Explorations. An Introduction to Astronomy. Higher Education. Thomas T. Arny. Me Grauu Hill

Transcription

1 Explorations An Introduction to Astronomy Thomas T. Arny Professor Emeritus Department of Astronomy University of Massachusetts, Amherst Me Grauu Hill Higher Education Boston Burr Ridge, IL Dubuque, IA Madison, Wl New York San Francisco St. Louis Bangkok Bogota Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal New Delhi Santiago Seoul Singapore Sydney Taipei Toronto

2 PREFACE xix PHOTO CREDITS FOR LOOKING UP FEATURES xxix Preview The Cosmic Landscape 1 P.I The Earth, Our Home 2 P.2 The Moon 3 P.3 The Planets 4 P.4 The Sun 5 P.5 The Solar System 6 P. 6 A Sense of Scale 6 P.7 The Astronomical Unit 7 P.8 The Milky Way Galaxy 7 P.9 The Light-Year 9 P. 10 Galaxy Clusters and the Universe 10 P. 11 Gravity 11 P. 12 Atoms and Other Forces 11 P. 13 The Still-Unknown Universe 12 P. 14 The Scientific Method 12 The Night Sky 19 Chapter 1 History of Astronomy Prehistoric Astronomy 20 The Celestial Sphere 21 Constellations 22 Motions of the Sun and Stars 23 Daily or Diurnal Motion 23 Annual Motion 24 The Ecliptic 25 Looking Up at Orion 26 The Seasons 28 The Ecliptic's Tilt 29 Solstices and Equinoxes 30 The Planets and the Zodiac 32 The Moon 34 Extending Our Reach: Visibility of the Moon 36 Eclipses Early Ideas of the Heavens: Classical Astronomy 40 The Shape of the Earth 40 The Size of the Earth 40 Distance and Size of the Sun and Moon 42 Extending Our Reach: Measuring the Diameter ofastronomical Objects 44 The Motion of the Planets 46 Ptolemy 47 Islamic Contributions 47 Asian Contributions Astronomy in the Renaissance 48 Copernicus 48 Tycho and Kepler 49 Galileo, Isaac Newton and the Birth of Astrophysics The Growth of Astrophysics 55 New Discoveries 55 New Technologies 55 The Nature of Matter and Heat 55 The Kelvin Temperature Scale 56 Projects 60 Essay 1 Backyard Astronomy 63 Learning the Constellations 63 Star Lore 64 Looking Up at the Summer Triangle 65 Amateur Astronomy 67 Small Telescopes 67 Star Charts 68 i Celestial Coordinates 69 Planetary Configurations 70 V Your Eyes at Night 72

3 xii Atoms, Forces, Light, and How We Learn About the Universe 75 Chapter 2 Gravity and Motion Chapter 3 Solving the Problem of Astronomical Motion 76 Inertia 76 Orbital Motion and Gravity 78 Newton's Second Law of Motion 79 Acceleration 79 Mass 80 The Law of Gravity 81 Newton's Third Law of Motion 82 Measuring an Object's Mass Using Orbital Motion 83 Extending Our Reach: Weighing the Sun 84 Surface Gravity 85 Escape Velocity 86 Light and Atoms Properties of Light 92 The Nature of Light Waves or Particles? 92 Light and Color 94 Characterizing Electromagnetic Waves by Their Frequency 95 White Light The Electromagnetic Spectrum: Beyond Visible Light 96 Infrared Radiation 96 Ultraviolet Light 97 Radio Waves 97 Other Wavelength Regions 98 Energy Carried by Electromagnetic Radiation 98 Radiation and Temperature 99 Extending Our Reach: Taking the Temperature of the Sun 100 Blackbodies and Wien's Law Atoms 101 Structure of Atoms 101 The Chemical Elements The Origin of Light Formation of a Spectrum 105 How a Spectrum Is Formed 105 Identifying Atoms by Their Light 107 Types of Spectra 109 Analyzing the Spectrum 111 Astronomical Spectra The Doppler Shift: Detecting Motion Absorption in the Atmosphere 114 Chapter 4 Telescopes Telescopes 120 Light-Gathering Power 120 Focusing the Light 120 Extending Our Reach: Refraction 121 Resolving Power Interferometers Observatories Detecting the Light Observing at Nonvisible Wavelengths 131 Extending Our Reach: Exploring New Wavelengths: Gamma Rays 134 Extending Our Reach: Observing the Crab Nebula at Many Wavelengths Observatories in Space 135 Looking Up at Taurus 136 Atmospheric Blurring 138 Space Observatories versus Ground- Based Observatories, Going Observing Computers 142 j Projects 145

