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Fifth Edition

21ST Century
Astronomy
FIFTH EDITION

21ST CENTURY
ASTRONOMY
LAURA KAY • Barnard College

STACY PALEN • Weber State University

GEORGE BLUMENTHAL • University of California—Santa Cruz

n
W. W. NORTON & COMPANY
NEW YORK • LONDON
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 four hundred 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 © 2016 by Laura Kay, Stacy Palen, and George Blumenthal.

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. © 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.

All rights reserved

Printed in Canada

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Library of Congress Cataloging-in-Publication Data

Kay, Laura.
21st century astronomy. — Fifth edition / Laura Kay, Barnard College, Stacy Palen, Weber State
University, George Blumenthal, University of California-Santa Cruz.
pages cm
Previous edition: 21st century astronomy (New York : W.W. Norton & Company, 2013).
Includes index.
ISBN 978-0-393-93899-9 (pbk.)
1. Astronomy—Textbooks. I. Palen, Stacy. II. Blumenthal, George (George Ray) III. Title. IV.
Title: Twenty-first century astronomy.
QB45.2.A14 2016
520—dc23
2015023646
W. W. Norton & Company, Inc., 500 Fifth Avenue, New York, NY 10110-0017
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 wife, M.P.M. She dedicates this
book to her late uncle, Lee Jacobi, for an early introduc-
tion to physics, and to her late colleagues at Barnard
College, Tally Kampen and Sally Chapman.
Stacy Palen thanks her husband, John Armstrong, for
his patient support during this project.
George Blumenthal gratefully thanks his wife, Kelly
Weisberg, and his children, Aaron and Sarah B
­ lumenthal,
for their support during this project. He also wants to
thank Professor Robert Greenler for stimulating his in-
terest in all things related to physics.
 Brief Contents

Part I Introduction to Astronomy

Chapter 1 Thinking Like an Astronomer 2
Chapter 2 Patterns in the Sky—Motions of Earth and the Moon 22
Chapter 3 Motion of Astronomical Bodies 58
Chapter 4 Gravity and Orbits 82
Chapter 5 Light 108
Chapter 6 The Tools of the Astronomer 142

Part II The Solar System

Chapter 7 The Birth and Evolution of Planetary Systems 172
Chapter 8 The Terrestrial Planets and Earth’s Moon 200
Chapter 9 Atmospheres of the Terrestrial Planets 234
Chapter 10 Worlds of Gas and Liquid—The Giant Planets 268
Chapter 11 Planetary Moons and Rings 296
Chapter 12 Dwarf Planets and Small Solar System Bodies 326

Part III Stars and Stellar Evolution

Chapter 13 Taking the Measure of Stars 358
Chapter 14 Our Star—The Sun 390
Chapter 15 The Interstellar Medium and Star Formation 420
Chapter 16 Evolution of Low-Mass Stars 448
Chapter 17 Evolution of High-Mass Stars 478
Chapter 18 Relativity and Black Holes 506

Part IV Galaxies, the Universe, and Cosmology

Chapter 19 Galaxies 534
Chapter 20 The Milky Way—A Normal Spiral Galaxy 564
Chapter 21 The Expanding Universe 590
Chapter 22 Cosmology 616
Chapter 23 Large-Scale Structure in the Universe 646
Chapter 24 Life 674

vi
Contents

Preface xxi
About the Authors xxxii

PART I Introduction to Astronomy

Chapter 1 Thinking Like an Astronomer 2

1.1 Earth Occupies a Small Place in the Universe 4
1.2 Science Is a Way of Viewing the Universe 7
Process of Science The Scientific Method 9
1.3 Astronomers Use Mathematics to Find Patterns 12
Working It Out 1.1 Mathematical Tools 13
Working It Out 1.2 Reading a Graph 14
Origins An Introduction 15
Reading Astronomy News Probe Detects Southern Sea
Under Ice on Saturnian Moon Enceladus 16

Summary 17
Unanswered Questions 17
Questions and Problems 18
Exploration: Logical Fallacies 21

Chapter 2 Patterns in the Sky—Motions of Earth and the Moon 22

2.1 Earth Spins on Its Axis 24
Working It Out 2.1 How to Estimate the Size of Earth 31
2.2 Revolution about the Sun Leads to Changes during the Year 33
Process of Science Theories Must Fit All the Known Facts 37
2.3 The Moon’s Appearance Changes as It Orbits Earth 40
2.4 Calendars Are Based on the Day, Month, and Year 43
2.5 Eclipses Result from the Alignment of Earth, Moon, and the Sun 45
Origins The Obliquity of Earth 51
Reading Astronomy News Thousands Expected in Hopkinsville for 2017 Solar Eclipse 52

Summary 53
Unanswered Question 53
Questions and Problems 54
Exploration: The Phases of the Moon 57

vii
viii Co n t e n t s

Chapter 3 Motion of Astronomical Bodies 58

3.1 The Motions of Planets in the Sky 60
Working It Out 3.1 How Copernicus Computed Orbital Periods and Scaled
the Solar System 64
3.2 Kepler’s Laws Describe Planetary Motion 64
Process of Science Theories Are Falsifiable 67
Working It Out 3.2 Kepler’s Third Law 68
3.3 Galileo’s Observations Supported the Heliocentric Model 69
3.4 Newton’s Three Laws Help to Explain the Motion of Celestial Bodies 71
Working It Out 3.3 Using Newton’s Laws 74
Origins Planets and Orbits 75
Reading Astronomy News NASA Spacecraft Take Spring Break at Mars 76

Summary 77
Unanswered Questions 77
Questions and Problems 77
Exploration: Kepler’s Laws 81

Chapter 4 Gravity and Orbits 82

4.1 Gravity Is a Force between Any Two Objects Due to Their Masses 84
Working It Out 4.1 Playing with Newton’s Laws of Motion and Gravitation 87
4.2 An Orbit Is One Body “Falling around” Another 89
Process of Science Universality 91
Working It Out 4.2 Circular Velocity and Escape Velocity 94
Working It Out 4.3 Calculating Mass from Orbital Periods 95
4.3 Tidal Forces Are Caused by Gravity 95
Working It Out 4.4 Tidal Forces 98
4.4 Tidal Forces Affect Solid Bodies 99
Origins Tidal Forces and Life 101
Reading Astronomy News Exploding Stars
Prove Newton’s Law of Gravity
Unchanged over Cosmic Time 102

Summary 103
Unanswered Question 103
Questions and Problems 103
Exploration: Newton’s Laws 107

Chapter 5 Light 108

5.1 Light Brings Us the News of the Universe 110
Process of Science Agreement between Fields 116
Working It Out 5.1 Working with Electromagnetic Radiation 117
5.2 The Quantum View of Matter Explains Spectral Lines 117
 Co n t e n t s ix

5.3 The Doppler Shift Indicates Motion Toward or Away from Us 125
Working It Out 5.2 Making Use of the Doppler Effect 127
5.4 Temperature Affects the Spectrum of Light That an Object Emits 127
Working It Out 5.3 Working with the Stefan-Boltzmann Law and Wien’s Law 132
5.5 The Brightness of Light Depends on the Luminosity and Distance of the Light
Source 132
Working It Out 5.4 Using Radiation Laws to Calculate
Equilibrium Temperatures of Planets 134
Origins Temperatures of Planets 135
Reading Astronomy News A Study in Scarlet 136

Summary 137
Unanswered Questions 137
Questions and Problems 137
Exploration: Light as a Wave, Light as a Photon 141

Chapter 6 The Tools of the Astronomer 142

6.1 The Optical Telescope Revolutionized Astronomy 144
Working It Out 6.1 Telescope Aperture and Magnification 146
Working It Out 6.2 Diffraction Limit 150
6.2 Optical Detectors and Instruments Used with Telescopes 152
6.3 Astronomers Observe in Wavelengths Beyond the Visible 155
6.4 Planetary Spacecraft Explore the Solar System 159
6.5 Other Tools Contribute to the Study of the Universe 161
Process of Science Technology and Science Are Symbiotic 163
Origins Microwave Telescopes Detect Radiation from the Big Bang 165
Reading Astronomy News Big Mirrors, High Hopes: Extremely Large Telescope is a Go 166

Summary 167
Unanswered Questions 167
Questions and Problems 168
Exploration: Geometric Optics and Lenses 171

PART II The Solar System

Chapter 7 The Birth and Evolution of Planetary Systems 172
7.1 Planetary Systems Form around a Star 174
Process of Science Converging Lines of Inquiry 176
7.2 The Solar System Began with a Disk 177
Working It Out 7.1 Angular Momentum 180
7.3 The Inner Disk and Outer Disk Formed at Different
Temperatures 181
7.4 The Formation of Our Solar System 185
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x Co n t e n t s

7.5 Planetary Systems Are Common 187

Working It Out 7.2 Estimating the Size of the Orbit of a Planet 189
Working It Out 7.3 Estimating the Radius of an Extrasolar Planet 190
Origins Kepler’s Search for Earth-Sized Planets 193
Reading Astronomy News Earth-Size Planet Found in the “Habitable Zone”
of Another Star 194

Summary 195
Unanswered Questions 195
Questions and Problems 196
Exploration: Exploring Extrasolar Planets 199

Chapter 8 The Terrestrial Planets and Earth’s Moon 200

8.1 Impacts Help Shape the Evolution of the Planets 202
Process of Science Certainty Is Sometimes Out of Reach 206
8.2 Radioactive Dating Tells Us the Age of the Moon and the Solar System 207
Working It Out 8.1 Computing the Ages of Rocks 208
8.3 The Surface of a Terrestrial Planet Is Affected by Processes
in the Interior 209
Working It Out 8.2 How Planets Cool Off 212
8.4 Planetary Surfaces Evolve through Tectonism 214
8.5 Volcanism Signifies a Geologically Active Planet 219
8.6 The Geological Evidence for Water 222
Origins The Death of the Dinosaurs 227
Reading Astronomy News Did Volcanoes
Erupt on the Moon while Dinosaurs
Roamed Earth? 228

