Essential Astrophysics

Undergraduate Lecture Notes in Physics

Essential

Astrophysics

Kenneth R. Lang

Undergraduate Lecture Notes in Physics

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Kenneth R. Lang

Essential Astrophysics

123

Kenneth R. Lang

Department of Physics and Astronomy

Tufts University

Medford, MA

USA

ISSN 2192-4791 ISSN 2192-4805 (electronic)

ISBN 978-3-642-35962-0 ISBN 978-3-642-35963-7 (eBook)

DOI 10.1007/978-3-642-35963-7

Springer Heidelberg New York Dordrecht London

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Springer-Verlag Berlin Heidelberg 2013

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Preface

Essential Astrophysics is a book to learn or teach from, as well as a fundamental

reference for anyone interested in astronomy and astrophysics. This unique volume

can be used as a textbook, teaching guide, or reference source for just about

anyone interested in astronomy and astrophysics.

It serves as a comprehensive, introductory text, which takes the student through

the field of astrophysics in lecture-sized chapters of basic physical principles

applied to the cosmos. Undergraduate students with an interest in the physical

sciences, such as astronomy, chemistry, engineering, or physics, will enjoy this

one-semester overview.

The text is of sufficient breadth and depth to prepare the interested student for

more advanced, specialized courses in the future. The clarity and comprehensive

nature of Essential Astrophysics make it a significant resource for the curious

reader that is unfamiliar with astrophysics or for professional astronomers who

may have forgotten the basics.

Astronomical examples are provided throughout the text, to reinforce the basic

concepts and physics, and to demonstrate the use of the relevant formulae. In this

way, the student learns to apply the fundamental equations and principles to

cosmic objects and situations. All of the example problems are solved with the

rough accuracy needed to portray the basic result. Such order-of-magnitude esti￾mates are commonly used in astronomy and astrophysics, where large numbers are

involved, and an understanding of the underlying physics does not require engi￾neering accuracy.

Essential Astrophysics is a serious introduction to astrophysics complete with

the necessary formulae. These equations sometimes include the calculus of inte￾gration, or adding up, and differentiation, that are found in the author’s classic

Astrophysical Formulae and more advanced textbooks. Nevertheless, the end

result in Essential Astrophysics is always a simple algebraic relationship that can

be applied to cosmic objects. These fundamental equations are given in the text

and collected at the end of the book in Appendix III, for future reference and use.

Therefore, only elementary algebra is required to solve any of the example

problems or other numerical conclusions in this book.

v

There are two types of intended readers. One type will be interested in broad,

general conclusions, without use of calculus. This reader will be content with the

existing text with no further elaboration. The more mathematically competent

reader will want to use Essential Astrophysics as a foundation for more advanced

considerations, with the guidance of the references, an instructor, or an advanced

textbook, using the formulae found in the text or within set aside Focus Elements

of Essential Astrophysics as a starting point.

The modern SI (International System) units are used in the equations and

example problems, which is another unique aspect of this book when compared to

most previous texts of astrophysics. A conversion table between the SI and c.g.s.

units is provided in the first chapter, to help the reader follow the details of many

papers and textbooks that use the older c.g.s. system. Astronomical and physical

constants, units, and fundamental equations are provided in appendices, for quick

reference.

Essential Astrophysics goes beyond the typical textbook by providing com￾prehensive access to astrophysical discoveries, concepts, and facts that are not

available in any other way. It gives us access to that long-forgotten formula, idea,

or reference, while also providing the material needed to introduce anyone to a

new area of astrophysics. Here, the reader can obtain the background required for a

general understanding and find guidance to the relevant literature including sem￾inal discoveries, original research, and comprehensive up-to-date reviews that will

enable the curious reader to delve deeper into a particular topic. A more extensive

reference compilation of developments in astrophysics, from then to now, can be

found in Astrophysical Formulae.

