Explorations Introduction to Astronomy 8 edition

by Stephen Schneider and Thomas Arny

Explorations Introduction to Astronomy 8 edition Author Stephen Schneider and Thomas Arny Isbn 9780073513911 File size 247MB Year 2016 Pages 592 Language English File format PDF Category Astronomy The eighth edition of Explorations An Introduction to Astronomy strives to share with students a sense of wonder about the universe and the dynamic ever changing science of astronomy Written for students of various educational backgrounds Explorations emphasizes current information a visually exciting art package accessible writing and accur

Publisher :

Author : Stephen Schneider and Thomas Arny

ISBN : 9780073513911

Year : 2016

Language: English

File Size : 247MB

Category : Astronomy


An Introduction to Astronomy
Eighth Edition

Thomas T. Arny & Stephen E. Schneider
The nine “Looking Up” figures on the following
pages explore a variety of the amazing objects
that can be spotted in the night sky. Brief
descriptions of each also list the chapter
where you can learn more about them.

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Delta Cephei
A pulsating variable
star (chapter 14) at a
distance of 980 ly.


Northern Circumpolar

For observers over most of the northern hemisphere,
there are five constellations that are circumpolar,
remaining visible all night long: Ursa Major (the Big
Bear), Ursa Minor (the Little Bear), Cepheus (the King),
Cassiopeia (the Queen), and Draco (the Dragon). The
brightest stars in Ursa Major and Ursa Minor form two
well-known asterisms: the Big and Little Dippers.



~12 ly


This is an open star
cluster (chapter 16). Its
distance is uncertain—
perhaps 3000 to 5000 ly.


Polaris —
The North Star
Little Dipper
∙170,000 ly

This star lies about 430 ly
away, almost directly above
the Earth’s North Pole,
making it an important aid for
navigation (chapter 1).


This spiral galaxy is ~27
million light-years away
from us (chapter 17).


Cassiopeia in 3-D

55 ly
100 ly

230 ly
550 ly

M81 and M82

Big Dipper

410 ly

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles

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This was the north star when
the pyramids were built in
ancient Egypt (chapter 6).


Gravitational interactions
between these two galaxies
have triggered star
formation (chapter 17).

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Ursa Major


Circling in the northern sky is the Big Dipper, part of the well-known
constellation Ursa Major, the Big Bear. The Big Dipper is technically not
a constellation, but just an asterism—a star grouping. It is easy to see in
the early evening looking north from mid-March through mid-September.
The Big Dipper can help you find the North Star, and with a telescope on a
dark, clear night, you can find several other intriguing objects as shown below.




~1.6 ly

Over the course of a night, stars appear to rotate
counterclockwise around the star Polaris, which
remains nearly stationary because it lies
almost directly above Earth’s North Pole.
Polaris is not especially bright, but you
can easily find Polaris by extending a
line from the two stars at the end
of the bowl of the Big Dipper,
the pointer stars, as shown
by the dashed yellow line
(chapter 1).
Location of the

Big Dipper

M97 — The Owl

This planetary nebula
(chapter 14) is ~2500 ly away.

Hubble Deep Field
(chapter 17)

Mizar and Alcor

Little Dipper

If you look closely at it, you may notice that the
middle star in the “handle” is actually two stars—
Mizar and Alcor. Despite appearing close together
in the sky, they are probably not in orbit around
each other. However, with a small telescope, you
can see that Mizar (the brighter of the star pair) has
a faint companion star. This companion does in fact
orbit Mizar. Moreover, each of Mizar‘s stars is itself
a binary star, making Mizar a quadruple system
(chapter 13).

Big Dipper in 3-D

80 ly
83 ly 123 ly
80 ly
83 ly
86 ly
104 ly

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles

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170,000 ly


The Whirlpool Galaxy can be seen as a dim patch
of light with a small telescope. M51 is about 37
million ly away from Earth (chapter 17).

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M31 & Perseus

The galaxy M31 lies in the constellation
Andromeda, near the constellations
Perseus and Cassiopeia. It is about 2.5
million ly from us, the most distant
object visible with the naked eye.
Northern hemisphere viewers can
see M31 in the evening sky from
August through December.