Summary 229
Unanswered Questions 229
Questions and Problems 230
Exploration: Exponential Behavior 233

Chapter 9 Atmospheres of the Terrestrial Planets 234

9.1 Atmospheres Change over Time 236
9.2 Secondary Atmospheres Evolve 238
Working It Out 9.1 Atmosphere Retention 239
9.3 Earth’s Atmosphere Has Detailed Structure 243
9.4 The Atmospheres of Venus and Mars Differ from Earth’s 251
9.5 Greenhouse Gases Affect Global Climates 255
Process of Science Thinking about Complexity 259
Origins Our Special Planet 260
 Co n t e n t s xi

Reading Astronomy News Mars Once Had an Entire Ocean—and then Lost It,
Scientists Say 261

Summary 262
Unanswered Questions 262
Questions and Problems 263
Exploration: Climate Change 267

Chapter 10 Worlds of Gas and Liquid—The Giant Planets 268

10.1 The Giant Planets Are Large, Cold, and Massive 270
Process of Science Scientific Laws Make Testable Predictions 272
10.2 The Giant Planets Have Clouds and Weather 275
Working It Out 10.1 Measuring Wind Speeds on Different Planets 280
10.3 The Interiors of the Giant Planets Are Hot and Dense 281
Working It Out 10.2 Internal Thermal Energy Heats the Giant Planets 282
10.4 The Giant Planets Are Magnetic Powerhouses 283
10.5 The Planets of Our Solar System Might Not Be Typical 287
Origins Giant Planet Migration and the Inner Solar System 289
Reading Astronomy News Hubble Sees Jupiter’s
Red Spot Shrink to Smallest Size Ever 290

Summary 291
Unanswered Questions 291
Questions and Problems 292
Exploration: Estimating Rotation Periods of the Giant Planets 295

Chapter 11 Planetary Moons and Rings 296

11.1 Many Solar System Planets Have Moons 298
Working It Out 11.1 Using Moons to Compute the Mass of a Planet 300
11.2 Some Moons Have Geological Activity and Water 301
Working It Out 11.2 Tidal Forces on the Moons 303
11.3 Rings Surround the Giant Planets 308
Working It Out 11.3 Feeding the Rings 312
11.4 Ring Systems Have a Complex Structure 312
Process of Science Following Up on the Unexpected 315
Origins Extreme Environments 319
Reading Astronomy News Possible New Moon Forming around Saturn 320

Summary 321
Unanswered Questions 321
Questions and Problems 322
Exploration: Measuring Features on Io 325
xii Co n t e n t s

Chapter 12 Dwarf Planets and Small Solar System Bodies 326

12.1 Dwarf Planets May Outnumber Planets 328
Process of Science How to Classify Pluto 330
Working It Out 12.1 Eccentric Orbits 331
12.2 Asteroids Are Pieces of the Past 332
12.3 Comets Are Clumps of Ice 337
12.4 Meteorites Are Remnants of the Early Solar System 344
12.5 Collisions Still Happen Today 348
Working It Out 12.2 Impact Energy 350
Origins Comets, Asteroids, Meteoroids, and Life 351
Reading Astronomy News Rosetta Spacecraft
Finds Water on Earth Didn’t Come
from Comets 352

Summary 353
Unanswered Questions 353
Questions and Problems 354
Exploration: Asteroid Discovery 357

PART III Stars and Stellar Evolution

Chapter 13 Taking the Measure of Stars 358

13.1 Astronomers Measure the Distance, Brightness, and Luminosity of Stars 360
Working It Out 13.1 Parallax and Distance 363
Working It Out 13.2 The Magnitude System 364
13.2 Astronomers Can Determine the Temperature, Size, and Composition
of Stars 365
Working It Out 13.3 Estimating the Sizes of Stars 370
13.3 Measuring the Masses of Stars in Binary Systems 371
Working It Out 13.4 Measuring the Mass of an Eclipsing Binary Pair 374
13.4 The Hertzsprung-Russell Diagram Is the Key to Understanding Stars 376
Process of Science Science Is Collaborative 378
Origins Habitable Zones 382
Reading Astronomy News NASA’s Hubble Extends Stellar Tape Measure
10 Times Farther into Space 383

Summary 384
Unanswered Questions 385
Questions and Problems 385
Exploration: The H-R Diagram 389
 Co n t e n t s xiii

Chapter 14 Our Star—The Sun 390

14.1 The Sun Is Powered by Nuclear Fusion 392
Working It Out 14.1 The Source of the Sun’s Energy 394
14.2 Energy Is Transferred from the Interior of the Sun 397
Process of Science Learning from Failure 401
14.3 The Atmosphere of the Sun 403
14.4 The Atmosphere of the Sun Is Very Active 405
Working It Out 14.2 Sunspots and Temperature 407
Origins The Solar Wind and Life 412
Reading Astronomy News Carrington-Class CME
Narrowly Misses Earth 413

Summary 414
Unanswered Questions 414
Questions and Problems 415
Exploration: The Proton-Proton Chain 419

Chapter 15 The Interstellar Medium and Star Formation 420

15.1 The Interstellar Medium Fills the Space between the Stars 422
Working It Out 15.1 Dust Glows in the Infrared 425
Process of Science All Branches of Science Are Interconnected 429
15.2 Molecular Clouds Are the Cradles of Star Formation 430
15.3 Formation and Evolution of Protostars 432
15.4 Evolution Before the Main Sequence 436
Working It Out 15.2 Luminosity, Surface Temperature, and Radius of
Protostars 438
Origins Star Formation, Planets, and Life 441
Reading Astronomy News Interstellar Dust Discovered Inside NASA
Spacecraft 442

Summary 443
Unanswered Questions 443
Questions and Problems 444
Exploration: The Stellar Thermostat 447

Chapter 16 Evolution of Low-Mass Stars 448

16.1 The Life of a Main-Sequence Star Depends on Its Mass 450
Working It Out 16.1 Estimating Main-Sequence Lifetimes 452
16.2 The Star Leaves the Main Sequence 453
16.3 Helium Burns in the Degenerate Core 456
16.4 Dying Stars Shed Their Outer Layers 460
Working It Out 16.2 Escaping the Surface of an
Evolved Star 461
xiv Co n t e n t s

16.5 Binary Star Evolution 466

Process of Science Science Is Not Finished 470
Origins Stellar Lifetimes and Biological Evolution 471
Reading Astronomy News Scientists Solve Riddle of Celestial
Archaeology 472

Summary 473
Unanswered Questions 473
Questions and Problems 474
Exploration: Low-Mass Stellar Evolution 477

Chapter 17 Evolution of High-Mass Stars 478

17.1 High-Mass Stars Follow Their Own Path 480
17.2 High-Mass Stars Go Out with a Bang 484
Working It Out 17.1 Binding Energy of Atomic Nuclei 485
17.3 The Spectacle and Legacy of Supernovae 489
Working It Out 17.2 Gravity on a Neutron Star 491
Process of Science Occam’s Razor 494
17.4 Star Clusters Are Snapshots of Stellar Evolution 495
Origins Seeding the Universe with New Chemical Elements 499
Reading Astronomy News We Are Swimming in a Superhot Supernova
Soup 500

Summary 501
Unanswered Questions 501
Questions and Problems 501
Exploration: The CNO Cycle 505

Chapter 18 Relativity and Black Holes 506

18.1 Relative Motion Affects Measured Velocities 508
18.2 Special Relativity Explains How Time and Space Are Related 510
Working It Out 18.1 Time Dilation 514
18.3 Gravity Is a Distortion of Spacetime 515
Process of Science New Science Can Encompass the Old 520
18.4 Black Holes 523
Working It Out 18.2 Masses in X-Ray Binaries 526
Origins Gamma-Ray Bursts 527
Reading Astronomy News After Neutron Star Death-Match, a Black Hole Is
Born 528

Summary 529
Unanswered Questions 529
Questions and Problems 530
Exploration: Black Holes 533
 Co n t e n t s xv

PART IV Galaxies, the Universe, and Cosmology

Chapter 19 Galaxies 534

19.1 Galaxies Come in Different Shapes and Sizes 536
19.2 Astronomers Use Several Methods to Find Distances to Galaxies 542
Working It Out 19.1 Finding the Distance from a Type Ia Supernova 544
Working It Out 19.2 Redshift—Calculating the Recession Velocity and
Distance of Galaxies 546
19.3 Galaxies Are Mostly Dark Matter 546
19.4 Most Galaxies Have a Supermassive Black Hole at the Center 549
Working It Out 19.3 The Size, Density, and Power of a Supermassive Black
Hole 553
Process of Science Finding the Common Thread 555
Origins Habitability in Galaxies 557
Reading Astronomy News Hubble Helps Find Smallest Known Galaxy with a
Supermassive Black Hole 558

Summary 559
Unanswered Questions 559
Questions and Problems 560
Exploration: Galaxy Classification 563

Chapter 20 The Milky Way—A Normal Spiral Galaxy 564

20.1 Astronomers Have Measured the Size and Structure of the Milky Way 566
20.2 The Components of the Milky Way Provide Clues about the Formation of
Spiral Galaxies 570
Process of Science Unknown Unknowns 571
20.3 Most of the Milky Way Is Unseen 576
Working It Out 20.1 The Mass of the Milky Way inside the Sun’s Orbit 578
Working It Out 20.2 The Mass of the Milky Way’s Central Black Hole 579
20.4 The History and Future of the Milky Way 580
Origins The Galactic Habitable Zone 583
Reading Astronomy News Dark Matter Half What
We Thought, Say Scientists 584

