Unit 5
Galaxies and Cosmology

Having completed our study of the properties of individual stars, we now examine the combined properties of billions of stars gathered together under the influence of gravity—the galaxies. Galaxies come in several shapes and sizes—spirals, irregulars, dwarfs, and giants. They can also be the centres of violent activity. Active galactic nuclei may produce prodigious amounts of radio and X-ray emission.

Galaxies also enable us to trace out the large-scale structure of the universe itself. As we observe galaxies at increasingly greater distances, we look backward toward the beginning of the universe itself. Since light travels at a finite speed, the more distant an object, the longer its light takes to reach us. Consequently, observations of very distant galaxies tell us about the properties of the early universe, and about the space-time structure of the universe that this light passes through on its way to us.