How did we come to understand that the Milky Way was just one of

billions of other galaxies in a vast Universe? This lecture

reviews the history of how we came to recognize that the spiral nebulae

were, in fact, other milky ways like our own: vast systems of

100s of billions of stars located millions of parsecs away. The

key to understanding their nature was finding the distances to the

spiral nebulae compared to the size of our Galaxy.

Recorded 2006 February 8 in 1008 Evans Laboratory on the Columbus campus

of The Ohio State University.