Summary 585
Unanswered Questions 585
Questions and Problems 586
Exploration: The Center of the Milky Way 589
xvi Co n t e n t s

Chapter 21 The Expanding Universe 590

21.1 The Cosmological Principle 592
Process of Science Data Are the Ultimate Authority 596
21.2 The Universe Began in the Big Bang 597
Working It Out 21.1 Expansion and the Age of the Universe 598
21.3 Expansion Is Described with a Scale Factor 601
Working It Out 21.2 When Redshift Exceeds One 603
21.4 Astronomers Observe Cosmic Microwave Background Radiation 604
Origins Big Bang Nucleosynthesis 608
Reading Astronomy News 50th Anniversay of the Big Bang Discovery 610

Summary 611
Unanswered Questions 611
Questions and Problems 611
Exploration: Hubble’s Law for Balloons 615

Chapter 22 Cosmology 616

22.1 Gravity and the Expansion of the Universe 618
Working It Out 22.1 Calculating the Critical Density 619
22.2 The Accelerating Universe 620
Process of Science Never Throw Anything Away 622
22.3 Inflation Solves Several Problems in Cosmology 626
22.4 The Earliest Moments of the Universe Connect the Very Largest Size Scales
to the Very Smallest 629
Working It Out 22.2 Pair Production in the Early Universe 632
22.5 String Theory and Multiverses 636
Origins Our Own Universe Must Support Life 639
Reading Astronomy News Cosmic Inflation: How Progress in Science Is
Achieved 640

Summary 641
Unanswered Questions 641
Questions and Problems 641
Exploration: Studying Particles 645

Chapter 23 Large-Scale Structure in the Universe 646

23.1 Galaxies Form Groups, Clusters, and Larger Structures 648
Working It Out 23.1 Mass of a Cluster of Galaxies 650
23.2 Gravity Forms Large-Scale Structure 651
Process of Science Multiple Streams of Evidence 656
23.3 First Light of Stars and Galaxies 657
Working It Out 23.2 Observing High-Redshift Objects 660
 Co n t e n t s xvii

23.4 Galaxies Evolve 662

Origins We Are the 4 or 5 Percent 667
Reading Astronomy News Welcome to Laniakea, Your Galactic Supercluster
Home 668

Summary 669
Unanswered Questions 669
Questions and Problems 669
Exploration: The Story of a Proton 673

Chapter 24 Life 674

24.1 Life Evolves on Earth 676
Working It Out 24.1 Exponential Growth 680
24.2 Life Involves Complex Chemical Processes 681
Process of Science All of Science Is Interconnected 682
24.3 Where Do Astronomers Look for Life? 684
24.4 Scientists Are Searching for Signs of Intelligent Life 689
Working It Out 24.2 Putting Numbers into the Drake Equation 690
Origins The Fate of Life on Earth 692
Reading Astronomy News Finding LIfe Beyond Earth Is within Reach 694

Summary 695
Unanswered Questions 695
Questions and Problems 696
Exploration: Fermi Problems and the Drake Equation 699

APPENDIX 1 Mathematical Tools A-1

APPENDIX 2 Physical Constants and Units A-6
APPENDIX 3 Periodic Table of the Elements A-8
APPENDIX 4 Properties of Planets, Dwarf Planets, and Moons A-9
APPENDIX 5 Space Missions A-13
APPENDIX 6 Nearest and Brightest Stars A-15
APPENDIX 7 Observing the Sky A-18
APPENDIX 8 Uniform Circular Motion and Circular Orbits A-27
APPENDIX 9 IAU 2006 Resolutions: “Definition of a Planet in the Solar System”
and “Pluto” A-29

Glossary G-1
Selected Answers SA-1
Credits C-1
Index I-1
Working It Out

1.1 Mathematical Tools 13 12.2 Impact Energy 350

1.2 Reading a Graph 14 13.1 Parallax and Distance 363
2.1 How to Estimate the Size of Earth 31 13.2 The Magnitude System 364
3.1 How Copernicus Computed Orbital Periods and Scaled 13.3 Estimating the Sizes of Stars 370
the Solar System 64 13.4 Measuring the Mass of an Eclipsing Binary Pair 374
3.2 Kepler’s Third Law 68 14.1 The Source of the Sun’s Energy 394
3.3 Using Newton’s Laws 74 14.2 Sunspots and Temperature 407
4.1 Playing with Newton’s Laws of Motion and 15.1 Dust Glows in the Infrared 425
Gravitation 87 15.2 L
 uminosity, Surface Temperature, and Radius of
4.2 Circular Velocity and Escape Velocity 94 Protostars 438
4.3 Calculating Mass from Orbital Periods 95 16.1 Estimating Main-Sequence Lifetimes 452
4.4 Tidal Forces 98 16.2 Escaping the Surface of an Evolved Star 461
5.1 Working with Electromagnetic Radiation 117 17.1 Binding Energy of Atomic Nuclei 485
5.2 Making Use of the Doppler Effect 127 17.2 Gravity on a Neutron Star 491
5.3 Working with the Stefan-Boltzmann Law and Wien’s 18.1 Time Dilation 514
Law 132 18.2 Masses in X-Ray Binaries 526
5.4 Using Radiation Laws to Calculate Equilibrium 19.1 Finding the Distance from a Type Ia Supernova 544
Temperatures of Planets 134
19.2 R
 edshift—Calculating the Recession Velocity and
6.1 Telescope Aperture and Magnification 146 Distance of Galaxies 546
6.2 Diffraction Limit 150 19.3 T
 he Size, Density, and Power of a Supermassive Black
7.1 Angular Momentum 180 Hole 553
7.2 Estimating the Size of the Orbit of a Planet 189 20.1 The Mass of the Milky Way inside the Sun’s Orbit 578
7.3 Estimating the Radius of an Extrasolar Planet 190 20.2 The Mass of the Milky Way’s Central Black Hole 579
8.1 Computing the Ages of Rocks 208 21.1 Expansion and the Age of the Universe 598
8.2 How Planets Cool Off 212 21.2 When Redshift Exceeds One 603
9.1 Atmosphere Retention 239 22.1 Calculating the Critical Density 619
10.1 Measuring Wind Speeds on Different Planets 280 22.2 Pair Production in the Early Universe 632
10.2 Internal Thermal Energy Heats the Giant Planets 282 23.1 Mass of a Cluster of Galaxies 650
11.1 Using Moons to Compute the Mass of a Planet 300 23.2 Observing High-Redshift Objects 660
11.2 Tidal Forces on the Moons 303 24.1 Exponential Growth 680
11.3 Feeding the Rings 312 24.2 Putting Numbers into the Drake Equation 690
12.1 Eccentric Orbits 331

xviii
AstroTours
AstroTour animations are available from the free Student Site at the Digital Landing Page,
and they are also integrated into assignable Smartwork5 exercises. Offline versions of the
animations for classroom presentation are available from the Instructor’s Resource USB Drive.
digital.wwnorton.com/astro5.

The Celestial Sphere and the Ecliptic 24 Traffic Circle Analogy 179
The View from the Poles 26 Processes That Shape the Planets 205
The Earth Spins and Revolves 35 Continental Drift 215
The Moon’s Orbit: Eclipses and Phases 40 Hot Spot Creating a Chain of Islands 220
Kepler’s Laws 65 Atmospheres: Formation and Escape 237
Velocity, Acceleration, Inertia 70, 73 Greenhouse Effect 241
Newton’s Laws and Universal Gravitation 89 Cometary Orbits 339
Elliptical Orbit 92 Stellar Spectrum 366
Tides and the Moon 95 H-R Diagram 377
Light as a Wave, Light as a Photon 115 The Solar Core 395
Atomic Energy Levels and the Bohr Model 118 Star Formation 431
Atomic Energy Levels and Light Emission and Absorption 122 Hubble’s Law 544, 597
The Doppler Effect 125 Dark Matter 546
Geometric Optics and Lenses 148 Active Galactic Nuclei 551
Solar System Formation 175 Big Bang Nucleosynthesis 608

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Vocabulary of The Celestial Sphere 26 Inverse Square Law 133

The Cause of Earth’s Seasons 36 Angular Momentum 178, 431
The Earth-Moon-Sun System 38 Charged Particles and Magnetic Forces 249
Phases of the Moon 40 Parallax 360
Velocity, Force, and Acceleration 73 Random Walk 398
Center of Mass 94 Type II Supernova 488
Tides 95 Pulsar Rotation 493
Emission and Absorption 119 Galaxy Shapes and Orientation 538
Doppler Shift 125, 188 Size of Active Galactic Nuclei 552
Changing Equilibrium 128, 241 Expanding Balloon Universe 597
Wien’s Law 131 Observable vs. Actual Universe 599

xix
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Lookback Time Simulator 8 Blackbody Curves 131

