One who studies the stars; an astronomer or astrologer (archaic usage).
From 'astre' (star) plus the agent suffix '-er', forming a noun for 'one who [does something with] stars.' This is an archaic term largely replaced by 'astronomer'.
Before we had 'astronomer' and 'astrologer' as separate words, people just called anyone who studied stars an 'astrer'—it shows how specialization in language follows specialization in actual fields!
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