Abstract
In an effort to provide additional opportunities for active, cooperative learning by students in introductory earth-science classes, I have developed an exercise on the use of cepheid-variable stars as distance indicators. Cepheid-variable stars were the key to Edwin P. Hubble's determination in 1924 that the so-called “spiral nebulae” (which were thought at the time by many astronomers to be located within our Milky Way galaxy) are, in fact, “spiral galaxies” that are located millions of light years away from the earth, well outside our galaxy. This determination by Hubble was one of the most significant astronomical discoveries ever made and enlarged our concept of the universe enormously. Five years later, Hubble demonstrated that the Universe is expanding.