Celestial and Horizon Systems Comparison 26 Snell’s Law Demonstrator 144
Rotating Sky Explorer 26 Telescope Simulator 145
Meridional Altitude Simulator 30 CCD Simulator 154
Declination Ranges Simulator 30 EM Spectrum Module 155
Big Dipper Clock 30 Influence of Planets on the Sun 188
Ecliptic (Zodiac) Simulator 34 Radial Velocity Graph 188
Seasons and Ecliptic Simulator 36 Exoplanet Radial Velocity Simulator 189
Daylight Hours Explorer 36 Exoplanet Transit Simulator 189
Lunar Phase Simulator 43 Gas Retention Simulator 238
Synodic Lag 45 Driving through Snow 344
Moon Inclinations 50 Parallax Calculator 362
Eclipse Shadow Simulator 50 Stellar Luminosity Calculator 365
Eclipse Table 51 Center of Mass Simulator 372
Obliquity Simulator 51 Eclipsing Binary Simulator 373
Ptolemaic Orbit of Mars 60 Hertzsprung-Russell Diagram Explorer 376
Retrograde Motion 61 Spectroscopic Parallax Simulator 379
Planetary Configurations Simulator 63 Proton-Proton Animation 396
Synodic Period Calculator 64 CNO Cycle Animation 480
Eccentricity Demonstrator 65 H-R Explorer 482
Planetary Orbit Simulator 68 H-R Diagram Star Cluster Fitting Explorer 497
Phases of Venus 70 Spectroscopic Parallax Simulator 542
Ptolemaic Phases of Venus 70 Supernova Light Curve Fitting Explorer 542
Gravity Algebra 87 Galactic Redshift Simulator 544
Earth Orbit Plot 92 Traffic Density Analogy 569
Tidal Bulge Simulation 95 Milky Way Rotational Velocity 576
EM Spectrum Module 114 Milky Way Habitability Explorer 583
Three Views Spectrum Demonstrator 121 Circumstellar Habitable Zone 687
Hydrogen Atom Simulator 122 Milky Way Habitability Explorer 688
Doppler Shift Demonstrator 126

xx
Preface

Dear Student
Why is it a good idea to take a science course, and in particular, why is astronomy
a course worth taking? Many people choose to learn about astronomy because Process of Science CONVERGING LINES OF INQUIRY
they are curious about the universe. Your instructor likely has two basic goals in Astronomers asked: Why is the Solar System a disk,
with all planets orbiting in the same direction?

mind for you as you take this course. The first is to understand some basic physi-
cal concepts and how they apply to the universe around us. The second is to think
like a scientist and learn to use the scientific method not only to answer questions
Stellar astronomers
Stellar astronomers test find dust and gas
the nebular hypothesis, around young stars.

in this course but also to make decisions in your life. We have written the fifth
Stellar
seeking evidence
astronomers
for or against.
observe this

edition of 21st Century Astronomy with these two goals in mind. Mathematicians suggest
the nebular hypothesis:
a collapsing rotating cloud
gas and dust
to be in the

Throughout this book, we emphasize not only the content of astronomy (for
shape of disks.
formed the Solar System.

example, the differences among the planets, the formation of chemical elements)
but also how we know what we know. The scientific method is a valuable tool that
you can carry with you and use for the rest of your life. One way we highlight Planetary scientists test
the nebular hypothesis,

the process of science is the Process of Science Figures. In each chapter, we seeking evidence
for or against.

have chosen one discovery and provided a visual representation illustrating the
discovery or a principle of the process of science. In these figures, we try to illus- Planetary scientists

trate that science is not a tidy process, and that discoveries are sometimes made
study meteorites that
show the Solar System
bodies formed from

by different groups, sometimes by accident, but always because people are trying many smaller bodies.

to answer a question and show why or how we think something is the way it is. Beginning from the same fundamental observations about the shape of the Solar System,
theorists, planetary scientists, and stellar astronomers converge in the nebular theory
that stars and planets form together from a collapsing cloud of gas and dust.
The most effective way to learn something is to “do” it.planetesimals
Whether playing thatan in- too sparsely distributed for large planets to grow. Icy
were
strument or a sport or becoming a good cook, reading “how” can only take you
planetesimals in the outer so Solar System that survived planetary accretion remain
far. The same is true of learning astronomy. We have writtentodaythis
asbook
comet to help
nuclei.youThe frozen, distant dwarf planets Pluto and Eris are espe-
“do” as you learn. We have created several tools in every chapter to make reading
cially large examples of these residents of the outer Solar System.
a more active process. At the beginning of each chapter, weMany have Solar
provided a setobjects show evidence of cataclysmic impacts that re-
System
of Learning Goals to guide you as you read. There is a lotshaped
of information in every
worlds, suggesting that the early Solar System must have been a remark-
chapter, and the Learning Goals should help you focusably on the mostand
violent important
chaotic place. The dramatic difference in the terrain of the
points. We present a big-picture question in association with the chapter-opening
northern and southern hemispheres on Mars, for example, has been interpreted
figure at the beginning of each chapter. For each of these,aswethe have tried
result of to
onepose a
or more colossal collisions. The leading theory for the origin
question that is not only relevant to its chapter but also something you may have
of our Moon is that it resulted from the collision of an object with Earth. Mercury
wondered about. We hope that these questions, plus the photographs
has a crater on thatitsaccom-
surface from an impact so devastating that it caused the crust
pany them, capture your attention as well as your imagination.
to buckle on the opposite side of the planet. In the outer Solar System, one of
In addition, there are Check Your Understanding Saturn’s questions at theMimas,
moons, end of has a crater roughly one-third the diameter of the moon
each chapter section. These questions are designed to be itself.
answered quickly
Uranus suffered if youone or more collisions that were violent enough literally to
have understood the previous section. The answers are knock provided in the back of
the planet on its side. Today, as a result, its equatorial plane is tilted at al-
the book so you can check your answer and decide if further mostreview
a rightisangle
necessary.
to its orbital plane. We will see other examples in subsequent
As a citizen of the world, you make judgments aboutchapters.
science, distinguishing
between good science and pseudoscience. You use
these judgments to make decisions in the grocery
CHECK YOUR UNDERSTANDING 7.4
store, pharmacy, car dealership, and voting booth.
Suppose that astronomers found a rocky, terrestrial planet beyond the orbit of
You may base these decisions on the presentation of
Neptune. What is the most likely explanation for its origin? (a) It formed close to
information you receive through the media, which
the Sun and migrated outward. (b) It formed in that location and was not dis-
is very different from the presentation in class. One
turbed by migration. (c) It formed later in the Sun’s history than other planets.
important skill is the ability to recognize what is
(d) It is a captured planet that formed around another star.
credible and to question what is not. To help you

xxi
7.5 Planetary Systems Are Common
 formation developed from the work of both planetary planets and other objects in our Solar System. In the cur-
and stellar scientists. Planets are a common by-product of rent model of the formation of the Solar System, solid ter-
star formation, and many stars are surrounded by planetary restrial planets formed in the inner disk, where temperatures
systems. Gravity pulls clumps of gas and dust together, caus- were high, and giant gaseous planets formed in the outer
xxii P R E FAC E ing them to shrink and heat up. Angular momentum must be disk, where temperatures were low. Dwarf planets such as
conserved, leading to both a spinning central star and an Pluto formed in the asteroid belt and in the region beyond
accretion disk that rotates and revolves in the same direc- the orbit of Neptune. Asteroids and comet nuclei remain to-

hone this skill, we have provided Reading Astronomy News sections at the end
tion as the central star. Solar System meteorites show that
larger objects build up from smaller objects.
day as leftover debris.
LG 5 List how astronomers find planets around other stars,
READING ASTRONOMY NEWS of LG
every chapter.
2 Discuss the role of These
gravity andfeatures
angular momentuminclude in a news article
and explain howwithwe know questions
that planetarytosystems
help you
explaining why planets orbit the Sun in a plane and why around other stars are common. Astronomers find plan-
Articles questions A system with five planets was observed by NASA’s Kepler space makethey senserevolve ofin how
the same science is the
direction that presented
Sun rotates. to you. It isother
ets around important thatof methods:
stars using a variety you learn the radialto be
telescope. As particles orbit the forming star, the cloud of dust and gas velocity method, the transit method, microlensing, astrom-
critical of into
flattens thea plane.
information
Conservation of you
angularreceive,
momentum and theseetry, and features
direct imaging. will helphas
As technology you do that.
improved, the
earth-size Planet Found in the “Habitable Zone” of Another star
While we inknow a lot about the universe,
determines both the speed and the direction of the revolution
of the objects the forming system. Dust grains in the proto- science is an withongoing
thousands process,
number and variety of known extrasolar planets has in-
creased dramatically, and we
of planets and planet
By Science@NASA “M dwarfs are the most numerous stars,” Although the size of Kepler-186f is known,