M31 —
Galaxy (chapter 17)


~150,000 ly
~200 ly


The Double Cluster


If you scan with binoculars from M31 toward the
space between Perseus and Cassiopeia, you will
see the Double Cluster—two groups of massive,
luminous but very distant stars. The Double
Cluster is best seen with binoculars. The two
clusters are about 7000 ly away and a few
hundred light-years apart (chapter 16).

California Nebula

An emission nebula (chapter 16) with
a shape like the state of California.


Algol, the “demon star,” dims for
about 10 hours every few days as its
companion eclipses it (chapter 13).

The brightest star in the
constellation Auriga, the
Charioteer. A binary star
(chapter 13).

M45 —

Perseus in 3-D
240 ly
113 ly
250 ly
34 ly
90 ly

880 ly
520 ly
520 ly

640 ly
750 ly

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles

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The Summer Triangle consists of the three bright stars Deneb, Vega, and
Altair, the brightest stars in the constellations Cygnus, Lyra, and Aquila,
respectively. They rise in the east shortly after sunset in late June and are
visible throughout the northern summer and into late October (when they set in
the west in the early evening). Vega looks the brightest to us, but Deneb produces
the most light, only looking dimmer because it is so much farther from us.



Summer Triangle


Epsilon Lyra

A double, double star

1 ly

M57 — Ring Nebula

This planetary nebula (chapter 14) is
about 2300 ly distant. From its
observed expansion rate it is estimated
to be 7000 years old.



Deneb is a blue supergiant
(chapter 13), one of the most
luminous stars we can see,
Deneb emits ~50,000 times
more light than the Sun.



M27 —
Dumbbell Nebula

Through a small telescope this
star pair shows a strong color
contrast between the orange red
giant and blue main-sequence star
(chapter 13). These stars may orbit each other
every few hundred thousand years, but they are
far enough apart that they may not be in orbit.
,2.5 lyly

Another planetary nebula
(chapter 14), the Dumbbell is
about 1200 ly distant and is
about 2.5 ly in diameter.

The Summer Triangle in 3-D

25 ly

430 ly

17 ly

1400 ly

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles

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Taurus, the Bull, is one of the constellations of the zodiac and one
of the creatures hunted by Orion in mythology. Taurus is visible in
the evening sky from November through March. The brightest star
in Taurus is Aldebaran, the eye of the bull. The nebula and two star
clusters highlighted below have been critical in the history of astronomy
for understanding the distances and fates of stars.

M45 — Pleiades

This open star cluster (chapter 16)
is easy to see with the naked eye
and looks like a tiny dipper.
It is about 400 ly from Earth.

~8 ly


Aldebaran is a red giant star (chapter 13).
It is about 67 ly away from Earth and has a
diameter about 45 times larger than the
Sun’s. Although it appears to be part of the
Hyades, it is less than half as distant.

~10 ly

M1 — Crab Nebula
The Crab Nebula is the remnant of
a star that blew up in the year A.D.
1054 as a supernova. At its center
is a pulsar (chapter 15). It is about
6500 ly away from us.

Taurus in 3-D

T Tauri
T Tauri is an erratically
varying pre-main-sequence
star, prototype of a class of
forming stars (chapter 14).
It is about 600 ly distant.
450 ly

400 ly

67 ly

151 ly

The “V” in Taurus is another nearby
star cluster, measured to be 151 ly
away by the Hipparcos satellite
(chapter 13). It is easy to see its
many stars with binoculars.

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles

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Mars’ orbit


Orion is easy to identify because of the
three bright stars of his “belt.” You can
see Orion in the evening sky from
November to April, and before dawn
from August through September.


Betelgeuse is a red supergiant star
(chapter 13) that has swelled to a size
that is larger than the orbit of Mars. Its
red color indicates that it is relatively
cool for a star, about 3500 kelvin.

10 ly

3 ly

Horsehead Nebula

M42 —
Orion Nebula

The horsehead shape is produced
by dust in an interstellar cloud
blocking background
light (chapter 16).

The Orion Nebula is an active
star-forming region rich with
dust and gas (chapter 14).


