There was always this question about a star, whether a star would crunch. Well, you pile on more and more mass, you shovel more and more gravel onto the surface of the Earth, you make it ultimately massive enough so it will crunch. Can you do anything to stop it? Well, what resists collapse is rigidity, but there's a limit to rigidity, because rigidity of a piano string governs the pitch of the string, rigidity of a steel rod governs the speed that sound will travel down the length of that steel rod. But we knew there's a limit to the speed of light, and therefore there must be a limit to rigidity. And if you factor that line of reasoning in, you see that you can't have a material rigid enough in the interior of a star to keep it from crunching, if you pile on enough mass. So I didn't see any way to escape the idea of a star crunching to a black hole. I've been impressed with my Dutch colleague, Jan Oort, he's no longer living, but he was the dean of astronomers while he lived. All through his life one theme stood uppermost, where is the mass that keeps going? Where is the mass that holds the stars together in the Milky Way? Where is the mass that keeps stars circulating around in tight orbits near the center of the Milky Way? Where is the mass that holds the universe together? And that search for the missing mass is one that goes on today. I'm fascinated by the recent work of our Polish colleague, Bohdan Paczynski, on Gravitational Lensing. He said maybe there's a lot of mass out there in space that we don't see. Stars may have burned out and become dead, they may have collapsed to black holes or maybe just cold, dead stars.