What is a Galaxy? Beth Willman Astronomy Department Haverford College Beth Willman is an assistant professor of astronomy at Haverford College. Working with Sloan Digital Sky Survey data, she and her team discovered the first two of a new class of ultra-faint galaxies around the Milky Way: Willman 1 and Ursa Major 1. Willman earned her PhD at the University of Washington in 2003. She earned her undergraduate degree in Astrophysics at Columbia University. Following her PhD, she was a James Arthur Postdoc at the Center for Cosmology and Particle Physics (CCPP) at New York University and a Clay Fellow at the Harvard-Smithsonian Center for Astrophysics. She recently earned a CAREER award from the NSF. In the past eight years, more than two dozen dwarf galaxies have been discovered around the Milky Way and M31. Many of these newly discovered galaxies are 100 times less luminous than any galaxy previously known, and a million times less luminous than the Milky Way itself. These discoveries have made astronomers question the very meaning of the word "galaxy", and hint that such ultra-faint dwarf galaxies may be the most numerous type of galaxy in the universe. This talk will highlight i. how we can see galaxies that are effectively invisible in images of the sky, ii. the brewing controversy over the definition of the term "galaxy", and iii. what ultra- faint galaxies can reveal about the distribution of dark matter in our Universe. Thursday, March 7, 2013 at 4:10PM in LL. 316 .