Using NASA’s Kepler space telescope, astrono-

said Elisa Quintana, research scientist at the
SETI Institute at NASA’s Ames Research Cen-
its mass and composition are not. Previous
research, however, suggests that a planet the
continue to search for new answers. To give candidates
planetary disk first stick together because of collisions and
static electricity. As these objects grow, they eventually have
you a glimpse of what we don’t know,
discovered orbiting other stars near the Sun
within the Milky Way Galaxy in just the past few years.
we provide
enough massan Unanswered Questions feature near the end of each chapter.
mers have discovered the first Earth-size ter in Moffett Field, California, and lead author size of Kepler-186f is likely to be rocky.
planet orbiting in the “habitable zone” of of the paper published today in the journal “The discovery of Kepler-186f is a signifi- to attract other objects gravitationally. Once
another star (see Figure 7.23). The planet, Science. “The first signs of other life in the gal- cant step toward finding worlds like our planet
this occurs, they begin emptying the space around them. Col-
Most lisions
of these questions represent topics that scientists are currently studying.
named “Kepler-186f,” orbits an M dwarf, or axy may well come from planets orbiting an Earth,” said Paul Hertz, NASA’s Astrophysics
red dwarf, a class of stars that makes up 70 M dwarf.” Division director at the agency’s headquarters of planetesimals lead to the formation of planets.
percent of the stars in the Milky Way Galaxy. However, “being in the habitable zone does in Washington.
The discovery of Kepler-186f confirms that not mean we know this planet is habitable,” The next steps in the search for distant life
planets the size of Earth exist in the habitable cautions Thomas Barclay, a research scientist include looking for true Earth-twins—Earth-
zone of stars other than our Sun. at the Bay Area Environmental Research Insti- size planets orbiting within the habitable zone
The “habitable zone” is defined as the tute at Ames, and coauthor of the paper. “The of a Sun-like star—and measuring their chemi-
range of distances from a star where liquid
water might pool on the surface of an orbiting
planet. While planets have previously been
found in the habitable zone, the previous finds
temperature on the planet is strongly depen-
dent on what kind of atmosphere the planet
has. Kepler-186f can be thought of as an Earth-
cousin rather than an Earth-twin. It has many
cal compositions. The Kepler space telescope,
which simultaneously and continuously mea-
sured the brightness of more than 150,000
stars, is NASA’s first mission capable of detect-
? UNANSWERED QUESTIONS
are all at least 40 percent larger in size than properties that resemble Earth.” ing Earth-size planets around stars like our
Earth, and understanding their makeup is Kepler-186f resides in the Kepler-186 sys- Sun. • How typical is the Solar System? Only within the past few • How Earth-like must a planet be before scientists declare it
challenging. Kepler-186f is more reminiscent tem, about 500 light-years from Earth in the Looking ahead, Hertz said, “future NASA
of Earth. constellation Cygnus. The system is also home missions, like the Transiting Exoplanet Survey
years have astronomers found other systems containing four to be “another Earth”? An editorial in the science journal
Kepler-186f orbits its parent M dwarf star to four companion planets: Kepler-186b, Satellite and the James Webb Space Telescope, or more planets, and so far the observed distributions of Nature cautioned that scientists should define “Earth-like”
once every 130 days and receives one-third the Kepler-186c, Kepler-186d, and Kepler-186e, will discover the nearest rocky exoplanets and
energy that Earth gets from the Sun, placing it whiz around their sun every four, seven, 13, determine their composition and atmospheric
large and small planets in these multiplanet systems have in advance—before multiple discoveries of planets “similar”
nearer the outer edge of the habitable zone. On and 22 days, respectively, making them too hot conditions, continuing humankind’s quest to looked different from those of the Solar System. Computer to Earth are announced and a media frenzy ensues. Must a
the surface of Kepler-186f, the brightness of its for life as we know it. These four inner planets find truly Earth-like worlds.”
star at high noon is only as bright as our Sun all measure less than 1.5 times the size of simulations of planetary system formation suggest that a planet be of similar size and mass, be located in the habitable
appears to us about an hour before sunset. Earth. system with an orbital stability and a planetary distribution zone, and have spectroscopic evidence of liquid water before
like those of the Solar System may develop only rarely. Im- we call it “Earth 2.0”?
Articles questions proved supercomputers can run more complex simulations,
1. This NASA press release was picked up by business and international news feeds. Why do you think coverage of this discovery was so
which can be compared with the observations to understand
widespread? better how solar systems are configured.
2. The planet is closer to its star than Earth is to the Sun yet receives much less energy. What does that imply about the temperature of the star?
3. Why is the mass of this planet not yet known? What method will be used to find its mass?
4. How will astronomers estimate the planet’s composition?
5. Why is this planet called a “cousin” of Earth?

The language of science is mathematics, and it can be as challenging to learn as

any other language. The choice to use mathematics as the language of science is
not arbitrary; nature “speaks” math. To learn about nature, you will need to speak
08_KAY_93899_ch7_172-199.indd 195 9/17/15 3:21 PM

its language. We don’t want the language of math to obscure the concepts, so we
have placed this book’s mathematics in Working It Out boxes to make it clear
08_KAY_93899_ch7_172-199.indd 194 9/17/15 3:21 PM

when we are beginning and ending a mathematical argument, so that you can
spend time with the concepts in the chapter text and then revisit the mathemat-
ics of the concept to study the formal language of the argument. You will learn
to work with data and identify when data aren’t quite right. We want you to be
comfortable reading, hearing, and speaking the language of science, and we will
provide you with tools to make it easier.
190 chapter 7 The Birth and Evolution of Planetary Systems

Origins: The Death of the Dinosaurs 227

7.3 Working It Out Estimating the Radius of an Extrasolar Planet
Origins
The Death of the Dinosaurs The masses of extrasolar planets can often be estimated using Then, to solve for the radius of the planet, astronomers need an esti-
When large impacts happen on Earth, Kepler’s laws and the conservation of angular momentum. When mate of the radius of the star and a measurement of the percentage
they can have far-reaching conse- planets are detected by the transit method, astronomers can esti- reduction in light during the transit. The radius of a star is estimated
quences for Earth’s climate and for ter-
restrial life. One of the biggest and mate the radius of an extrasolar planet. In this method, astronomers from the surface temperature and the luminosity of the star.
most significant impacts happened at
look for planets that eclipse their stars and observe how much the Let’s consider an example. Kepler-11 is a system of at least six plan-
the end of the Cretaceous Period,
which lasted from 146 million years star’s light decreases during this eclipse (see Figure 7.19). In the So- ets that transit a star. The radius of the star, Rstar, is estimated to be
ago to 65 million years ago. At the end
of the Cretaceous Period, more than
lar System when Venus or Mercury transits the Sun, a black circular 1.1 times the radius of the Sun, or 1.1 3 (7.0 3 105 km) 5 7.7 3 105 km.
50 percent of all living species, includ- disk is visible on the face of the circular Sun. During the transit, the The light from planet Kepler-11c is observed to decrease by 0.077 per-
ing the dinosaurs, became extinct. This
mass extinction is marked in Earth’s
amount of light from the transited star is reduced by the area of the cent, or 0.00077 (see Figure 7.19). What is Kepler-11c’s size?
fossil record by the Cretaceous-Tertiary circular disk of the planet divided by the area of the circular disk of
boundary, or K-T boundary (the K comes
R2Kepler-11c R2Kepler-11c
the star: 0.00077 5 5
from Kreide, German for “Cretaceous”). Figure 8.30 This artist’s rendition depicts an asteroid or comet, perhaps 10 km R2star 17.7 3 105 km2 2
Fossils of dinosaurs and other now- across, striking Earth 65 million years ago in what is now the Yucatán Peninsula in
extinct life-forms are found in older Area of disk of planet
layers below the K-T boundary. Fossils
Mexico. The lasting effects of the impact might have killed off most forms of
Percentage reduction in light 5 R2Kepler-11c 5 4.5 3 108 km2
in the newer rocks above the K-T
terrestrial life, including the dinosaurs.
Area of disk of star
boundary lack more than half of all surveys and rocks from drill holes in cold and dark “impact winter.” Recent RKepler-11c 5 2.1 3 104 km
previous species but contain a record this area show a deeply deformed sub- measurements of ancient microbes in pR2planet R2planet
of many other newly evolving species. surface rock structure, similar to that ocean sediments suggest that Earth 5 5 Dividing Kepler-11c’s radius by the radius of Earth (6,400 km) shows
Big winners in the new order were seen at known impact sites. These re- may have cooled by 7°C. The fire-
pR2star R2star
the mammals—distant ancestors of sults provide compelling evidence that storms, temperature changes, and de- that the planet Kepler-11c has a radius of 3.3 R Earth.
humans—that moved into ecological 65 million years ago, an asteroid about creased food supplies could have led to
niches vacated by extinct species. 10 km in diameter struck the area, a mass starvation that would have been
How do scientists know that an im- throwing great clouds of red-hot dust especially hard on large animals such
pact was involved? The K-T boundary and other debris into the atmosphere as the dinosaurs.
is marked in the fossil record in many (Figure 8.30) and possibly igniting a Not all paleontologists believe
areas by a layer of clay. Studies at more worldwide conflagration. The energy that this mass extinction was the re-

Each chapter concludes with an Origins section, which relates material or

than 100 locations around the world of the impact is estimated to have been sult of an impact; some think volcanic
have found that this layer contains more than that released by 5 billion activity was important as well. How-
Planets can be More than a thousand extrasolar planets have been detected
large amounts of the element iridium, nuclear bombs. ever, the evidence is compelling that
distinguished by: from ground-based and space telescopes using the transit method.
subjects found in the chapter to•• questions about the origin of the universe and the
as well as traces of soot. Iridium is very An impact of this energy clearly a great impact did occur at the end
Different periods
rare in Earth’s crust but is common in
meteorites. The soot at the K-T bound-
would have had a devastating effect on
terrestrial life. In addition to the pos-
of the Cretaceous Period. Life on our
planet has had its course altered by Different depths Current ground-based technology limits the sensitivity of the tran-
ary possibly indicates that widespread
fires burned the world over. The thick-
ness of the layer of clay at the K-T
sible firestorm ignited by the impact,
computer models suggest there would
have been earthquakes and tsunamis.
sudden and cataclysmic events when
asteroids and comets have slammed
into Earth. It seems very possible
origin of life in the universe and• Different
on Earth.
durations
Astrobiologists have made much prog-
sit method to about 0.1 percent of a star’s brightness. Amateur as-
tronomers have confirmed the existence of several extrasolar
boundary and the concentration of
iridium increases toward what is today
Dust from the collision and soot from
the firestorms thrown into Earth’s up-
that we owe our existence to the
luck of our remote ancestors—small ress
1.0
in recent years on understanding how conditions planets in the
by observing transits universe
using charge-coupledmaydevicehave
(CCD)
the Yucatán Peninsula in Mexico. Al- per atmosphere would have remained rodent-like mammals—that could live cameras mounted on telescopes with apertures as small as 20 cen-
helped or hindered the origin of life, and in each Origins we explore
improve thean example
Relative brightness

