This is an excellent question. Let's consider the sentence
Mathematici Graeci saepe lunam astraque intuiti sunt.
The -que at the end of "astra" is called the enclitic -que. It is a special type of suffix that attaches to the second word in a pair of words, or to the last word in a series of words.
In the example above, the enclitic -que attaches to the second of a pair of words (lunam and astra).
The enclitic -que gets translated as "and".
When translating, we insert the "and" between the penultimate (second-to-last) word in the series, and the ultimate (last) word in the series. In this case it gets inserted between "moon" and "stars".
So we translate the phrase "lunam astraque" as "moon and stars".
There is a lot of discussion of the enclitic -que on this website (https://dcc.dickinson.edu/grammar/latin/uses-conjunctions) and this website (https://dcc.dickinson.edu/grammar/goodell/proclitics-and-enclitics).
It is also interesting to point out a frequently used enclitic in the English language. The possessive 's (apostrophe s) is an enclitic in the English language.
The -que in the phrase "Senatus populusque Romanus" is another example. This phrase famously gets abbreviated as SPQR. A translation would be: The Roman senate and people.
Having talked at length about the enclitic -que, we are now ready to translate the sentence.
Mathematici Graeci saepe lunam astraque intuiti sunt.
The verb intueor, intueri, intuitus sum means "to look at, look upon, admire". The phrase intuiti sunt has a passive form but an active meaning, since the verb is deponent. It's in the perfect tense, so we can translate it as "they looked at" or "they looked upon" or "they admired".
The adverb "saepe" means often. (In case you were wondering, what part of speech is the enclitic -que? It's a conjunction, since it serves to connect two or more words.)
Putting all of this together, we arrive at the translation
Mathematici Graeci saepe lunam astraque intuiti sunt.
The Greek mathematicians often looked at the moon and the stars.
To answer your question, the word "astra" does not modify "lunam", but gets combined with "lunam" by means of the enclitic -que. So the two nouns — lunam and astra — are joined together into a single direct object phrase. The word "lunam" is a first declension singular noun in the accusative case, and it means "moon". The word "astra" is a second declension plural noun in the accusative case, and it means "stars".