We are the benefactors of 300 years of cumulative discovery in astronomy and

astrophysics, and Essential Astrophysics helps pass on these fundamental insights

to the next generation. It also reveals both the exciting moments of the past and

relatively recent discoveries. Historical aspects are illuminated through a pro￾gressive flow of chapter topics and by guidance to the earliest ideas, with reference

to the original sources as well as contemporary reviews. Perhaps because of the

rapid pace of modern research, contemporary texts often focus on specialized

topics and overlook these broader perspectives that Essential Astrophysics

provides.

There are 50 set-aside focus elements that enhance and amplify the discussion

with fascinating details. They include the intriguing development of particular

themes, which is missing in most astrophysics textbooks, or provide further

astrophysics or equations for use in examples, problems or further investigations.

In Essential Astrophysics we can rediscover basic physical concepts such as

space, time, radiation, mass, gravity, motion, heat, atoms, radioactivity, and cos￾mic rays, which are required to understand the observable universe. These fun￾damental topics are discussed in the first seven chapters, beginning with the

introductory chapter that describes how astronomers observe the contents of the

universe and how astrophysicists interpret them. The SI units of distance, mass,

time, energy, and luminosity are introduced, together with their astronomical units

vi Preface

such as the Ångström, light-year, parsec, and the Sun’s mass, luminosity, and

radius. The magnitude unit is also defined, but used sparingly in examples.

Chapter 2 describes radiation, of both the visible and invisible sort, which

carries messages from the cosmos and tells us much of what we know about it.

Chapter 3 discusses gravity, together with mass that helps determine its strength,

and related tidal phenomena and space curvature. Chapter 4 discusses cosmic

motion, and its balanced equilibrium with gravitation. Chapter 5 discusses the

motion of particles in a gas, together with the related concepts of speed distri￾bution, heat, temperature, and pressure. The inside of the atom is explored in

Chap. 6, where the reader learns about atomic spectral lines and their use in

determining the composition of stars and the measurement of motions and mag￾netic fields. The transformation of elements in both radioactivity and by sub￾atomic bombardment is presented in Chap. 7.

The fundamental concepts described in these first seven chapters provide a

necessary prelude to the rest of the book. It includes the discoveries that the

universe is predominantly hydrogen; that the stars shine by nuclear fusion; that the

stars live and die while new ones continue to be formed; that the interstellar spaces

are not empty but filled with dust, atoms, and molecules; and that the observable

universe is expanding and has a history. The last half of Essential Astrophysics

also includes relatively recent discoveries, such as pulsars, black holes, the three￾degree cosmic microwave background, the formation of stars and galaxies,

invisible dark matter, and the dark energy that is now accelerating the expansion of

the universe.

Chapter 8 provides an account of the nuclear fusion reactions that make the Sun

shine. This is followed in Chap. 9 by modern discoveries of the Sun’s expanding

atmosphere, the solar winds, explosions on the Sun, the solar flares and coronal

mass ejections, and their space–weather threats to spacecraft and humans in space.

Chapter 10 presents an overview of the stars, telling us how far away, bright,

luminous, hot, big, and massive they are. It also includes discussions of stellar

spectra, as well as the evolution of stars and their role in the origin of the chemical

elements.

The space between the stars is discussed in Chap. 11, beginning with bright

stars that illuminate nearby space and continuing with the dust, gas, radio emis￾sion, and molecules within interstellar space. This is naturally followed in

Chap. 12 by the ongoing formation of stars and their planets; recent discoveries of

protoplanetary disks and planets around nearby stars can also be found in this

chapter.

The final destiny of stars, when they have depleted their nuclear resources, is

presented in Chap. 13. It includes planetary nebulae, white dwarf stars, degenerate

pressure, novae, supernovae, neutron stars, pulsars, and stellar black holes.

Our last two chapters discuss the observable universe in its entirety, including

the Milky Way, the receding galaxies, the big bang with its background radiation,

the first atoms, stars, and galaxies, the evolution of galaxies, dark matter and dark

energy, and the ultimate destiny of the universe.

