Before 1931, astronomers could gain information about the cosmos only by visual means, using the naked eye aided by magnifying lenses and reflective mirrors. In the early part of the twentieth century, however, a new way of assessing the nature of the stars and the planets was found.

Visible light is a small segment of a vast electromagnetic spectrum that includes X rays, gamma rays, infrared light, and radio waves. Radio waves are light waves that have a longer frequency than the visible red and the invisible infrared portions of the electromagnetic spectrum. Many of the most important objects in the cosmos are invisible, or very dim, at visible wavelengths, yet extremely active at lower electromagnetic frequencies. Such objects can be detected and studied easily with a receiver tuned to the appropriate frequency.

In 1928 Karl G. Jansky, an engineer employed by Bell Telephone Laboratories, was given the...