Neptune’s orbit

3 ly

Protoplanetary disk

Rigel is a Blue Supergiant star
(chapter 13). Its blue color
indicates a surface temperature of
about 10,000 kelvin.

This is the beginning of a star; our
early Solar System may have looked
like this (chapter 8).

Orion in 3-D
640 ly
250 ly

690 ly
740 ly
650 ly

1300 ly
860 ly

1340 ly

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles

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M16 —
Eagle Nebula


Sagittarius marks the direction to the center
of the Milky Way. It can be identified by its
“teapot” shape, with the Milky Way
seeming to rise like steam from the spout.
From northern latitudes, the constellation
is best seen July to September, when it is
above the southern horizonin the evening.
Many star-forming nebulae are visible in
this region (chapter 16).

This young star cluster
and the hot gas around
it lie about 7000 ly
from Earth.

~1 ly

~70 ly


~50 ly

M22 is one of many globular
clusters (chapter 16)
concentrated toward the
center of our Galaxy. Easy to
see with binoculars, it is just
barely visible to the naked eye. It
is about 11,000 ly away from us.

~100 ly

M17 —

M20 —
Trifid Nebula
The “teapot”
of Sagittarius

Center of the
Milky Way

Sagittarius in 3-D
78 ly

97 ly

M8 —

The name Trifid was
given because of the
dark streaks that
divide it into thirds.
The distance of this
nebula is uncertain,
approximately 5000
ly away, making its
size uncertain too.

(chapter 16)

230 ly 240 ly

122 ly
88 ly 143 ly

350 ly

1 light year (ly) ≈ 10 trillion km ≈ 6 trillion miles


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These constellations are best observed from the southern
hemisphere. Northern hemisphere viewers can see Centaurus
low in the southern sky during evenings in May–July, but the
Southern Cross rises above the horizon only for viewers south of
latitude ~25°N (Key West, South Texas, and Hawaii in the U.S.).


entaurus and
d Crux,
The Southern Cross

Proxima Centauri

This dim star is the nearest star
to the Sun, 4.22 ly distant
(chapter 13).



Omega Centauri

~50 ly

The Jewel Box

NGC 4755, an open star cluster
(chapter 16) ~500 ly from us.

~200 ly

This is the largest globular cluster
(chapter 16) in the Milky Way,
~16,000 ly distant and
containing millions of stars.

The Coal Sack

An interstellar dust cloud
(chapter 16)


The Southern Cross

,50,000 ly

Centaurus A

This active galaxy (chapter 17),
~11 million ly distant, is one of the
brightest radio sources in the sky.

Southern Cross in 3-D
Eta Carinae

At over 100 times the mass of the Sun,
this is one of the highest-mass stars
known and doomed to die young
(chapter 14). It is about 8000 ly distant.

280 ly

89 ly

230 ly

320 ly
345 ly

1 light-year (ly) ≈ 10 trillion km ≈ 6 trillion miles


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The Southern Cross

hern Circumpol

Most of the constellations in this part of
the sky are dim, but observers in much of
the southern hemisphere can see the
Magellanic Clouds circling the south
celestial pole throughout the night.



~0.5 ly

A planetary nebula
(chapter 14)
~8000 ly distant


The constellation closest to the
south celestial pole is named
after a navigational
instrument, the

The South
Celestial Pole


No bright stars lie near the south
celestial pole (chapter 1), but the
Southern Cross points toward it.

A Bok globule (chapter 14)
about 600 ly distant



A dwarf galaxy orbiting the
Milky Way at a distance of
∼200,000 ly (chapter 17).

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A small galaxy orbiting the
Milky Way at a distance of
∼160,000 ly (chapter 17).

~1000 ly

Tarantula Nebula

A star-formation region
(chapter 16) in the Large
Magellanic Cloud larger than
any known in the Milky Way.

