A PhD in physics and research in astrophysics could get you there. So could a PhD in astronomy or astrophysics, but be sure to take a lot of physics as well. I'm getting a PhD in physics and studying astrophysics for my dissertation (early universe cosmology) so I guess I'm doing all of it - and a PhD in physics makes you a lot more employable than a PhD in astronomy. So major in physics in college, take classes in astronomy, math, and computer science, and find a university with a combined physics and astronomy program for grad school.