which is located in an active zinc mine 2,700 meters under Mount Ikena, near Kamioka, Japan It has a 50,000-ton tank of ultrapure water, surrounded by 13,000 detectors capable of registering extremely faint flashes of light When a neutrino interacts with an atom in the tank, a faint conical flash of blue light is produced This flash is seen by some of the detectors The SNO+ experiment at the Sudbury Neutrino Observatory utilizes a chemical used in biodegradable detergents contained in a 12-meter sphere surrounded by light detectors (Figure 14.10b) and buried deep in a nickel mine near Sudbury, Ontario The Double Chooz experiment uses neutrinos from the Chooz nuclear power plant in France to measure neutrino oscillations An objective of still-newer neutrino telescopes is to collect higher-energy neutrinos that originate from the most distant objects in space Figure 14.10c shows the ANTARES experiment, which detects neutrinos passing through the Mediterranean Sea The IceCube neutrino detector at the South Pole has optical sensors buried far beneath the surface, at depths of up to 2.5 km within the Antarctic ice (see Figure 6.33) Neutrino telescopes observe neutrinos produced in the heart of the Sun, enabling astronomers to directly observe the results of the nuclear reactions going on there Although these observations have provided crucial confirmation that stars are powered by nuclear reactions, they have also challenged models of the solar interior and led to changes in ideas about the nature of the neutrino itself In addition to solar neutrinos, a number of experiments detected neutrinos from Supernova 1987A As we will discuss in Chapter 17, this explosion marked the end of the life of a massive star located 160,000 light-years away in a small galaxy called the Large Magellanic Cloud Neutrino astronomy was one of the great innovations of 20th century astronomy, and it will significantly benefit from the new neutrino detectors under construction (b) (a) ʹʼ ˰ʹʸ Neutrino “telescopes” not look much like visible-light telescopes (a) The first neutrino telescope, the Homestake neutrino detector, was a 100,000-gallon tank of dry-cleaning fluid located deep in a mine in South Dakota (b) The SNO+ Sudbury Neutrino Observatory, buried km deep in a Canadian nickel mine (c) Artist’s conception of the ANTARES neutrino observatory in the Mediterranean Sea (c) ˃ˆ˰ˆ˥The Atmosphere of the Sunˆˆ˃ FLASHBACK TO FIGURE 5.17 Waves that reach this observer are spread out to longer “redshifted” wavelengths (lower frequency) Waves that reach this observer are squeezed to shorter “blueshifted” wavelengths (higher frequency) v Speed of light c Moving source of light This observer sees no Doppler shift solar interior This technique provides a powerful test of an understanding of the solar interior, and it has led both to some surprises and to improvements in the modHelioseismology confirms els For example, some scithe predictions of entists had proposed that solar models the solar neutrino problem might be solved if the models were found to have too much helium in the Sun—an explanation that was ruled out by analysis of the waves that penetrate to the core of the Sun Helioseismology showed that the value for opacity used in early solar models was too low This realization led astronomers to recalculate the location of the bottom of the convective zone Both theory and observation now put the base of the convective zone at 71.3 percent of the way out from the center of the Sun, with an uncertainty in this number of less than half a percent 14.4 The Atmosphere of the Sun The Sun is a large ball of gas, and so, unlike Earth, it has no solid surface Instead, it has the kind of surface that a fog bank on Earth does; it is a gradual thing—an illusion really Imagine watching some people walking into a fog bank When they disappeared from view, you would say they were definitely inside the fog bank, even though they never passed through a definite boundary The apparent surface of the Sun is defined by the same effect Light from the Sun’s surface can escape into space, so you can see it Light from below the Sun’s surface cannot escape directly into space, so you cannot see it ʹʼ ˰ʹ ʹ The interior of the Sun rings like a bell as helioseismic waves move through it Waves with the right wavelength amplify and sustain the vibrations, while those with the wrong wavelength are damped out and disappear In the particular “mode” of the Sun’s vibration shown here, red indicates regions where gas is traveling inward; blue, where gas is traveling outward Astronomers observe these motions by using Doppler shifts An overview of the Sun’s atmosphere shows that it is made of several layers that lie above the top of the convective zone (Figure 14.12) The Sun’s atmosphere is where all visible solar phenomena take place At the base of the atmosphere is the photosphere: the Sun’s apparent surface This is where features such as sunspots can be seen Above this photosphere is the chromosphere, a region of strong emission lines The top layer is the corona, which can be viewed during a solar eclipse as a halo around the Sun Solar prominences, caused by the Sun’s magnetic field, poke out into the corona In the Sun’s atmosphere the density of the gas drops very rapidly with increasing altitude Figure 14.12 shows how pressure and temperature change across the atmosphere of the Sun In this section we will explore each of these layers, beginning at the bottom, with the photosphere The Sun’s apparent surface—the photosphere—has an effective temperature (the temperature of a blackbody that would emit at the same peak wavelength as the object) of 5780 K, ranging from 6600 K at the bottom to 4400 K at the top It is a zone about 500 km thick, across which the The apparent surface density and opacity of the of the Sun is called the Sun increase sharply The photosphere reason the Sun appears to have a well-defined surface and a sharp outline (but note that you should never look at the Sun directly) is that this zone is relatively shallow; 500 km does not look very thick when viewed from a distance of 150 million km Look at the photograph of the Sun in Figure 14.13a and notice that the Sun appears to be fainter near its edges than near its center This effect, called limb darkening, is ... 21st CENTURY ASTRONOMY FOURTH EDITION Laura Kay, Stacy Palen, Brad Smith, and George Blumenthal FOURTH EDITION 21ST CENTURY ASTRONOMY F O U RT H E D ITIO N 21ST CENTURY ASTRONOMY LAURA... Preface xxv About the Authors PART I xxxiv Introduction to Astronomy Chapter Why Learn Astronomy? 1.1 Getting a Feel for the Neighborhood 1.2 Astronomy Involves Exploration and Discovery 1.3 Science... interest in all things related to physics Brief Contents Part I Introduction to Astronomy Chapter 1 Why Learn Astronomy? Chapter 2 Patterns in the Sky—Motions of Earth Chapter 3 Motion of Astronomical