4 ,«-. res xiii The Earth and Moon 147 Chapter 5 The Earth The Earth as a Planet 148 Shape and Size of the Earth 148 Composition of the Earth 149 Density of the Earth The Earth's Interior 151 Probing the Interior with Earthquake Waves 151 Extending Our Reach: Measuring the Earth's Mass 151 Heating of the Earth's Core The Age of the Earth Motions in the Earth's Interior 157 Convection in the Earth's Interior 157 Plate Tectonics The Earth's Atmosphere 161 Composition of the Atmosphere 161 Origin of the Atmosphere 161 Extending Our Reach: Measuring the Motion of Plates Across Time 162 The Ozone Layer 165 The Greenhouse Effect 165 Structure of the Atmosphere The Earth's Magnetic Field 167 Origin of the Earth's Magnetic Field 168 Magnetic Effects in the Upper Atmosphere Motions of the Earth 170 Air and Ocean Circulation: The Coriolis Effect 170 Precession 172 Looking Up at the Northern Circumpolar Constellations 174 Project 177 Essay 2 Keeping Time 179 Length of the Daylight Hours 179 The Day 179 Time Zones 182 Chapter 6 Universal Time 182 Daylight Saving Time 182 The Month The Calendar 184 Leap Year 184 Religious Calendars 184 Other Calendars 185 Names of the Months and Days 185 The Abbreviations A.M., P.M., B.C., anda.d. 185 Summary 186 The Moon Description of the Moon 190 General Features 190 Surface Features 190 Origin of Lunar Surface Features Structure of the Moon Crust and Interior 195 The Absence of a Lunar Atmosphere Orbit and Motions of the Moon 197 The Moon's Rotation 198 Oddities of the Moon's Orbit Origin and History of the Moon Eclipses 201 Rarity of Eclipses 202 Appearance of Eclipses Tides 206 Cause of Tides 207 Solar Tides 209 Tidal Braking Moon Lore 211 Project 214 PART IV The Solar System 215 Chapter 7 Survey of the Solar System Components of the Solar System 217 The Sun 217 The Planets 217

5 XIV Two Types of Planets 220 Satellites 221 Asteroids and Comets 221 Composition Differences Between the Inner and Outer Planets 222 Density as a Measure of a Planet's Composition 222 Extending Our Reach: Bode's Law: The Search for Order 223 Age of the Solar System Origin of the Solar System 225 Interstellar Clouds 225 Formation of the Solar Nebula 226 Condensation in the Solar Nebula 227 Accretion and Planetesimals 228 Formation of the Planets 228 Science at Work: Direct Formation of Giant Planets 229 Formation of Moons 230 Final Stages of Planet Formation 230 Formation of Atmospheres 232 Cleaning up the Solar System Other Planetary Systems 232 Science at Work: Migrating Planets 234 Chapter 8 The Terrestrial Planets Portraits of the Terrestrial Planets Mercury 242 Mercury's Temperature and Atmosphere 244 Mercury's Interior 244 Mercury's Rotation Venus 246 The Venusian Atmosphere 247 The Greenhouse Effect 248 The Surface of Venus 248 The Interior of Venus 251 Rotation of Venus Mars 252 The Martian Atmosphere 256 The Martian Interior 259 The Martian Moons 259 Life on Mars? Why Are the Terrestrial Planets So Different? 261 Role of Mass and Radius 261 Role of Internal Activity 261 Role of Sunlight 261 Role of Water Content 262 Role of Biological Processes Update: Exploring Mars 263 Chapter 9 The Outer Planets Jupiter 272 Jupiter's Appearance and Physical Properties 272 Jupiter's Interior 273 Jupiter's Atmosphere 274 Jupiter's Ring 276 Jupiter's Moons Saturn 281 Saturn's Appearance and Physical Properties 281 Saturn's Rings 282 Origin of Planetary Rings 284 The Roche Limit 284 Saturn's Moons Uranus 285 Uranus's Structure 287 Uranus's Rings and Moons 288 Uranus's Odd Tilt Neptune 290 Neptune's Structure 291 Neptune's Atmosphere 291 Neptune's Rings and Moons Pluto 293 Chapter 10 Meteors, Asteroids, and Comets Meteors, Meteoroids, and Meteorites 300 Heating of Meteoroids 300 Meteorites 301

