| Galaxies | 479 |
| 16.1Discovering Galaxies | 480 |
| Early Observations of Galaxies | 480 |
| Types of Galaxies | 481 |
| Differences in the Stellar and Gas Content of Galaxies | 485 |
| The Cause of Galaxy Types | 487 |
| Galaxy Collisions and Mergers | 488 |
| 16.2Measuring Properties of Galaxies | 490 |
| Galaxy Distances | 490 |
| Extending Our Reach: Measuring the Distance of a Galaxy Using Cepheid Variables | 491 |
| The Redshift and the Hubble Law | 492 |
| Measuring the Diameter of a Galaxy | 493 |
| Extending Our Reach: Other Ways to Measure a Galaxy's Distance | 494 |
| Measuring the Mass of a Galaxy | 494 |
| 16.3Dark Matter | 495 |
| 16.4Active Galaxies | 496 |
| Radio Galaxies | 496 |
| Seyfert Galaxies | 498 |
| Quasars | 498 |
| Cause of Activity in Galaxies | 499 |
| Extending Our Reach: Measuring the Diameter of Astronomical Objects
by Using Their Light Variability | 500 |
| 16.5Quasars As Probes of Intergalactic Space | 502 |
| Gravitational Lenses | 503 |
| Extending Our Reach: Dark Matter and Gravitational Lenses | 504 |
| 16.6Galaxy Clusters | 504 |
| The Local Group | 505 |
| Rich and Poor Galaxy Clusters | 506 |
| Superclusters | 507 |
| Project | 511 |