noun
The scientific study of celestial objects and phenomena.
彼は大学で天文学を専攻している。
He is majoring in astronomy at university.
天文学 is the full term for 'astronomy' as a field of study, while 天文 is a shorter form often used in compounds like 天文台 (observatory).
From Middle Chinese 天文 (then mjun), meaning 'heavenly patterns' or 'astronomy'. The kanji 天 means 'sky, heavens' and 文 means 'pattern, writing'.