Preface vii

A total of 69 tables provide vital facts and physical information for the main

types of cosmic objects; students, teachers, and researchers may also consult this

information throughout their careers. In alphabetical order, they include the

physical properties of atmospheres, clusters of galaxies, the cosmic microwave

background radiation, the Earth, emission nebulae, galaxies, our Galaxy, giant

molecular clouds, H I regions, H II regions, interstellar molecules, the Milky Way,

our Moon, neutron stars, novae, planetary nebulae, planets, pulsars, radioactive

isotopes, the Sun, stars, star clusters, supernova explosions, and supernova

remnants.

Our tables also include information about cosmic magnetic fields, cosmic rays,

cosmological parameters, and nuclear fusion processes, as well as the range of

cosmic pressures, cosmic temperatures and stellar luminosity, and the spectral

lines of active galaxies, emission nebulae, stars, the Sun’s corona, and the Sun’s

photosphere.

There are also excellent line drawings, prepared by Kacha Bradonjich, and

several images of astronomical objects from the ground and space that help cement

our newfound knowledge together. They help crystallize a new concept with a

visual excitement that adds another dimension to our understanding.

The author also writes another sort of popular book, filled with personal

anecdotes, the lives of contributors to the field, and human metaphors, without an

equation or reference in sight. For this complementary approach, the reader is

referred to the author’s two books The Life and Death of Stars and Parting the

Cosmic Veil, which deal with many of the same general topics as Essential

Astrophysics in a different, lighter perspective.

I am indebted to Gayle Grant for help in assembling this book, and to the Tufts

Faculty Research Committee for modest support for typing some equations in it.

And last, but not least, the author thanks Ramon Khanna for his skillful editorial

suggestions that have made Essential Astrophysics a better book.

Medford, November 2012 Kenneth R. Lang

viii Preface

Contents

1 Observing the Universe................................ 1

1.1 What Do Astronomers and Astrophysicists Do? . . . . . . . . . . . 1

1.2 Our Place on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Location in the Sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Measuring Angle and Size. . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 The Locations of the Stars are Slowly Changing . . . . . . . . . . 10

1.6 What Time is It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.7 Telling Time by the Stars . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.8 Optical Telescopes Observe Visible Light . . . . . . . . . . . . . . . 19

1.9 Telescopes that Detect Invisible Radiation. . . . . . . . . . . . . . . 23

1.10 Units Used by Astronomers and Astrophysicists. . . . . . . . . . . 27

1.11 Physical Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2 Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.1 Electromagnetic Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.2 The Electromagnetic Spectrum. . . . . . . . . . . . . . . . . . . . . . . 37

2.3 Moving Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.4 Thermal (Blackbody) Radiation . . . . . . . . . . . . . . . . . . . . . . 44

