By measuring the rapid orbits of the stars near the center of our galaxy, Dr. Andrea Ghez of UCLA and her colleagues have moved the case for a supermassive black hole at the heart of our Milky Way Galaxy from a possibility to a certainty.  She reports on her pioneering observations of stars near our galaxy's center (that orbit the monster black hole) and discusses some of the surprising results this work has led to.  The talk was recorded in January 2017; in 2020, Dr. Ghez won the Nobel Prize in physics for this work.