Written in an informal and engaging style, this volume traces the discoveries that led to our understanding of the size and structure of the Milky Way, and the conclusive evidence for a massive black hole at its center. Robert H. Sanders, an astronomer who witnessed many of these developments, describes how we parted the veil of interstellar dust to probe the strange phenomena within. We now know that the most luminous objects in the Universe – quasars and radio galaxies – are powered by massive black holes at their hearts. But how did black holes emerge from being a mathematical peculiarity, a theoretical consequence of Einstein's theory of gravity, to become part of the modern paradigm that explains active galactic nuclei and galaxy evolution in normal galaxies such as the Milky Way? This story, aimed at non-specialist readers and students and historians of astronomy, will both inform and entertain.

Acknowledgements page ix

1 Introduction: The Luminous Pathway 1

2 The Discovery of the Milky Way Galaxy 13

2.1 Kapteyn’s Universe 13

2.2 Pickering’s Harem 16

2.3 Shapley’s Supergalaxy 18

2.4 The Legacy ofJ. C. Kapteyn 23

3 The New Physics 25

3.1 Revolution 25

3.2 The Power ofPrinciple 28

3.3 Arthur Eddington Boosts Einstein to World Acclaim 32

3.4 The Solution from the Trenches 34

4 Parting the Veil with Radio Astronomy 38

4.1 Kapteyn’s Famous Student 38

4.2 Radio Astronomy and the 21-cm Line 42

4.3 Toward the Galactic Center 47

4.4 Jan Oort and the Galaxy: An Assessment 49

5 The Violent Universe 51

5.1 A New Constituent ofthe Universe 51

5.2 Active Galaxies: Radio Galaxies and Seyferts 56

5.3 Twinkle, Twinkle Little Quasar: Early Models ofthe QSO Phenomenon 59

5.4 Where Have All the Quasars Gone? 64

5.5 Summing Up a Decade ofDiscovery 67

6 New Windows on the Galactic Center 69

6.1 The Near Infrared and the Distribution ofStars in the

Galactic Center 69

6.2 A Unique Source at a Unique Location 73

6.3 Mid- and Far-Infrared Radiation 76

6.4 The “Mini-Spiral” in Sgr A 79

6.5 Molecular Clouds in the Central Region 80

7 The Milky Way as a Barred Spiral Galaxy 83

7.1 Ejections and Explosions 83

7.2 Gas Motion in Barred Spiral Galaxies 89

7.3 Controversy in Astronomy 97

8 The Evolving View of Active Galactic Nuclei 99

8.1 The Jet Set 99

8.2 The View from Cambridge 101

8.3 A Massive Black Hole in NGC 4258 106

8.4 Are There Alternatives to a Black Hole? 110

8.5 Reflections 113

9 The “Paradox of Youth”: Young Stars in the Galactic Center 115

9.1 The View from Groningen, 1983 115

9.2 A Nuclear Starburst 118

9.3 Gas Distribution and Motions Near Sgr A* 121

9.4 Distribution and Motion ofYoung Stars Near the Galactic Center 123

9.5 Star Formation in the Near Tidal Field ofa Black Hole 126

10 Stellar Orbits in the Galactic Center, QED 129

10.1 Two Mountains, Two Legends 129

10.2 New Technology for a New Millennium 131

10.3 Inward Bound: Leapfrog to the Black Hole 137

10.4 Individual Star Orbits: The Black Hole Gains Weight 139

10.5 Youth, Even More Paradoxical 143

10.6 Pele and the Alicanto 145

10.7 Summing Up: What Have We Learned? 146

11 Black Holes Here, Black Holes There... 149

11.1 Nearby, “Normal” Galaxies 149

11.2 Bigger Black Holes Live in Bigger Spheroids 153

11.3 Blast from the Past 158

11.4 The Early Formation ofBlack Holes 161

12 Traces of Activity: Past, Present, and Future 165

12.1 Effects ofBlack Hole Outbursts on the Galactic Center Gas 165

12.2 X-Ray Echos 169

12.3 Fermi Bubbles 171

12.4 A Snack for Pele 173

12.5 The Face ofPele: The Event Horizon Telescope 175

13 After Words: Progress in Astronomy 177

References 185

Index 193

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