Outline

Galaxies 301

10.1 Discovering Galaxies 302

Early Observations of Galaxies 302

Types of Galaxies 303

Differences in the Stellar and Gas Content of Galaxies 307

The Cause of Galaxy Types 309

Galaxy Collisions and Mergers 310

10.2 Measuring Properties of Galaxies 312

Galaxy Distances 312

Extending Our Reach: Measuring the Distance of a Galaxy Using Cepheid Variables 313

The Redshift and the Hubble Law 314

Measuring the Diameter of a Galaxy 315

Extending Our Reach: Other Ways to Measure a Galaxy's Distance 316

Measuring the Mass of a Galaxy 316

10.3 Dark Matter 317

10.4 Active Galaxies 318

Radio Galaxies 318

Seyfert Galaxies 320

Quasars 320

Cause of Activity in Galaxies 321

Extending Our Reach: Measuring the Diameter of Astronomical Objects by Using Their Light Variability 322

10.5 Quasars As Probes of Intergalactic Space 324

Gravitational Lenses 325

Extending Our Reach: Dark Matter and Gravitational Lenses 326

10.6 Galaxy Clusters 326

The Local Group 327

Rich and Poor Galaxy Clusters 328

Superclusters 329

Project 333

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