2.5 How Far Away is the Sun, and How Bright,

Big and Hot is it?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

2.5.1 Distance of the Sun . . . . . . . . . . . . . . . . . . . . . . . . 50

2.5.2 How Big is the Sun?. . . . . . . . . . . . . . . . . . . . . . . . 54

2.5.3 The Unit of Energy. . . . . . . . . . . . . . . . . . . . . . . . . 54

2.5.4 The Sun’s Luminosity. . . . . . . . . . . . . . . . . . . . . . . 55

2.5.5 Taking the Sun’s Temperature . . . . . . . . . . . . . . . . . 55

2.5.6 How Hot are the Planets? . . . . . . . . . . . . . . . . . . . . 56

2.6 The Energy of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

2.7 Radiation Scattering and Transfer. . . . . . . . . . . . . . . . . . . . . 61

2.7.1 Why is the Sky Blue and the Sunsets Red? . . . . . . . . 61

2.7.2 Rayleigh Scattering. . . . . . . . . . . . . . . . . . . . . . . . . 62

ix

2.7.3 Thomson and Compton Scattering . . . . . . . . . . . . . . 63

2.7.4 Radiation Transfer . . . . . . . . . . . . . . . . . . . . . . . . . 65

3 Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

3.1 Ceaseless, Repetitive Paths Across the Sky . . . . . . . . . . . . . . 69

3.2 Universal Gravitational Attraction . . . . . . . . . . . . . . . . . . . . 73

3.3 Mass of the Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

3.4 Tidal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

3.4.1 The Ocean Tides . . . . . . . . . . . . . . . . . . . . . . . . . . 81

3.4.2 Tidal Locking into Synchronous Rotation . . . . . . . . . 85

3.4.3 The Days are Getting Longer. . . . . . . . . . . . . . . . . . 85

3.4.4 The Moon is Moving Away from the Earth . . . . . . . . 87

3.4.5 A Planet’s Differential Gravitational Attraction

Accounts for Planetary Rings. . . . . . . . . . . . . . . . . . 90

3.5 What Causes Gravity?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4 Cosmic Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

4.1 Motion Opposes Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

4.1.1 Everything Moves . . . . . . . . . . . . . . . . . . . . . . . . . 99

4.1.2 Escape Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

4.2 Orbital Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

4.3 The Moving Stars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

4.3.1 Are the Stars Moving? . . . . . . . . . . . . . . . . . . . . . . 105

4.3.2 Components of Stellar Velocity . . . . . . . . . . . . . . . . 105

4.3.3 Proper Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

4.3.4 Radial Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

4.3.5 Observed Proper Motions of Stars . . . . . . . . . . . . . . 109

4.3.6 Motions in Star Clusters . . . . . . . . . . . . . . . . . . . . . 111

4.3.7 Runaway Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

4.4 Cosmic Rotation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4.4.1 Unexpected Planetary Rotation. . . . . . . . . . . . . . . . . 116

