You would study astrophysics as a graduate student, not as an undergraduate. At the undergraduate level, you would be studying physics as the second major (possibly, but less likely, astronomy). as others have noted, with a Chemical Engineering degree, you can get a job right out of college. For astrophysics, a Ph.D is required, followed by one or more postdoctoral research positions. All that being said, if you really want to do the two degrees, go for it.