6 XV 10.2 Asteroids 302 Size and Shape 302 Composition 304 Origin of Asteroids 305 Unusual Asteroids Comets 307 Structure of Comets 307 Composition of Comets 309 Origin of Comets 309 Formation of the Comet's Tail 310 Light from the Comet's Tail 311 Short-Period Comets and the KuiperBelt 312 Science at Work: Kuiper Belt Objects and Beyond 313 Fate of Short-Period Comets 313 Meteor Showers Giant Impacts 315 Giant Meteor Craters 316 Mass Extinction and Asteroid/Comet Impacts 317 Science at Work: Ghost Craters or No Tell-Tale Fragments 317 Science at Work: Meteorites Can Be Deadly 318 Projects 321 Stars 323 Chapter 11 The Sun, Our Star Size and Structure 324 \ Measuring the Sun's Properties 325 The Solar Interior 325 Energy Flow in the Sun 326 The Solar Atmosphere How the Sun Works 328 Internal Balance (Hydrostatic Equilibrium) 329 Pressure in the Sun 329 Powering the Sun 330 Nuclear Fusion 331 The Structure of Hydrogen and Helium 331 The Proton-Proton Chain Probing the Sun's Core 332 Solar Neutrinos 333 Science at Work: Solving the Neutrino Puzzle 334 Solar Seismology Solar Magnetic Activity 335 Sunspots 335 Solar Magnetic Fields 335 Prominences and Flares 336 Heating of the Chromosphere and Corona 338 Extending Our Reach: Detecting Magnetic Fields: The Zeeman Effect 339 The Solar Wind The Solar Cycle 341 Cause of the Solar Cycle 342 Changes in the Solar Cycle 343 Links Between the Solar Cycle and Terrestrial Climate 344 Projects 347 Chapter 12 Measuring the Properties of Stars Measuring a Star's Distance 350 Measuring Distance by Triangulation and Parallax 350 Extending Our Reach: Measuring the Distance to Sirius 353 Measuring Distance by the Standard-Candles Method Measuring the Properties of Stars from Their Light 354 Temperature 354 Luminosity 355 The Inverse-Square Law and Measuring a Star's Luminosity 356 Radius 357 The Stefan-Boltzmann Law 357 Extending Our Reach: Measuring the Radius of the Star Sirius 359 The Magnitude System 360 i 12.3 Spectra of Stars 361 Measuring a Star's Composition^ 362 How Temperature Affects a Star's Spectrum 362

7 XVI Classification of Stellar Spectra 363 Science at Work: New Spectrum Classes 364 Definition of the Spectral Classes 364 Measuring a Star's Motion Binary Stars 368 Visual and Spectroscopic Binaries 368 Looking Up at Ursa Major 369 Measuring Stellar Masses with Binary Stars 370 Eclipsing Binary Stars Summary of Stellar Properties The H-R Diagram 373 Constructing the H-R Diagram 373 Analyzing the H-R Diagram 373 Giants and Dwarfs 374 The Mass-Luminosity Relation 375 Luminosity Classes 375 Summary of the H-R Diagram Variable Stars Finding a Star's Distance by the Method of Standard Candles 379 Project 383 Chapter 13 Stellar Evolution The Evolution of a Star 386 The Sun's Life Story 387 The Life Story of a High-Mass Star 388 The Importance of Gravity Star Formation 390 Interstellar Gas Clouds 390 Protostars 390 Bipolar Flows from Young Stars 392 Stellar Mass Limits Main-Sequence Stars 394 Why a Star's Mass Determines Its Core Temperature 395 Structure of High-Mass and Low-Mass Stars 395 Main-Sequence Lifetime of a Star Giant Stars 396 Leaving the Main Sequence 396 Nuclear Fuels Heavier Than Hydrogen 397 Degeneracy in Low-Mass Stars Yellow Giants and Pulsating Stars 398 Why Do Stars Pulsate? 399 The Period-Luminosity Law Death of Stars Like the Sun 401 Ejection of a Low-Mass Star's Outer Layers 401 Science at Work: Planetary Nebulas 401 Planetary Nebulas Old Age of Massive Stars 404 Formation of Heavy Elements: Nucleosynthesis 404 Core Collapse of Massive Stars 405 Supernova Explosions 406 Supernova Remnants History of Stellar Evolution Theories Testing Stellar Evolution Theory 409 Extending Our Reach: Measuring the Age of a Star Cluster 410 Chapter 14 Stellar Remnants: White Dwarfs, Neutron Stars, and Black Holes White Dwarfs 416 General Properties, Origin, and Fate 416 Structure of White Dwarfs 417 Degeneracy and the Chandrasekhar Limit 418 White Dwarfs in Binary Systems: Novas and Supernovas of Type I Neutron Stars 421 General Properties and Origin 421 Pulsars and the Discovery of Neutron Stars 421 Emission from Neutron Stars 423 Structure of Neutron Stars 425 Neutron Stars in Binary Systems 425 X-Ray Binary Stars Black Holes 426 The Formation and Observation of Black Holes 429 Gravitational Waves from Double Compact Stars 430 Hawking Radiation 431