4.4.2 The Sun’s Differential Rotation . . . . . . . . . . . . . . . . 120

4.4.3 Stellar Rotation and Age . . . . . . . . . . . . . . . . . . . . . 124

5 Moving Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

5.1 Elementary Constituents of Matter . . . . . . . . . . . . . . . . . . . . 125

5.2 Heat, Temperature, and Speed . . . . . . . . . . . . . . . . . . . . . . . 130

5.2.1 Where Does Heat Come From? . . . . . . . . . . . . . . . . 130

5.2.2 Thermal Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . 132

5.2.3 Collisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

5.2.4 The Distribution of Speeds . . . . . . . . . . . . . . . . . . . 135

5.3 Molecules in Planetary Atmospheres. . . . . . . . . . . . . . . . . . . 138

x Contents

5.4 Gas Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

5.4.1 What Keeps Our Atmosphere Up? . . . . . . . . . . . . . . 141

5.4.2 The Ideal Gas Law . . . . . . . . . . . . . . . . . . . . . . . . . 142

5.4.3 The Earth’s Sun-Layered Atmosphere . . . . . . . . . . . . 144

5.4.4 Pressure, Temperature, and Density Inside the Sun. . . 148

5.5 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

5.5.1 Ionized Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

5.5.2 Plasma Oscillations and the Plasma Frequency . . . . . 152

5.5.3 Atoms are Torn Apart into Plasma Within the Sun. . . 153

5.6 Sound Waves and Magnetic Waves . . . . . . . . . . . . . . . . . . . 154

5.6.1 Sound Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

5.6.2 Magnetic Waves. . . . . . . . . . . . . . . . . . . . . . . . . . . 156

6 Detecting Atoms in Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

6.1 What is the Sun Made Out Of? . . . . . . . . . . . . . . . . . . . . . . 159

6.2 Quantization of Atomic Systems . . . . . . . . . . . . . . . . . . . . . 165

6.3 Some Atoms are Excited Out of Their Lowest-Energy

Ground State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

6.4 Ionization and Element Abundance in the Sun

and Other Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

6.5 Wavelengths and Shapes of Spectral Lines . . . . . . . . . . . . . . 180

6.5.1 Radial Motion Produces a Wavelength Shift . . . . . . . 180

6.5.2 Gravitational Redshift . . . . . . . . . . . . . . . . . . . . . . . 181

6.5.3 Thermal Motion Broadens Spectral Lines . . . . . . . . . 183

6.5.4 Rotation or Expansion of the Radiating Source

can Broaden Spectral Lines . . . . . . . . . . . . . . . . . . . 184

6.5.5 Curve of Growth . . . . . . . . . . . . . . . . . . . . . . . . . . 185

6.5.6 Magnetic Fields Split Spectral Lines. . . . . . . . . . . . . 186

7 Transmutation of the Elements . . . . . . . . . . . . . . . . . . . . . . . . . . 191

7.1 The Electron, X-rays and Radium. . . . . . . . . . . . . . . . . . . . . 191

7.2 Radioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

7.3 Tunneling Out of the Atomic Nucleus. . . . . . . . . . . . . . . . . . 196

7.4 The Electron and the Neutrino . . . . . . . . . . . . . . . . . . . . . . . 199

7.5 Cosmic Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

7.6 Nuclear Transformation by Bombardment . . . . . . . . . . . . . . . 209

8 What Makes the Sun Shine? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

8.1 Can Gravitational Contraction Supply

the Sun’s Luminosity?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

8.2 How Hot is the Center of the Sun?. . . . . . . . . . . . . . . . . . . . 217