though the original crater has largely there for years, blocking out sunlight amid the destruction after such an im-
been erased by erosion, geophysical and plunging Earth into decades of a pact 65 million years ago. timeters (cm). Telescopes in space sensitivity because
smaller dips in brightness can be measured. The small French
from
0.99
its chapter that relates to how the universe and life formed and evolved.
COROT telescope (27 cm) discovered 32 planets during its 6 years
of operation (2007–2013). NASA’s 0.95-meter Kepler telescope has
Transit durations are greatly exaggerated
discovered many planets and has found thousands more candidates
09_KAY_93899_ch8_200-233.indd 227 9/29/15 12:53 PM that are being investigated further. Figure 7.20 illustrates how
0.98
0 20 40 60 80 100 multiplanet systems are identified with this method: if one planet is
Time (days) found, then observations of the variations in timing of the transit
Figure 7.20 Multiple planets can be detected by multiple transits with can indicate that there are other planets orbiting the same star.
different brightness changes. The arrows point to the changes in the
 mass (2.33 MJup) are known. The density provides a clue about
the science goals? Have some planets been found?
whether the object is gaseous or rocky.
a. What is the mass of this planet in kilograms? 49. Citizen science projects:
b. What is the planet’s radius in meters? a. Go to the “PlanetHunters” website at
c. What is the planet’s volume? http://planethunters.org. PlanetHunters is part of the
d. What is the planet’s density? How does this density com- Zooniverse, a citizen science project that invites individu- P R E FAC E xxiii
pare to the density of water (1,000 kg/m3)? Is the planet als to participate in a major science project using their own
likely to be rocky or gaseous? computers. To participate in this or any of the other Zooni-
verse projects mentioned in later chapters, you will need to

At the end of each chapter, we have pro- Using the Web

sign up for an account. Read through the sections under

EXPLORATION
“About,” including the FAQ. What are some of the advan-

vided several types of questions, problems, Exploring Extrasolar Planets

tages to crowdsourcing Kepler data analysis? Back on the
PlanetHunters home page, click on “Tutorial” and watch
digital.wwnorton.com/astro5
the “Introduction” and “Tutorial Video.” When you’re
and activities for you to practice your skills.
46. Go to the “Extrasolar Planets Global Searches” Web page
(http://exoplanet.eu/searches.php) of the Extrasolar Planets ready toVisit
trythe
looking for planets, click on “Classify” and be-
Student Site at the Digital Landing Page, and open the Exo- 8 When is the star moving fastest: when the planet is close to it or
Encyclopedia. Click on one ongoing project under “Ground” gin. Save a copy
Radialof yourSimulator
stars for your homework.
The Test Your Understanding questions focus
planet Velocity in Chapter 7. This applet has a num- when the planet is far away?
ber of different panels that allow you to experiment with the variables
and one ongoing project under “Space.” What method is used b. Go to the “Disk Detective” website at http://www
that are important for measurement of radial velocities. First, in the
to detect planets in each case? Has the selected project found .diskdetective.org/, another Zooniverse
window labeled “Visualization Controls,” checkproject
the box tofor
showwhich
mul-

on more detailed facts and concepts from the

9 Explain how an astronomer would determine, from a radial ve-
any planets, and if so, what type are they? Now click on one of you willtiple
need toCompare
views. make the an views
account
shownas in part
in panels (a).the
1–3 with Incolored
this proj- locity graph of the star’s motion, whether the orbit of the planet was in
arrows in the last panel to see where an observer would stand to see a circular or elongated orbit.
the future projects. When will the one you chose be ready to ect, youthe
will
viewlook
shown.atStart
observations ofthe
the animation (in young stars
“Animation to see if
Controls”

chapter. Thinking about the Concepts ques- begin? What will be the method of detection? there is panel),
“Science”
evidence foritatoplanetary
and allow run while you disk. Under
watch the “Menu,”
planet orbit its star read
from each of the views shown. Stop the animation, and in the “Pre-
and
sets” “About,”
panel, and
select “Option A” then “Classify.”
and then click “set.” Work through
47. Using the exoplanet catalogs:
tions ask you to synthesize information and
10 Study the “Earth view” panel at the top of the window. Would
an example, and then
1 Is Earth’s view ofclassify
this systemamost
fewnearly
images.
a. Go to the “Catalog” Web page (http://exoplanet.eu/catalog) like the “side view” or this planet be a good candidate for a transit observation? Why or why
most nearly like the “orbit view”? not?
of the Extrasolar Planets Encyclopedia and set to “All Plan- 50. Go to the “Super Planet Crash” Web page (http://www
explain the “how” or “why” of a situation. ets detected.” Look for a star that has multiple planets.
Make a graph showing the distances of the planets from
.stefanom.org/spc/ or http://apod.nasa.gov/apod/ap150112
2 Is the orbit of this planet circular or elongated?
.html). Read “Help” to see the rules. First build a system like
Applying the Concepts problems give you a
In the “System Orientation” panel, change the inclination to 0.0.
that star, and note the masses and sizes of the planets. Put ours with four Earth-sized planets in the inner 2 AU—is this
3 Study the radial velocity graph in the upper right panel. The blue 11 Now is Earth’s view of this system most nearly like the “side
the Solar System planets on the same axis. How does this stable? What happens
curve if you
shows the radial addofinthesuper-Earths
velocity or “ice
star over a full period. gi-
What is view” or most nearly like the “orbit view”?

chance to practice the quantitative skills you extrasolar planet system compare with the Solar System?
b. Go to the “Exoplanets Data Explorer” website (http://
ants”? Build theup a fewradial
maximum completely different
velocity of the star?

and see what happens. What types of situations cause insta-

planetary systems
12 How does the radial velocity of the star change as the planet

learned in the chapter and to work through a

exoplanets.org) and click on “Table.” This website lists bility in the4 inner 2 AU of
The horizontal axisthese systems?
of the graph shows the “phase,” or fraction of orbits?
planets that have detailed orbital data published in scientific the period. A phase of 0.5 is halfway through a period. The vertical
red line indicates the phase shown in views in the upper left panel.
journals, and it may have a smaller total count than the web-
situation mathematically. The Using the Web
Start the animation to see how the red line sweeps across the graph as
site in part (a). Pick a planet that was discovered this year or the planet orbits the star. The period of this planet is 365 days. How
many days pass between the minimum radial velocity and the maxi-
last, as specified in the “First Reference” column. What is
questions and Explorations represent other
mum radial velocity? 13 Click the box that says “show simulated measurements,” and
the planet’s minimum mass? What is its semimajor axis and change the “noise” to 1.0 m/s. The gray dots are simulated data, and
the period of its orbit? What is the eccentricity of its orbit? If your instructor assigns homework in Smartwork5, access your the blue line is the theoretical curve. Use the slider bar to change the

opportunities to “learn by doing.” Using the

5 When the planet moves away from Earth, the star moves toward inclination. What happens to the radial velocity as the inclination in-
Click on the star name in the first column to get more assignments atEarth.
digital.wwnorton.com/astro5.
The sign of the radial velocity tells the direction of the motion creases? (Hint: Pay attention to the vertical axis as you move the slid-
(toward or away). Is the radial velocity of the star positive or negative er, not just the blue line.)
at this time in the orbit? If you could graph the radial velocity of the

Web sends you to websites of space missions, observatories, experiments, or ar- planet at this point in the orbit, would it be positive or negative?

chives to access recent observations, results, or press releases. Other sites are for In the “Presets” window, select “Option B” and then click “set.”

6 What has changed about the orbit of the planet as shown in the 14 What is the smallest inclination for which you would find the

“citizen science” projects in which you can contribute to the analysis of new data.
views in the upper left panel? data convincing? That is, what is the smallest inclination for which the
theoretical curve is in good agreement with the data?
08_KAY_93899_ch7_172-199.indd 198 9/17/15 3:22 PM

Explorations show you how to use the concepts and skills you learned in an 7 When is the planet moving fastest: when it is close to the star or
when it is far from the star?

interactive way. Most of the book’s Explorations ask you to use animations and
simulations on the Student Site, while the others are hands-on, paper-and-pencil Student Site : digital.wwnorton.com/astro5 199

activities that use everyday objects such as ice cubes or balloons. 08_KAY_93899_ch7_172-199.indd 199 9/17/15 3:22 PM

The resources outside of the book (at the Student Site) can help you understand
and visualize many of the physical concepts described in the book. AstroTours
and Nebraska Simulations are represented by icons in the margins of the book.
There is also a series of short Astronomy in Action videos that are represented
by icons in the margins and available at the Student Site. These videos feature one
of the authors (and several students) demonstrating physical concepts at work.
Your instructor might assign these videos to you or you might choose to watch
them on your own to create a better picture of each concept in your mind.
Astronomy gives you a sense of perspective that no other field of study offers.
The universe is vast, fascinating, and beautiful, filled with a wealth of objects
that, surprisingly, can be understood using only a handful of principles. By the
end of this book, you will have gained a sense of your place in the universe.

