SAO[L1 ] is a "research institute" of the Smithsonian Institution[L2 ] headquartered in Cambridge, MA, where it is joined with the Harvard College Observatory (HCO[L3 ]) to form the Harvard-Smithsonian Center for Astrophysics (CfA[L4 ]). More than 300 scientists at the CfA are engaged in a broad program of research in astronomy, astrophysics, earth and space sciences, and science education.