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An Introduction to Astronomy
Eighth Edition

Thomas T. Arny
Professor Emeritus
Department of Astronomy
University of Massachusetts, Amherst

Stephen E. Schneider
Professor of Astronomy
University of Massachusetts, Amherst

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Published by McGraw-Hill Education, 2 Penn Plaza, New York, NY 10121. Copyright © 2017 by McGraw-Hill Education. All
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Library of Congress Cataloging-in-Publication Data
Arny, Thomas.
Explorations : an introduction to astronomy / Thomas T. Arny, professor
emeritus, Department of Astronomy, University of Massachusetts, Amherst,
Stephen E. Schneider, professor of astronomy, University of Massachusetts,
Amherst.—Eighth edition.
pages cm
Includes index.
ISBN 978-0-07-351391-1 (alk. paper)
1. Astronomy—Textbooks. I. Schneider, Stephen E. (Stephen Ewing), 1957II. Title.
QB45.2.A76 2017
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Brief Contents
Looking Up Illustrations ii
Preface xxiii

Chapter 13 Measuring the Properties of
Stars 324
Chapter 14 Stellar Evolution 356


The Cosmic Landscape 1

Chapter 1

The Cycles of the Sky 14

Chapter 2

The Rise of Astronomy 36

Chapter 15 Stellar Remnants: White
Dwarfs, Neutron Stars, and
Black Holes 386

Essay 1

Backyard Astronomy 60

Chapter 16 The Milky Way Galaxy 408

Chapter 3

Gravity and Motion 70

Chapter 17 Galaxies 440

Chapter 4

Light and Atoms 86

Chapter 18 Cosmology 476

Essay 2

Special and General
Relativity 114

Chapter 5

Telescopes 122

Chapter 6

The Earth 144

Essay 3

Keeping Time 170

Chapter 7

The Moon 178

Chapter 8

Survey of Solar
Systems 196

Chapter 9

The Terrestrial Planets 222

Chapter 10 The Outer Planets 252
Chapter 11 Small Bodies Orbiting the
Sun 276

Essay 4

Life in the Universe 504

Answers to Test Yourself 516
Scientific Notation A-1
Metric Prefixes A-1
Solving Distance, Velocity, Time
(d, V, t) Problems A-2
Some Useful Formulas A-2
Glossary G-1
Credits C-1
Index I-1

Chapter 12 The Sun, Our Star 302


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Looking Up Illustrations ii
#1: Northern Circumpolar
Constellations ii
#2: Ursa Major iii
#3: M31 & Perseus iv
#4: Summer Triangle v
#5: Taurus vi
#6: Orion vii
#7: Sagittarius viii
#8: Centaurus and Crux, The Southern
Cross ix
#9: Southern Circumpolar Constellations x

Preface xxiii





The Cosmic Landscape 1
The Earth, Our Home 1
The Moon 2
The Planets 2
The Sun 3
The Solar System 4
Astronomical Sizes 5
Astronomy by the Numbers: The Size of a
Light-Year 5
The Milky Way 6
Galaxy Clusters and the Universe 7
Forces and Matter 8
The Still-Unknown Universe 9
The Scientific Method 9


The Rise of Astronomy 36


The Cycles of the Sky 14

The Celestial Sphere 15
Constellations 16
Daily Motions of the Sun and Stars 17
Annual Motion of the Sun 18

The Ecliptic and the Zodiac 19
Extending Our Reach: Are You an
Ophiuchan? 20
The Seasons 20
Solstices, Equinoxes, and the Ecliptic’s
Tilt 22
Tracking the Sun’s Changing Position 22
Astronomy by the Numbers: The Angle of the
Sun at Noon 24
The Moon 26
Astronomy by the Numbers: Estimating When
the Moon Will Rise 27
Extending Our Reach: Observing the Moon 28
Eclipses 29
Appearance of Eclipses 29
Rarity of Eclipses 32
Precession of the Moon’s Orbit 33


Early Ideas of the Heavens: Classical
Astronomy 37
The Shape of the Earth 37
Distances and Sizes of the Sun and
Moon 38
Extending Our Reach: The Moon Illusion 40
Arguments for an Earth-Centered
Universe 40
The Size of the Earth 41
Astronomy by the Numbers: The Diameter–
Distance Relation of Astronomical
Objects 43
The Planets 44
Explaining the Motion of the Planets 46
Ptolemy 46
Islamic Astronomy 47
Asian Astronomy 47
Astronomy in the Renaissance 48
Nicolaus Copernicus 48


