Is it possible to go straight into an Astrophysics or Astronomy Ph.D. program after obtaining a B.S. in Physics (general), or would you have to more or less specialize at the undergrad level and/or shoot for a masters prior to the Ph.D. in order to make sure that you know "enough" about Astrophysics/Astronomy before moving into a Ph.D. Program? I hope all of that made sense. Thanks!