Outline

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