The Impact of Massive Stars on Cluster Formation by Dr. S. T. Megeath (CfA) (CfA)   Massive OB stars play an important role in the evolution of molecular clouds and star forming regions. The OB stars both photo-ionize molecular gas as well as sweep up and compress interstellar gas through winds, ionization fronts, and supernovae. I will examine the effect massive stars have on the formation of young stellar clusters. I will first discuss the processes by which OB stars destroy cluster-forming molecular cores, and overview observational evidence that OB stars forming in young stellar clusters destroy their parental cores on a timescale of 0.1 Myr. I will then summarize extensive observations of the NGC 281 molecular cloud complex, and present evidence that two types of triggered star formation are occurring in this complex. My goal is to underscore the impact massive stars have on cluster formation over distances ranging from 0.1 pc to 300 pc.