The Milky Way 267 9.1 Discovering the Milky Way 269 Shape of the Milky Way 269 Size of the Milky Way 270
9.2 Overview of the Milky Way 272 Structure and Contents 272 Mass of the Milky Way and the Number of Stars 273 Age of the Milky Way 274
9.3 Stars of the Milky Way 274 Stellar Censuses and the Mass Function 274 Two Stellar Populations: Population I and Population II 275 Star Clusters 277
9.4 Gas and Dust in the Milky Way 279 Interstellar Dust: Obscuration and Reddening 280 Interstellar Gas 282 Visible Emission from Interstellar Gas 282 Radio Waves from Cold Interstellar Gas 284
Extending Our Reach: Mapping the Milky Way with Radio Waves 284
9.5 Motion of Stars and Gas in the Milky Way 285 9.6 Measuring the Milky Way 288 Diameter of the Milky Way 288 Mass of the Milky Way 288 Extending Our Reach: Measuring the Mass of the Milky Way 289 Extending Our Reach: Measuring the Sun's Speed Around the Milky Way 291
9.7 The Galactic Center 291 9.8 History of the Milky Way 293 Formation of Our Galaxy 293 Collapse of the Proto-Milky Way and the Birth of Population I and II Stars 293 Population III 296 The Future of the Milky Way 296 Project 299
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