AstroTour

Astronomy in Action Nebraska Simulation

xxiv P reface

Dear Instructor
We wrote this book with a few overarching goals: to inspire students, to make
the material interactive, and to create a useful and flexible tool that can support
multiple learning styles.
As scientists and as teachers, we are passionate about the work we do. We
hope to share that passion with students and inspire them to engage in science
on their own. Through our own experience, familiarity with education research,
and surveys of instructors, we have come to know a great deal about how students
learn and what goals teachers have for their students. We have explicitly ad-
dressed many of these goals and learning styles in this book, sometimes in large,
immediately visible ways such as the inclusion of features but also through less
obvious efforts such as questions and problems that relate astronomical concepts
to everyday situations or a fresh approach to organizing material.
For example, many teachers state that they would like their students to be-
come “educated scientific consumers” and “critical thinkers” or that their stu-
dents should “be able to read a news story about science and understand its sig-
nificance.” We have specifically addressed these goals in our Reading Astronomy
News feature, which presents a news article and a series of questions that guide a
student’s critical thinking about the article, the data presented, and the sources.
In nearly every chapter, we have Visual Analogy figures that compare astrono-
my concepts to everyday events or objects. Through these analogies, we strive to
make the material more interesting, relevant, and memorable.
Education research shows that the most effective way to learn is by doing.
Exploration activities at the end of each chapter are hands-on, asking students to
take the concepts they’ve learned in the chapter and apply them as they interact
with animations and simulations on the Student Site or work through pencil-
and-paper activities. Many of these Explorations incorporate everyday objects
and can be used either in your classroom or as activities at home. The Using the
Web problems direct students to “citizen science” projects, where they can con-
tribute to the analysis of new astronomical data. Other problems send students
to websites of space missions, observatories, collaborative projects, and catalogs
to access the most current observations, results, and news releases. These Web
problems can be used for homework, lab exercises, recitations, or “writing across
the curriculum” projects.
We also believe students should be exposed to the more formal language of
science—mathematics. We have placed the math in Working It Out boxes, so it
does not interrupt the flow of the text or get in the way of students’ understanding
of conceptual material. But we’ve gone further by beginning with fundamental
ideas in early Working It Out boxes and slowly building in complexity through
the book. We’ve also worked to remove some of the stumbling blocks that affect
student confidence by providing calculator hints, references to earlier Working It
Out boxes, and detailed, fully worked examples. Many chapters include problems
on reading and interpreting graphs. Appendix 1, “Mathematical Tools,” has also
been reorganized and expanded.
Discussion of basic physics is contained in Part I to accommodate courses that
use the Solar System or Stars and Galaxies volumes. A “just-in-time” approach to
introducing the physics is still possible by bringing in material from Chapters 2–6
as needed. For example, the sections on tidal forces in Chapter 4 can be taught
along with the moons of the Solar System in Part II, or with mass transfer in
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My Sorrow diligent would sweep
That dingy room infest
With dust (thereby I mean my soul)
Because she hath a Guest
Who doth require that self-same room
Be garnished for His rest.

And Sorrow (who had washed His feet

Where He before had been)
Took the long broom of Memory
And swept the corners clean,
Till in the midst of the fair floor
The sum of dust was seen.

It lay there, settled by her tears,

That fell the while she swept—
Light fluffs of grey and earthy dregs;
And over these she wept,
For all were come since last her Guest
Within the room had slept.

And, for nor broom nor tears had power

To lift the clods of ill,
She called one servant of her Guest
Who came with right good will,
For, by his sweet Lord’s bidding, he
Waiteth on Sorrow still;

Who, seeing she had done her part

As far as in her lay
And had intent to keep the place
More cleanly from that day,
Did with his Master’s dust-pan come
And take the dust away.
She thankèd him, and Him who sent
Such succour, and she spread
Fair sheets of Thankfulness and Love
Upon her Master’s bed,
Then on the new-scoured threshold stood
And listened for His tread.
EPITAPH ON A CHILD
RUN OVER AND KILLED BY A

MOTOR-CAR IN THE STREET

Here lies A. B. who, four years from her birth,
Found there was nowhere left to play on earth.
Strange, for her mother’s child had ever grown
In the quaint precincts of a country town,
Yet was she one whose small predestined feet
Learnt nor forgot to walk upon the street.
She might not ramble where the farmer spanned
With consecrated quickset all his land
To fill her pinafore when mushrooms swell;
Nor dare she scale the lovely citadel
Of brambles in the lane, for their sweet prize
Was spoilt with dust that dimmed the children’s eyes
When local gods dispersed the timid crowd
And went before in pillars of grey cloud.
Nor might a bigger child frequent the edge
Of the pebbled stream to plait the flowering sedge,
For aught of native life was kept without
The chosen haunt of Dives and his trout;
His pheasants held the coppice and its nuts,
Where bearded men played peep behind their butts
And wolvish keepers prowling through the woods
Had a short way with all Red Riding Hoods.
No blade of wholesome grass shot through the hard
And greasy flagstones of the narrow yard
At home, nor might the children ever play
Through the allotments where, a mile away,
The civic cabbages congested stood,
Reluctant tenants of a stony rood.
One playground, one alone, for such as she,
Had planned a grave adult humanity,
There where grey asphalt hid the ruder ground
And serried spikes begirt the place around;
At the one end, of yellow brick and slate,
Was reared a sort of female Traitors’ Gate,
At t’other end the piety of a nation
Had raised a shrine of tin to sanitation.
This, thanks to man, was all the children’s share
And Nature was allowed to tender air.
Hence did it chance (as now and then it may)
The Powers that Be decreed a holiday.
And reckless childhood, whom it ever galls
To sit within the compass of four walls,
Loosed from its wonted pen conspired to run
At random through the town beneath the sun,
Rashly disporting in the common street
Its rude hands and unnecessary feet.
That day, so many a hooting corner crost,
The marvel is that one alone was lost,
She to whom poverty no tomb assigns
But a low mound and these unworthy lines.—
Mourn not at all that Her whose burnished wing
Flies on the blissful errands of her King,
Whom (by a heavenly law too young to err,
Accounted on the earth a Trespasser)
He hath resumèd and her footfall white
Enfranchised of the liberties of light:
But for all those who play the part of Fate
To engineer this poor and mirthless state
Weep,—and for all who loved that childish hair
And saw it stained with Tragedy—one prayer.
 THE WATER-MEADS OF
MOTTISFONT

On the painted bridge at Mottisfont above the Test I’ve stood

Where the dab-chick from a rushy raft directs her little brood,
Where fringed with sedge and willow-weed the waters spread about
And linger in pellucid glooms the sleepy spotted trout.

I’ve seen the tawny tumult of the headlong Highland spate,

And the ebb round Hair-brush Island (which the map calls Chiswick
Ait)
Where the withy bristles shimmer and the purple mud-banks gleam
And the lights come out by Thornycroft’s and glisten in the stream.

’Twere good to be at Abergeirch: the little brook again

Greets the brine among the shingle on the beetling coast of Lleyn,—
O the shallows on the sand-banks where the dozing flat-fish lie
And the heather surging inland till it breaks against the sky!

But the chalky scaurs of Compton hold the shadows; and between
Lie the water-meads of Mottisfont enamelled with such green
As discolours all I’ve looked upon in valleys far apart—
For the water-meads of Mottisfont lie nearest to my heart.
THE SENIOR MISTRESS OF BLYTH
[“Blyth Secondary School.—The Governors of
the above School invite applications for the post
of Senior Mistress. Candidates must be
Graduates in Honours of a British University and
must be well qualified in Mathematics, Latin,
and English. Ability to teach Art will be a
recommendation.”—Advertisement in The
Spectator.]
It is told of the painter Da Vinci,
Being once unemployed for a span,
At the menace of poverty’s pinch he
Sought work at the Court of Milan.
Having shown himself willing and able
To perform on the curious lyre,
He presented the Duke with a table
Of the talents he proffered for hire.

“I can raze you a fortress,” it ran on,

“Quell castles, drain ditches and moats,
Make shapely and competent cannon,
Build aqueducts, bridges and boats;
In peace I can mould for your Courts a
Few models in marble or clay
And paint the illustrious Sforza
With anyone living to-day.”

Leonardo is dead, they asseverate,

He has left no successor behind,
For the days of the specialist never rate
At its value the versatile mind.
Is Lord Brougham, then, our latest example?
No, Time, the old churl with his scythe,
Shall spare us a notable sample
In the Senior Mistress of Blyth.

She shall guide Standard Three through Progressions,

Study Statics and Surds with the Fourth,
She shall dwell on De Quincey’s Confessions,
Donne, Caedmon and Christopher North;
And no class-room shall boast of a quicker row
When her classical pupils rehearse
Their prose, which is modelled on Cicero,
And their more than Horatian verse
And their more than Horatian verse.

She shall lead them to love Cimabue,

To distinguish with scholarship ripe
’Twixt the texture of Clausen and Clouet,
And the values of Collier and Cuyp.
Nay, all Blyth shall reflect her ability
As its brushes acquire by her aid
Or South Kensington’s pretty facility
Or the terrible strength of the Slade.

Yes, her duties are diverse, and this’ll

Suggest to each candidate why
They should read Leonardo’s epistle
Before they sit down to apply;
For his style is itself a credential
Though truly he has not a tithe
Of the qualifications essential
To the Senior Mistress of Blyth.
THE FIRST PARTY
Follow, my Betsey-Jane, as best you can,
Clutching your Mother’s fingers in firm hold,
The sable progress of the serving-man,
Nor stumble on your shawl’s imperial fold;
Whose ceremonious pin of jade and gold
Bringeth such rosy awe into your face
As the white frock, the stockings silken-soled
And the white shoes (with pompons) which will grace
The lightness of your feet in this illumined place.

Shawls being shed, descend the ample stair

And greet our Hostess. Now you’re set to see
The Conjurer, nor think to leave your chair
For safer eyrie of your Mother’s knee;—
Still, as his tricks are tedious to Three
And strange the flounce-clad children in their tiers,
Turn your shy back on wiles and wizardry
To hug, for comfort’s sake, two homely bears
And a prepost’rous poodle, white with knitted ears.

For tea, gramercie to a thoughtful choice

And nice derangement of the chairs, your seat
Faces a fair acquaintance known as Joyce;—
What glances under glossy tresses greet
The fellow-connoisseur of cake and sweet
Till the last cracker’s pulled on the last plate.
Now sidle through the dancers’ tortuous feet
And come at last, for the time waxes late,
Where in their cloudy breath the shadowy horses wait.

