I'll list the top grad schools for astronomy below, but while these would be great for undergrad as well, they're not the only good schools - you might want to consider a liberal arts college as well. In addition to a physics major and classes in astronomy, math, and computer science, you need research experience at the undergrad level to get into grad school, and you don't have to compete with grad students to work with a professor at a liberal arts college. So don't limit yourself to these schools.
Top grad schools for physics and astronomy - Harvard, Berkeley, CalTech, MIT, Stanford, Yale, Princeton, Columbia, Cornell, U Chicago, UCLA, UC Santa Cruz, UT Austin, U Washington, Rice, U Wisconsin, U Arizona, U Virginia, Purdue, UMass Amherst, SUNY Stony Brook, and others.
