Is it good that taking 1 year for iit jam preparation for msc? or wanna i do any other courses?. i like in astronomy.

I am graduate in physics with 72%. if i am taking my 1 year for iiit jam preparation(self study) without any coaching, can be it successful?. i like in astronomy. is it any diploma courses in astronomy or astrophysics?. what is the difference b/w experimental physics and theoretical physics?