Yes. A PhD is regarded as the level of expertise in a subject. You'll be eligible for professorship provided you have at least a Master's in Astrophysics or similar field. In order to apply for PhD in a course, you need to have a previous degree and research papers in that particular course. So look for pursuing a Master's course in astrophysics before a PhD. This will make you more qualified and a better equipped candidate.
Hope this helps. Good luck.