Mathematical astrophysics subjects for math graduate students
As a first step Newtonian Mechanics is very important. Both at an undergrad level and graduate level. So a calculas based first course is very important. Also very useful is a source on undergrad Atomic Physics and a course in Nuclear Physics. There are several others besides these. I would recommend the series of courses in a Halliday and Resnick type of text as a start. This will help set you up for many other courses in Physics. IMHO.