、。
There was a time I wanted to be an astrophysicist.
Literal
I [topic-は] one-period, astrophysicist [target-に] become-want-[past-だった].
~たい inflects like an i-adjective: present ~たい, past ~たかった, negative ~たくない. So なりたかった is 'wanted to become' — a past desire. 一時期 ('one period of time') is a fixed expression meaning 'at one point/for a while in the past.' 天体物理学者 'celestial-body physics-scholar' = astrophysicist; a heavy Sino-Japanese compound that nicely shows how the language builds technical vocabulary.