8 xvii The Milky Way and Other Galaxies 435 Chapter 15 The Milky Way Galaxy Discovering the Milky Way 435 Shape of the Milky Way 437 Size of the Milky Way 438 Looking Up at Sagittarius Overview of the Milky Way 441 Structure and 441 Mass of the Milky Way and the Number of Stars 442 Age of the Milky Way Stars of the Milky Way 443 Stellar Censuses 443 Two Stellar Populations: Population I and Population II 444 Star Clusters Gas and Dust in the Milky Way 448 Distribution and Composition of Interstellar Matter 448 Interstellar Dust: Dimming and Reddening 448 Interstellar Gas 451 Visible Emission from Interstellar Gas 451 Radio Waves from Cold Interstellar Gas Motion of Stars and Gas in the Milky Way 453 Extending Our Reach: Mapping the Milky Way with Radio Waves Measuring the Milky Way 457 Diameter of the Milky Way 457 Mass of the Milky Way 458 Extending Our Reach: Measuring the Mass of the Milky Way 458 Extending Our Reach: Measuring the Sun's Speed Around the Milky Way The Galactic Center Formation of the Milky Way 462 Birth of Population I and II Stars 462 Building by Mergers 463 Population III 464 The Future of the Milky Way 465 Project 468 Chapter 16 Galaxies Discovering Galaxies 470 Early Observations of Galaxies 470 Looking Up at M31 & Perseus 471 Types of Galaxies 473 Differences in the Stellar and Gas Content of Galaxies 476 The Cause of Galaxy Types 478 Galaxy Collisions and Mergers Measuring the Properties of Galaxies 482 Galaxy Distances 482 The Redshift, and the Hubble Law 482 Extending Our Reach: Measuring the Distance of a Galaxy Using Cepheid Variables 483 Measuring the Diameter of a Galaxy 485 Extending Our Reach: Other Ways to Measure a Galaxy's Distance 485 Measuring the Mass of a Galaxy Dark Matter Active Galaxies 488 Radio Galaxies 488 Seyfert Galaxies 489 Quasars 489 Cause of Activity in Galaxies 490 Extending Our Reach: Measuring the Diameter of Astronomical Objects by Using Their Light Variability 492 Science at Work: Explaining Jets Quasars as Probes of Intergalactic Space 494 Gravitational Lenses 494 Extending Our Reach: Dark Matter and Gravitational Lenses Galaxy Clusters 496 The Local Group 496 Rich and Poor Galaxy Clusters 497 Superclusters 499 Project 504 Chapter 17 Cosmology Observations of the Universe 506 Distribution of Galaxies 506

9 XVIII Motion of Galaxies 507 Age of the Universe 508 Extending Our Reach: Estimating the Age of the Universe 509 The Cosmic Horizon 509 The Size of the Universe 510 Are We at the Center of the Universe? 510 Olbers'Paradox 511 The Cosmic Microwave Background 512 Composition of the Oldest Stars 514 Deductions from Basic Observations of the Universe Evolution of the Universe: Expansion Forever or Recollapse? 515 The Density of the Universe 515 A Cosmological Repulsion? 516 Science at Work: The Universe's Fate The Shape of the Universe The Origin of the Universe 520 Radiation, Matter, and Antimatter in the Early Universe 521 History of Matter and Radiation in the Early Universe 521 The Formation of Galaxies The Inflationary Universe 524 The Flatness Problem 524 The Horizon Problem 525 Extending Our Imagination: Other Universes? 525 Essay 3 Life in the Universe 529 Life on Earth 529 History of Life on Earth 529 Unity of Living Beings 530 Deductions from the Unity of Life and the Time Line 532 Origin of Life on Earth 532 Origin of Complex Organisms 533 Panspermia 533 Life Elsewhere in the Universe 533 Are We Alone? 534 Arguments for Many Worlds 534 Loners 535 Searching for Life Elsewhere 535 Radio Searches 535 The Gaia Hypothesis 536 The Anthropic Principle 537 Answers to "Test Yourself" 539 Appendix 541 Powers-of-Ten Notation 541 Some Useful Formulas 541 Solving Distance, Velocity, Time (D, V,t) Problems 542 Table 1 Physical and Astronomical Constants 542 Table 2 Conversion Between American and Metric Units 543 Table 3 Physical Properties of the Planets 543 Table 4 Orbital Properties of the Planets 543 Table 5 Satellites of the Solar System 544 Table 6 Meteor Showers 545 Table 7 The Constellations 546 Table 8 The Brightest Stars 548 Table 9 The Nearest Stars 549 Table 10 Properties of Main-Sequence Stars 550 Table 11 Periodic Table of Elements 551