Basically, an adjective clause is a fancy term for an adjective that has several words to it, including a subject and a verb. Clue alert: an adjective clause will often (but not always) start with a pronoun that generally begins with a "w." Clue pronouns include who/whom, which, when, and that.
In this case, the adjective clause is "which is not a required course." (Note that "which" clue word.) This clause is further describing the noun "astronomy" by providing more information about it.
