Published in 2010 by Britannica Educational Publishing (a trademark of Encyclopædia Britannica, Inc.) in association with Rosen Educational Services, LLC 29 East 21st Street, New York, NY 10010. Copyright © 2010 Encyclopædia Britannica, Inc. Britannica, Encyclopædia Britannica, and the Thistle logo are registered trademarks of Encyclopædia Britannica, Inc. All rights reserved. Rosen Educational Services materials copyright © 2010 Rosen Educational Services, LLC. All rights reserved. Distributed exclusively by Rosen Educational Services. For a listing of additional Britannica Educational Publishing titles, call toll free (800) 237-9932. First Edition Britannica Educational Publishing Michael I. Levy: Executive Editor Marilyn L. Barton: Senior Coordinator, Production Control Steven Bosco: Director, Editorial Technologies Lisa S. Braucher: Senior Producer and Data Editor Yvette Charboneau: Senior Copy Editor Kathy Nakamura: Manager, Media Acquisition Erik Gregersen: Associate Editor, Astronomy and Space Exploration Rosen Educational Services Jeanne Nagle: Senior Editor Nelson Sá: Art Director Matthew Cauli: Designer Introduction by Greg Roza Library of Congress Cataloging-in-Publication Data The Milky Way and beyond / edited by Erik Gregersen.—1st ed. p. cm.—(An explorer’s guide to the universe) “In association with Britannica Educational Publishing, Rosen Educational Services.” Includes index. ISBN 978-1-61530-053-2 (eBook) 1. Milky Way—Popular works. 2. Galaxies—Popular works. I. Gregersen, Erik. QB857.7.M55 2010 523.1'13—dc22 2009037980 On the cover: Thousands of sparkling young stars are nestled within the giant nebula NGC 3603, one of the most massive young star clusters in the Milky Way Galaxy. NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration 17 CONTENTS Introduction 12 Chapter 1: The Milky Way Galaxy 21 The Structure and Dynamics of the Milky Way Galaxy 21 Structure of the Spiral System 22 The Nucleus 22 The Central Bulge 22 The Disk 23 The Spiral Arms 24 The Spherical Component 25 The Massive Halo 25 Magnetic Field 25 23 Rotation 26 Mass 26 Star Populations and Movement 28 Stars and Stellar Populations 28 Principal Population Types 28 The Stellar Luminosity Function 31 Density Distribution 35 Density Distribution of Various Types of Stars 36 Variations in the Stellar Density 36 Variation of Star Density with z Distances 37 Stellar Motions 38 Proper Motions 38 Radial Velocities 39 Space Motions 39 High-Velocity Stars 40 Solar Motion 40 Solar Motion Calculations from 33 Radial Velocities 41 Solar Motion Calculations from Proper Motions 41 Solar Motion Calculations from Space Motions 41 Solar Motion Solutions 42 Chapter 2: Stars 45 The Nature of Stars 45 64 Size and Activity 45 Variations in Stellar Size 46 Stellar Activity and Mass Loss 46 The 20 Brightest Stars 47 Distances to the Stars 49 Determining Stellar Distances 49 Nearest Stars 50 The 20 Nearest Stars 51 Stellar Positions 52 Basic Measurements 52 Stellar Motions 52 Light from the Stars 53 Stellar Magnitudes 53 66 Stellar Colours 54 Magnitude Systems 54 Bolometric Magnitudes 55 Stellar Spectra 55 Line Spectrum 56 Spectral Analysis 56 Classification of Spectral Types 57 Bulk Stellar Properties 58 Stellar Temperatures 59 Stellar Masses 59 Visual Binaries 60 Spectroscopic Binaries 61 Eclipsing Binaries 61 Binaries and Extrasolar Planetary Systems 63 Stellar Radii 67 Average Stellar Values 67 69 Stellar Statistics 68 Hertzsprung-Russell Diagram 68 Estimates of Stellar Ages 71 Numbers of Stars Versus Luminosity 72 Mass-Luminosity Correlations 72 Variable Stars 73 Classification 73 95 Pulsating Stars 74 Explosive Variables 76 Peculiar Variables 78 Stellar Atmospheres 80 Stellar Interiors 82 Distribution of Matter 83 Source of Stellar Energy 85 Chapter 3: Star formation and Evolution 89 Birth of Stars and Evolution to the Main Sequence 89 Subsequent Development on the Main 111 Sequence 92 Later Stages of Evolution 93 Evolution of Low-Mass Stars 93 Origin of the Chemical Elements 96 The Most Abundant Chemical Elements 96 Evolution of High-Mass Stars 99 End States of Stars 102 White Dwarfs 103 Neutron Stars 104 Black Holes 108 Chapter 4: Star Clusters 110 Open Clusters 110 Globular Clusters 114 OB and T Associations 117 Dynamics of Star Clusters 119 Clusters in External Galaxies 120 124 Notable Stars and Star Clusters 122 51 Pegasi 122 Alpha Centauri 122 Arcturus 123 Betelgeuse 123 Deneb 123 Fomalhaut 123 Pleiades 124 Polaris 124 125 Sirius 125 Vega 126 Chapter 5: Nebulae 127 Classes of Nebulae 129 Early Observations of Nebulae 131 The Work of the Herschels 131 Advances Brought by Photography and Spectroscopy 132 20th-Century Discoveries 132 Chemical Composition and Physical Processes 133 Interstellar Dust 134 145 Turbulence 136 Galactic Magnetic Field 137 Molecular Clouds 137 Composition 138 Formation of Stars 139 Hydrogen Clouds 140 Reflection Nebulae 141 H II Region 141 Ultracompact H II Regions 144 