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: -)
The use of the definite article "-ul" here means that we are talking about a certain astronomer (who has been mentioned before). "An astronomer" would be "un astronom" in Romanian.
This is correct but if no astronomer was mentioned before it could mean the whole category (all astronomers in this case)
As a matter of fact, this affirmation is not entirely correct. Astronomers have to know and understand lots of physics in order to be classified or qualified as ”astronomers”.