Returning to school for a masters and PHD in Astronomy.... with an undergraduate in Agricultural Business.?

I have an obsession with anything dealing with astronomy and astrophysics and I would like to return to school to become either a professor in Astronomy or anything related. I currently have an undergraduate in Agricultural Business. I also hate the job that I currently work and would enjoy teaching Astronomy a lot more than what I'm doing right now. Not sure how much I would get paid but I really don't care. Since my undergraduate is in an unrelated field, would I have to go back for a second undergraduate degree in astronomy or is it as simple are taking a few prerequisites? Any advice here would be greatly appreciated.