Details from the Cosmic Origins web site:
Origins of Black Holes: Gravity at Its Extreme
Feryal Özel, Associate Professor, Astronomy/Steward Observatory, University of Arizona
Gravity is the most important force in the universe, holding together planetary systems, stars, and galaxies. It is what makes the stars hot enough to shine and what keeps the Earth close enough to the Sun for life to form. It is also what ends the life of every massive star with a spectacular collapse and the formation of a black hole. Finding and studying hundreds of black holes within the Milky Way and in other galaxies brings us closer to understanding gravity at its extreme.