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Astronomy by the Numbers: How Copernicus
Calculated the Distances to the Planets 50
Tycho Brahe 50
Johannes Kepler 51
Astronomy by the Numbers: Using Kepler ’s
Third Law for Orbit Calculations 53
The Birth of Astrophysics 54
Galileo Galilei 54
Isaac Newton 56
Extending Our Reach: Astronomy and
Astrology 56
New Discoveries 57
New Technologies 57


Light and Atoms 86


Learning the Constellations 60
Celestial Mapping 62
Planetary Configurations 64
Your Eyes at Night 65
Imaging the Sky 66
Small Telescopes 67


Gravity and Motion 70



Inertia 71
Orbital Motion and Gravity 73
Newton’s Second Law of Motion 74
Acceleration 74
Mass 75
Newton’s Third Law of Motion 76
The Law of Gravity 77
Measuring an Object’s Mass Using Orbital
Motion 79
Astronomy by the Numbers: Weighing the
Sun 80
Surface Gravity 81
Astronomy by the Numbers: The Surface
Gravity of the Earth and Moon 81

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Escape Velocity 82
Astronomy by the Numbers: The Escape
Velocity from the Moon 83



Backyard Astronomy 60





Properties of Light 87
The Nature of Light—Waves or
Particles? 88
Light and Color 89
Characterizing Electromagnetic Waves
by Their Frequency 90
Astronomy by the Numbers: Wavelength and
Frequency 90
White Light 91
The Electromagnetic Spectrum: Beyond
Visible Light 92
Infrared Radiation 93
Ultraviolet Light 93
Radio Waves and Microwaves 94
X Rays and Gamma Rays 94
Energy Carried by Electromagnetic
Radiation 94
The Nature of Matter and Heat 95
The Kelvin Temperature Scale 96
Temperature and Radiation 96
Astronomy by the Numbers: Taking the
Temperature of the Sun 97
Radiation from Individual Atoms 98
The Chemical Elements 99
Electron Orbitals 99
The Generation of Light by Atoms 101
Formation of a Spectrum 102
How a Spectrum Is Formed 103
Identifying Atoms by Their Light 104
Types of Spectra 106
Astronomical Spectra 107
Absorption in the Atmosphere 108
Extending Our Reach: Observing the Crab
Nebula at Many Wavelengths 109
The Doppler Shift: Detecting Motion 110

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Special and General Relativity 114

The Earth 144

Rest Frames 114
The Speed of Light from Moving
Objects 115
The Michelson-Morley Experiment 115
Einstein’s Theory of Special
Relativity 116
Special Relativity and Space Travel 117
The Twin Paradox 118
Rethinking Gravity 119
General Relativity 120
Astronomy by the Numbers: A Lorentz Factor
of a Million 120





Telescopes 122





Telescope Fundamentals 123
Light-Gathering Power 124
Astronomy by the Numbers: Light-Gathering
Power of a Telescope 124
Focusing the Light 125
Extending Our Reach: Refraction 126
Resolving Power 129
Astronomy by the Numbers: Resolving Power
of a Telescope 130
Interferometers 130
Detecting Light 131
Visible Light 131
Detecting Other Wavelengths 132
Observatories on the Ground and in
Space 134
Extending Our Reach: Exploring New
Wavelengths: Gamma Rays 137
Going Observing 138
Challenges and New Directions in GroundBased Observing 139
Atmospheric Blurring 139
Extending Our Reach: Distortion of the Sun’s
Shape 140
Light Pollution 141

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The Earth as a Planet 145
Shape and Size of the Earth 145
Composition of the Earth 146
Density of the Earth 147
Astronomy by the Numbers: Determining the
Internal Composition of the Earth 147
The Earth’s Interior 148
Probing the Interior with Earthquake
Waves 148
Heating and Differentiation of the Earth’s
Core 150
The Age of the Earth 151
Motions in the Earth’s Interior 152
Convection in the Earth’s Interior 152
Plate Tectonics 153
The Earth’s Magnetic Field 156
Extending Our Reach: Measuring Reversals of
the Earth’s Magnetic Field 157
Origin of the Earth’s Magnetic Field 157
Magnetic Effects on Cosmic
Particles 158
The Earth’s Atmosphere 159
Structure of the Atmosphere 159
Composition of the Atmosphere 160
The Greenhouse Effect 160
The Ozone Layer 162
Origin of the Atmosphere 162
The Spin of the Earth 164
Air and Ocean Circulation: The Coriolis
Effect 164
Precession 166