8.3 Nuclear Fusion Reactions in the Sun’s Core . . . . . . . . . . . . . 219

8.3.1 Mass Lost is Energy Gained . . . . . . . . . . . . . . . . . . 219

8.3.2 Understanding Thermonuclear Reactions . . . . . . . . . . 225

Contents xi

8.3.3 Hydrogen Burning . . . . . . . . . . . . . . . . . . . . . . . . . 231

8.3.4 Why Doesn’t the Sun Blow Up? . . . . . . . . . . . . . . . 237

8.4 The Mystery of Solar Neutrinos . . . . . . . . . . . . . . . . . . . . . . 237

8.4.1 The Elusive Neutrino . . . . . . . . . . . . . . . . . . . . . . . 237

8.4.2 Solar Neutrino Detectors Buried

Deep Underground . . . . . . . . . . . . . . . . . . . . . . . . . 239

8.4.3 Solving the Solar Neutrino Problem . . . . . . . . . . . . . 242

8.5 How the Energy Gets Out . . . . . . . . . . . . . . . . . . . . . . . . . . 244

8.6 The Faint-Young-Sun Paradox . . . . . . . . . . . . . . . . . . . . . . . 252

8.7 The Sun’s Destiny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

9 The Extended Solar Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . 255

9.1 Hot, Volatile, Magnetized Gas . . . . . . . . . . . . . . . . . . . . . . . 255

9.1.1 The Million-Degree Solar Corona. . . . . . . . . . . . . . . 255

9.1.2 Varying Sunspots and Ever-Changing

Magnetic Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

9.1.3 Coronal Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

9.1.4 What Heats the Corona? . . . . . . . . . . . . . . . . . . . . . 266

9.1.5 Coronal Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268

9.2 The Sun’s Varying Winds . . . . . . . . . . . . . . . . . . . . . . . . . . 268

9.2.1 The Expanding Sun Envelops the Earth . . . . . . . . . . 268

9.2.2 Properties of the Solar Wind . . . . . . . . . . . . . . . . . . 271

9.2.3 Where Do the Two Solar Winds Come From? . . . . . . 274

9.2.4 Where Does the Solar Wind End? . . . . . . . . . . . . . . 275

9.3 Explosions on the Sun. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

9.3.1 Solar Flares . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

9.3.2 Coronal Mass Ejections. . . . . . . . . . . . . . . . . . . . . . 281

9.4 Space Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

9.4.1 Earth’s Protective Magnetosphere. . . . . . . . . . . . . . . 283

9.4.2 Trapped Particles . . . . . . . . . . . . . . . . . . . . . . . . . . 287

9.4.3 Earth’s Magnetic Storms . . . . . . . . . . . . . . . . . . . . . 288

9.4.4 Solar Explosions Threaten Humans

in Outer Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

9.4.5 Disrupting Communication . . . . . . . . . . . . . . . . . . . 290

9.4.6 Satellites in Danger . . . . . . . . . . . . . . . . . . . . . . . . 291

9.4.7 Forecasting Space Weather . . . . . . . . . . . . . . . . . . . 292

10 The Sun Amongst the Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

10.1 Comparisons of the Sun with Other Stars . . . . . . . . . . . . . . . 293

10.1.1 How Far Away are the Stars? . . . . . . . . . . . . . . . . . 293

10.1.2 How Bright are the Stars? . . . . . . . . . . . . . . . . . . . . 296

10.1.3 How Luminous are the Stars? . . . . . . . . . . . . . . . . . 298

10.1.4 The Temperatures of Stars. . . . . . . . . . . . . . . . . . . . 303

10.1.5 The Colors of Stars. . . . . . . . . . . . . . . . . . . . . . . . . 304

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10.1.6 The Spectral Sequence . . . . . . . . . . . . . . . . . . . . . . 305

