The Cosmic Rosetta Stone

The discovery of the Rosetta Stone profoundly changed our knowledge of ancient civilizations. On one tablet a message was found written in a number of different languages. From this, archaeologists were able to decipher hitherto unknown languages. In a metaphoric way astronomy has its own "Rosetta Stones". In today's lecture we will introduce one of the most fundamental of these Rosetta Stones. In so doing we will have entered the era of modern astrophysics which began in the early part of this century.