My father studies astronomy — that is, the study of stars.
Literal
Father [topic-は] astronomy, that-is stars [concerning-に関する] study [object-を] researching.
つまり means 'that is, in other words' — a discourse connector that clarifies or rephrases. 天文学 (astronomy) is defined for the listener as 星に関する学問 (the study related to stars).