To "the" or not to "the," that is the question. Apparently in Romanian a (or the) definite article is correct here, but in English we would probably say "an astronomer," if I understand this correctly. The astronomer spoken of is not actually a certain person, but one that represents all. Correct me if I am wrong.
You are right. If you want to say "all astronomers" you can either say "astronomii sunt oameni deştepți" or "astronomul este un om deştept". A famous Romanian idiom "codrul e frate cu românul" can be translated by "romanians are brothers to the woods" because from ancient times, if in trouble, the only solution was to go there: -)