10.1.7 Radius of the Stars . . . . . . . . . . . . . . . . . . . . . . . . . 306

10.1.8 How Massive are the Stars?. . . . . . . . . . . . . . . . . . . 310

10.2 Main-Sequence and Giant Stars . . . . . . . . . . . . . . . . . . . . . . 318

10.2.1 The Hertzsprung–Russell Diagram . . . . . . . . . . . . . . 318

10.2.2 The Luminosity Class . . . . . . . . . . . . . . . . . . . . . . . 321

10.2.3 Life on the Main Sequence . . . . . . . . . . . . . . . . . . . 323

10.2.4 The Red Giants and Supergiants. . . . . . . . . . . . . . . . 326

10.3 Nuclear Reactions Inside Stars . . . . . . . . . . . . . . . . . . . . . . . 329

10.3.1 The Internal Constitution of Stars. . . . . . . . . . . . . . . 329

10.3.2 Two Ways to Burn Hydrogen

in Main-Sequence Stars. . . . . . . . . . . . . . . . . . . . . . 335

10.3.3 Helium Burning in Giant Stars. . . . . . . . . . . . . . . . . 340

10.4 Using Star Clusters to Watch How Stars Evolve . . . . . . . . . . 343

10.5 Where did the Chemical Elements Come From? . . . . . . . . . . 348

10.5.1 Advanced Nuclear Burning Stages

in Massive Supergiant Stars. . . . . . . . . . . . . . . . . . . 348

10.5.2 Origin of the Material World . . . . . . . . . . . . . . . . . . 349

10.5.3 The Observed Abundance of the Elements . . . . . . . . 350

10.5.4 Synthesis of the Elements Inside Stars . . . . . . . . . . . 351

10.5.5 Big-Bang Nucleosynthesis . . . . . . . . . . . . . . . . . . . . 353

10.5.6 The First and Second Generation of Stars . . . . . . . . . 354

10.5.7 Cosmic Implications of the Origin of the Elements. . . 355

11 The Material Between the Stars . . . . . . . . . . . . . . . . . . . . . . . . . 357

11.1 Gaseous Emission Nebulae . . . . . . . . . . . . . . . . . . . . . . . . . 357

11.2 Solid Dust Particles in Interstellar Space . . . . . . . . . . . . . . . . 366

11.3 Radio Emission from the Milky Way . . . . . . . . . . . . . . . . . . 369

11.4 Interstellar Hydrogen Atoms . . . . . . . . . . . . . . . . . . . . . . . . 375

11.5 Interstellar Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

12 Formation of the Stars and Their Planets . . . . . . . . . . . . . . . . . . 381

12.1 How the Solar System Came into Being . . . . . . . . . . . . . . . . 381

12.1.1 The Nebular Hypothesis . . . . . . . . . . . . . . . . . . . . . 381

12.1.2 Composition of the Planets . . . . . . . . . . . . . . . . . . . 382

12.1.3 Mass and Angular Momentum

in the Solar System. . . . . . . . . . . . . . . . . . . . . . . . . 385

12.2 Star Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388

12.2.1 Giant Molecular Clouds . . . . . . . . . . . . . . . . . . . . . 388

12.2.2 Gravitational Collapse. . . . . . . . . . . . . . . . . . . . . . . 389

12.2.3 Triggering Gravitational Collapse . . . . . . . . . . . . . . . 392

12.2.4 Protostars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

12.2.5 Losing Mass and Spin. . . . . . . . . . . . . . . . . . . . . . . 398

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12.3 Planet-Forming Disks and Planets Around Nearby Stars . . . . . 400

12.3.1 The Plurality of Worlds. . . . . . . . . . . . . . . . . . . . . . 400

12.3.2 Proto-Planetary Disks . . . . . . . . . . . . . . . . . . . . . . . 400

12.3.3 The First Discoveries of Exoplanets . . . . . . . . . . . . . 403

12.3.4 Hundreds of New Worlds Circling Nearby Stars . . . . 408

12.3.5 Searching for Habitable Planets . . . . . . . . . . . . . . . . 409

13 Stellar End States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

13.1 A Range of Destinies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

13.2 Planetary Nebulae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412

13.3 Stars the Size of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . 418

13.3.1 The Discovery of White Dwarf Stars . . . . . . . . . . . . 418

13.3.2 Unveiling White Dwarf Stars . . . . . . . . . . . . . . . . . . 419

13.3.3 The High Mass Density of White Dwarf Stars . . . . . . 420

13.4 The Degenerate Electron Gas. . . . . . . . . . . . . . . . . . . . . . . . 423

13.4.1 Nuclei Pull a White Dwarf Together

as Electrons Support It . . . . . . . . . . . . . . . . . . . . . . 423

13.4.2 Radius and Mass of a White Dwarf . . . . . . . . . . . . . 427

13.5 Exploding Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

13.5.1 Guest Stars, the Novae . . . . . . . . . . . . . . . . . . . . . . 429

13.5.2 What Makes a Nova Happen? . . . . . . . . . . . . . . . . . 430

13.5.3 A Rare and Violent End, the Supernovae . . . . . . . . . 433

13.5.4 Why do Supernova Explosions Occur? . . . . . . . . . . . 436

13.5.5 When a Nearby Star Detonates Its Companion. . . . . . 437

13.5.6 Stars that Blow Themselves Up . . . . . . . . . . . . . . . . 438

13.5.7 Light of a Billion Suns, SN 1987A. . . . . . . . . . . . . . 439

13.5.8 Will the Sun Explode? . . . . . . . . . . . . . . . . . . . . . . 443

13.6 Expanding Stellar Remnants . . . . . . . . . . . . . . . . . . . . . . . . 443

13.7 Neutron Stars and Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . 450

13.7.1 Neutron Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450

13.7.2 Radio Pulsars from Isolated Neutron Stars. . . . . . . . . 453

13.7.3 X-ray Pulsars from Neutron Stars in Binary

Star Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460

13.8 Stellar Black Holes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465

13.8.1 Imagining Black Holes . . . . . . . . . . . . . . . . . . . . . . 465

13.8.2 Observing Stellar Black Holes . . . . . . . . . . . . . . . . . 466

13.8.3 Describing Black Holes. . . . . . . . . . . . . . . . . . . . . . 467

14 A Larger, Expanding Universe . . . . . . . . . . . . . . . . . . . . . . . . . . 471

14.1 The Milky Way . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

14.1.1 A Fathomless Disk of Stars . . . . . . . . . . . . . . . . . . . 471

14.1.2 The Sun is Not at the Center of Our

Stellar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

14.1.3 The Rotating Galactic Disk . . . . . . . . . . . . . . . . . . . 479

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