Clusters and superclusters of galaxies
  Rich clusters of galaxies are the most massive virialized systems known. Even though they contain only a small fraction of all galaxies, rich clusters provide a powerful tool for the study of galaxy formation, dark matter, large-scale structure, and cosmology. Superclusters, the largest known systems of galaxies, extend to (approx) 100h^{-1} Mpc in size and highlight the large-scale structure of the universe. This large-scale structure reflects initial conditions in the early universe and places strong contraints on models of galaxy formation and on cosmology. Some of the questions that can be addressed with clusters and superclusters of galaxies include: How did galaxies and larger structures form and evolve? What is the amount, composition, and distribution of matter in clusters and larger structures? How does the cluster mass density relate to the matter density in the universe? What constraints can the cluster and supercluster data place on cosmology? I will discuss in these lectures some of the properties of clusters and superclusters of galaxies that can be used to investigate these topics. 
Neta A. Bahcall (Princeton University Observatory)