Keeping Time 170
The Day 170
Hours of Daylight 172
Time Zones 173
Universal Time 173
Daylight Saving Time 173
The Week 174

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The Month and Lunar Calendars 174
The Mayan Calendar 174
The Common Calendar 175
Leap Year 175
Moon Lore 176
The Abbreviations a.m., p.m., b.c., a.d.,
b.c.e., and c.e. 176



The Moon 178





The Surface of the Moon 179
Surface Features 179
Origin of Lunar Surface Features 181
Astronomy by the Numbers: The Limits of
Telescopic Observations of the Moon 182
Structure of the Moon 184
Crust and Interior 184
The Absence of a Lunar Atmosphere 185
Extending Our Reach: Is the Moon Completely
Dead? 186
Orbit and Motions of the Moon 186
The Moon’s Rotation 187
Oddities of the Moon’s Orbit 187
Origin and History of the Moon 188
Extending Our Reach: The Moon Landing
“Hoax” 188
Tides 190
Cause of Tides 190
Solar Tides 192
Tidal Braking 192
Astronomy by the Numbers: The Distance of
the Moon in the Past 193

The Terrestrial Planets 222


Survey of Solar Systems 196
Components of the Solar System 197
The Sun 197
The Planets 198
Asteroids and Comets 199
The Orbits and Spins of the Planets 200
Astronomy by the Numbers: Bode’s Rule: The
Search for Order 201

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Composition Differences Between the
Inner and Outer Planets 202
Age of the Solar System 204
Other Planetary Systems 205
The Discovery of Planets Beyond the
Solar System 205
Transiting Exoplanets 208
Composition of Exoplanets 210
Formation of Planetary Systems 211
Interstellar Clouds 212
Condensation in the Solar Nebula 213
Accretion and Planetesimals 214
Formation of the Planets 214
Extending Our Reach: Direct Formation of Gas
Giants 215
Final Stages of Planet Formation 216
Formation of Atmospheres 217
Formation of Satellite Systems 218
Cleaning Up the Solar System 218
Migrating Planets and the Late Heavy
Bombardment 218






Mercury 223
The Surface of Mercury 224
Mercury’s Temperature and
Atmosphere 226
Mercury’s Interior 227
Mercury’s Rotation 228
Venus 229
The Venusian Atmosphere 229
The Runaway Greenhouse Effect 230
The Surface of Venus 230
The Interior of Venus 233
Rotation of Venus 233
Mars 234
The Surface of Mars 234
Water on Mars 237
Extending Our Reach: Analyzing Martian
Geology 239
The Martian Atmosphere 241
The Martian Interior 243

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The Martian Moons 243
Life on Mars? 244
Why Are the Terrestrial Planets So
Different? 245
Role of Mass and Radius 245
Role of Internal Activity 246
Role of Sunlight 246
Role of Water Content 247
Role of Biological Processes 248


The Outer Planets 252
10.1 Jupiter 253
Jupiter’s Outer Atmosphere 254
Jupiter’s Interior 254
Circulation of Jupiter’s Atmosphere 255
Jupiter’s Rings 257
Jupiter’s Moons 258
10.2 Saturn 261
Saturn’s Appearance and Structure 261
Saturn’s Rings 262
Origin of Planetary Rings 264
The Roche Limit 264
Saturn’s Moons 265
10.3 Uranus 268
Uranus’s Structure 268
Uranus’s Odd Tilt 269
Uranus’s Rings and Moons 270
10.4 Neptune 271
Neptune’s Structure and Atmosphere 271
Neptune’s Rings and Moons 272

Small Bodies Orbiting the Sun 276
11.1 Meteors, Meteoroids, and Meteorites 277
Heating of Meteoroids 277
Types of Meteorites 278
11.2 Asteroids 280
Size and Shape 280
Composition 282
Origin of Asteroids 282