Glow the two tawny lanterns on the hedge,

Gleam the ungainly boughs the window blurs,
And Betsey nodding on the seat’s soft edge
Holds to her heart those pompon’d shoes of hers;
Till in my arms most spent of revellers
Till in my arms, most spent of revellers,
I lift her slumb’ring whom nor lifting grieves
Nor sudden stay nor the cold night wind stirs,
Borne up the path through fragrance of box-leaves,
Up to her drowsy cot under dependent eaves.
SOUVENIR OF MICHAEL DRAYTON

Scarce hath the crookèd scythe

Duly been whetted
When all the mowers blithe
(By the storm letted,
Crouching the shed beneath
At the field’s margent)
See the first fallen swathe
Pelted with argent.
White mist the valley blurs,
White the horizon,
Since the cloud skirmishers
Sent their first spies on.
Haste away,
Waters grey,
Spare of your shedding,
Till we bestow our hay
Safe in the steading.
 II
Gild, sun, the pendent leaves
Silverly dripping,
Call the swifts from the eaves
Screaming and dipping,
Raise the green docks that be
To the ground beaten,
All the washed earth we see
Comfort and sweeten;
Till at soft interval
On the small flowers,
Drops from the thatch-ends fall—
Spent are the showers.
Haste away,
Waters grey,
Spare of your shedding,
Till we bestow our hay
Safe in the steading.
 III
Soon may the whisp’ring blade
Bow the grey grasses,
Lo, the lush edge unfrayed
Where the scythe passes!
All with a stately speed
Shorn and soft whistle
Muted on nought of weed,
Burdock nor thistle.—
Grace hath possessed the sky,
Hope hath o’er-spanned it,
Parteth he hurriedly,
Storm, the black bandit.
Haste away,
Waters grey,
Spare of your shedding,
Till we bestow our hay
Safe in the steading.
“FOUR-PAWS”
Four-paws, the kitten from the farm,
Is come to live with Betsey-Jane,
Leaving the stack-yard for the warm
Flower-compassed cottage in the lane,
To wash his idle face and play
Among chintz cushions all the day.

Under the shadow of her hair

He lies, who loves him nor desists
To praise his whiskers and compare
The tabby bracelets on his wrists,—
Omelet at lunch and milk at tea
Suit Betsey-Jane and so fares he.

Happy beneath her golden hand

He purrs contentedly nor hears
His Mother mourning through the land,
The old grey cat with tattered ears
And humble tail and heavy paw
Who brought him up among the straw.

Never by day she ventures nigh,

But when the dusk grows dim and deep
And moths flit out of the strange sky
And Betsey has been long asleep—
Out of the dark she comes and brings
Her dark maternal offerings;—

Some field-mouse or a throstle caught

Near netted fruit or in the corn,
Or rat, for this her darling sought
In the old barn where he was born;
And all lest on his dainty bed
Four-paws were faint or under-fed.
Only between the twilight hours
Under the window-panes she walks
Shrewdly among the scented flowers
Nor snaps the soft nasturtium stalks,
Uttering still her plaintive cries
And Four-paws, from the house, replies,

Leaps from his cushion to the floor,

Down the brick passage scantly lit,
Waits wailing at the outer door
Till one arise and open it—
Then from the swinging lantern’s light
Runs to his Mother in the night.
“FOUR-PAWS” IN LONDON
Four-paws, we know the sun is white
At dawn in Hampshire when the night
Deserts those frozen miles,
When robin creaks from wintry bush
And early milk-boy’s breeches brush
The hoar-frost from the stiles;

Yet shall you never hear him more

Insistent at our cottage door
Nor of his spoils partake,
Alas, poor puss who stir and yawn
Uneasy in the London dawn
And, in a flat, awake.

Four-paws, forgive us! When apprised

Of our departure you devised,
No doubt, some darling plan
Of exodus that should surpass
His who removed last Michaelmas—
Your friend the dairy-man:—

A mightier waggon on the road

You pictured and so vast a load
That all should turn and look,—
Betsey precarious on the shaft,
Master and Mistress fore and aft,
The carter and the cook,

Nurse, with her knitting, in mid-air,

Carpets in bales, your favourite chair
And (the progressive path
With added glory to invest)
Our Four-paws couchant on the crest
Of an inverted bath.
Alas, what difference disgraced
Our flight! An obscure van replaced
The customary wain;
And you, with many a mournful cry,
Fettered by Betsey in the fly
And hampered in the train.

And now you’re here. Well, it may be

The sun does rise in Battersea
Although to-day be dark,
Life is not shorn of loves and hates
While there are sparrows on the slates
And keepers in the Park:

And you yourself will come to learn

The ways of London and in turn
Assume your cockney cares,
Like other folk who live in flats,
Chasing your purely abstract rats
Upon the concrete stairs.
TO MY SISTER DOROTHY,
A PASTE BROOCH

Time, cunning smith, hath set you in my heart

Like stones in silver none may wrest apart;
Not counterfeit as these our loves shall stay
When sullen-footed Time hath paced away.
SESTINA
TO D. E.
I saw myself encircled in the grey
Of your grey eyes, Dear Love, as in a glass;
In place of lurking glooms I come their way
As idle ghosts through magic mirrors pass
Or shifty clouds bewilder a spring day
Or windy shadows dusk the summer grass.

And as swift sickles lop the hedge-row grass,

As ghosts scent out the dawn with faces grey
And flee before the stirring feet of day,
As magic shivers in a splintered glass,
So all the shaken pictures of me pass
Even with the moving of your head away.

Yet would your head be ever turned my way,

Only our peace is fugitive as grass:—
Beyond the clapping lintels footsteps pass,
Shake the snared joy from quiet’s cobweb grey—
O who drinks silence from a jolted glass,
Who deals in stillness on a market-day?

Our joys go begging for a gentle day,

They are swayed as weed-stems in a water-way,
Hurt as blind lips that drain a broken glass,
Blown down by breath as petals flung on grass,
Thinned as gold hair dull sorrow braids with grey,
Lopped short as willow-tufts where cattle pass.

This noisy horde of minutes never pass,

This patchwork crew;—they throng us day by day,
Hint of silk linings to their cloaks of grey,
Cleave out strong-elbowed their ungentle way,
Bruise the poor joy as legions tread the grass,
Or as wet fingers rub a moaning glass.
There is no day ringed round with seas of glass,
No island day, where like-faced minutes pass
Fingered on gathered mouth through breathless grass
With close-girt garment lest the bloom of day
Be brushed or pollen spilt along their way,—
Or lest my face be shook from your eyes’ grey.

O dear grey eyes, though ruder minutes pass

And dusk the glass, your heart is turned my way
Wherein all day my face springs up like grass.
LULLABY FOR A LITTLE GIRL

Now candle-flames disperse the rout

Of shadows and their giant wars;
And though the roof of night without
Be spanned with dusk and set with stars,
’Tis lullaby,
The elm-tops cry,
And lullaby, the leaves that pass
In stealth across the window-glass.

The comb shall sleek your drooping head

And through the darling tangles go
And all your night attire is spread
Before the fire to face the glow,
And lullaby,
The cinders sigh,
For ev’ry rosy palace gone,
Fall’n in their dwarfish Ilion.

Now rest, your prayers said aright

And timely supped your milky bowl,
Your little body all as white
And sweet as your unsoilèd soul;
And lullaby,
Her melody,
Who from the quilted bedside goes,
A-tiptoe, when your eye-lids close.
RONDEAU OF SARUM CLOSE

In Sarum Close, when she had said her say,

He stood bare-headed where dim vapours lay
Heavy on vacant lawn, athwart the stone
Of that great pile that stands unsought, alone,—
Himself as still and derelict as they.
Here, when morn’s gleaming hand had rolled away
From the green plot of this their week-old play
Her misty curtain, each to each was shown,
In Sarum Close.

Void the discoloured fane before him lay,

Void the dark-sodded precincts,—far away
One closed a window, night’s appeal had grown
Perchance too urgent, even as his own
Had seemed to her whose friendship did with day
In Sarum close.
 THE KNOBBY-GREEN

O thou who ’neath the umbrageous trees

That line the Avenue Louise
Did’st spread in Belgian sun and breeze
Thy buds about,
I come to weep thy destinies
My Brussels Sprout:

Who, on this drear December day,

Rearest above mine Essex clay
Thy wand of buds as green as they
Who spend their Yule
Hearing remoter church-bells play
In St. Gudule.

Hail, noble alien, I see

Thou bear’st in exile and for me
A neat-curl’d row of progeny,
(Not all unlike
Some purse-proud donor’s family,
By John van Eyck)

For me unmindful of thy place

(Comrade of carpets and of lace)
Who class thee with the vulgar race
Of Beet and Bean,
And call thee—to thy very face—
The Knobby-green.
 THE CARCANET

The world’s a quarry for whose spoils

Love, the untiring miner, toils
Early and late, such stones to get
As may be cut devised and set
Into his mistress’ carcanet.

Alack that love can never choose

But bring thee pebbles of no use:—
Glance at the gift and thou shalt see
Each facet in his treasury
Of stones doth but diminish thee.
 TO A TOWN CRIER
“Whiffin, proclaim silence!”—Pickwick

Whiffin, with all thy faults, I love thee still,

Thee and thine ancient office and the sweet
Metallic peal that quelled the popular heat
When party strife ran high in Eatanswill;
Who now with quavering eloquence would’st fill,
And tidings of a pilfered purse, the street
Maddened with motors and the armoured fleet
Of base mechanical engines out to kill.
Go, thou sole arbiter of Buff and Blue,
Time hath prevailed against thee, yield the floor,
Toll, on bare sufferance, from door to door,
The hooters hold the highway;—as for you,
You voice the missing ha’pence of the poor,
And they the incomes of the well-to-do.