Supergiant Nebulae 145 Chemical Composition of H II Regions 146 Planetary Nebulae 148 Forms and Structure 148 The Distances of Planetary Nebulae 150 Chemical Composition 150 Positions in the Galaxy 151 Evolution of Planetary Nebulae 151 157 Central Stars 152 The Nature of the Progenitor Stars 153 Supernova Remnants 154 The Crab Nebula 157 The Cygnus Loop 158 Diffuse Ionized Gas 158 Notable Nebulae 159 Cassiopeia A 159 161 Coalsack 160 Great Rift 160 Gum Nebula 160 Horsehead Nebula 160 Lagoon Nebula 161 North American Nebula 161 Orion Nebula 161 R Monocerotis 162 Ring Nebula 162 Trifid Nebula 162 Chapter 6: Galaxies 163 The Evolution of Galaxies 165 164 Historical Survey of the Study of Galaxies 167 The Problem of the Magellanic Clouds 168 Novae in the Andromeda Nebula 169 The Scale of the Milky Way Galaxy 169 The van Maanen Rotation 171 The Shapley-Curtis Debate 172 Hubble’s Discovery of Extragalactic Objects 173 The Distance to the Andromeda Nebula 175 The Golden Age of Extragalactic Astronomy 176 Types of Galaxies 177 Principal Schemes of Classification 177 Elliptical Galaxies 177 Irregular Galaxies 182 Other Classification Schemes and Galaxy Types 182 The External Galaxies 183 181 The Extragalactic Distance Scale 183 Physical Properties of External Galaxies 186 Size and Mass 186 Luminosity 186 Age 187 Composition 187 Structure 188 The Spheroidal Component 188 The Disk Component 189 196 Spiral Arms 189 Gas Distribution 189 Galaxy Clusters 189 Types of Clusters 190 Distribution 190 Interactions Between Cluster Members 191 Galaxies as a Radio Source 191 Radio Galaxies 191 X-ray Galaxies 193 Clusters of Galaxies as Radio and X-ray Sources 193 Quasars 193 Gamma-Ray Bursters 195 203 Notable Galaxies and Galaxy Clusters 195 Andromeda Galaxy 195 Coma Cluster 196 Cygnus A 197 Great Attractor 197 Magellanic Clouds 198 M81 Group 198 Maffei I and II 199 Virgo A 199 Virgo Cluster 200 Appendix: Other Stars and Star Clusters 201 Glossary 212 Further Reading 213 Index 214 207 INTRODUCTION Introduction | 13 or thousands of years, astronomers ergs) of luminosity per second, making it Fhave worked to unlock the mysteries average in size, mass, and brightness. of the heavens. As they observed the stars Scientists use these measurements as and planets that parade majestically benchmarks when discussing other across the sky, one great source of won- stars, which can be bigger or smaller, der was a huge faint trail of light that was more or less bright, depending on their called the Milky Way Galaxy because it type and age. looked like milk, spilled across the dark- While 150 million kilometres may ness of night. When scientists started sound like a very long distance, it is tiny using telescopes in the 1600s, they began in cosmic terms. The next closest star to to understand more about the Milky Way. Earth is Proxima Centauri, which is the But it wasn’t until the 20th century that smallest of three stars making up the triple new technology allowed scientists to take star Alpha Centauri. Proxima Centauri is the full measure of this galaxy and others approximately 4.22 light years from scattered across the universe. What they Earth—or about 3.99 × 1016 metres (2.48 × have uncovered is richly detailed in the 1013 miles). Even the fastest modern pages of this book. spacecraft would take countless lifetimes The primary elements of any galaxy to reach this distant star. are stars. A star is a massive body of gas Scientists have used several diff er- that shines by radiation resulting from ent methods to determine the distances internal energy sources. There are so from Earth to the stars. The earliest many stars in the universe it would be method, which is still used to determine impossible to count them all. Only a very the distances of closer stars, is a trigo- small fraction are actually visible to the nometric parallax, which involves unaided eye. In the Milky Way Galaxy observing a star from two points on alone there are hundreds of billions of opposite sides of Earth’s orbit. This stars. The easiest of these for scientists to technique depends on using the rela- study is the Sun, the star closest to Earth. tively unchanged backdrop of distant It is about 150 million kilometres (93 space for precise measurements. More- million miles) away from us. It has a distant stars require other techniques, radius of about 700,000 km (430,000 most of which rely on comparing miles.). Its mass is about the same as that luminosities. of 330,000 Earth masses. It creates Depending on its mass, a star may approximately 4 x 1023 kilowatts (4 × 1033 “live” 10 million years or 10 billion years. Milky white stars and interstellar dust cascade across the sky to form the Milky Way Galaxy, a spiral galaxy that is home to the solar system. www.istockphoto.com/Shaun Lowe 14 | The Milky Way and Beyond: Stars, Nebulae, and Other Galaxies During this time, it goes through many the outer layers of the star to expand.