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Asteroid Orbits 282
11.3 Pluto, Plutoids, and Beyond 284
Pluto 284
Astronomy by the Numbers: Pluto’s Escape
Velocity 285
The Plutoids 285
11.4 Comets 287
The Appearance and Structure of
Comets 287
Formation of the Comet’s Tails 288
Astronomy by the Numbers: Calculating Comet
Halley’s Orbit 290
Composition of Comets 290
Origin of Comets 292
Short-Period Comets and the Kuiper
Belt 293
Fate of Short-Period Comets 293
Meteor Showers 294
11.5 Giant Impacts 295
Meteor Impacts on Earth 295
Astronomy by the Numbers: The Energy of
Impacts 296
Science at Work: Ghost Craters, or No Telltale
Fragments 297
Mass Extinction and Asteroid/Comet
Impacts 298

The Sun, Our Star 302
12.1 Size and Structure 303
Measuring the Sun’s Properties 303
The Solar Interior 304
Energy Flow in the Sun 305
The Solar Atmosphere 306
12.2 How the Sun Works 307
Internal Balance (Hydrostatic
Equilibrium) 307
Powering the Sun 308
Nuclear Fusion 309
The Proton–Proton Chain 310
Astronomy by the Numbers: The Mass Lost in
Hydrogen-to-Helium Fusion 311

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12.3 Probing the Sun’s Core 311
Solar Neutrinos 311
Science at Work: Solving the Solar Neutrino
Puzzle 313
Solar Seismology 313
12.4 Solar Magnetic Activity 313
Solar Magnetic Fields 314
Sunspots, Prominences, and Flares 314
Extending Our Reach: Detecting Magnetic
Fields: The Zeeman Effect 315
Heating of the Chromosphere and
Corona 317
The Solar Wind 317
12.5 The Solar Cycle 318
Cause of the Solar Cycle 318
Changes in the Solar Cycle 319
Links Between the Solar Cycle and
Terrestrial Climate 320

Measuring the Properties of
Stars 324
13.1 Measuring a Star’s Distance 325
Measuring Distance by Triangulation and
Parallax 326
Astronomy by the Numbers: Deriving the
Parallax Formula 328
13.2 The Luminosities of Stars 329
Luminosity 329
The Inverse-Square Law and Measuring a
Star’s Luminosity 329
Finding a Star’s Distance by the Method
of Standard Candles 330
Astronomy by the Numbers: Finding the
Distance of a Distant Star from a Nearby
Star 331
The Magnitude System 331
13.3 Determining the Temperatures and Radii of
Stars 333
Temperature 333
Astronomy by the Numbers: The Surface
Temperatures of Rigel and Betelgeuse 334
Radius 334

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The Stefan-Boltzmann Law 334
Direct Measurements of Radius 335
Astronomy by the Numbers: Finding the
Radius of the Star Sirius 336
13.4 Spectra of Stars 337
Measuring a Star’s Composition 338
How Temperature Affects a Star’s
Spectrum 338
Classification of Stellar Spectra 339
Science at Work: New Spectral Types 340
Definition of the Spectral Types 340
Measuring a Star’s Motion 342
Astronomy by the Numbers: Calculating a
Star ’s Radial Velocity 343
13.5 Binary Stars 344
Visual and Spectroscopic Binaries 344
Measuring Stellar Masses with Binary
Stars 345
Eclipsing Binary Stars 346
Astronomy by the Numbers: The Combined
Mass of Alpha Centauri 347
13.6 The H-R Diagram 347
Constructing the H-R Diagram 348
Interpreting the H-R Diagram 348
Giants and Dwarfs 349
Luminosity Classes 349
The Mass–Luminosity Relation 350
Astronomy by the Numbers: Calculating Stellar
Properties 352

Stellar Evolution 356
14.1 Overview of Stellar Evolution 357
The Importance of Gravity 358
The Life Story of the Sun—A Low-Mass
Star 359
The Life Story of a High-Mass Star 360
Stellar Recycling 361
14.2 Star Formation 362
Interstellar Gas Clouds 362
Protostars 363
Bipolar Flows from Young Stars 364
Stellar Mass Limits 365

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