το φως του αστεριού - the light of the star (i.e. one star)
starlight - το φως των αστέρων, (maybe also η αστροφεγγιά?)
You have a typo των αστεριών of the stars. Αστροφεγγιά is starlight but also ξαστεριά very bright starlight on a cloudless sky
Well, των αστέρων isn't really a typo, but the genitive plural of ο αστέρας. But you are right—in line with του αστεριού I should have used των αστεριών.
'The light of the stars' was a wrong answer, really?
That would be "το φως των αστεριών".
If I wanted to say we went for a walk in the starlight, would the singular αστεριού or the plural αστεριών be used?
Plural, "... στο φως των αστεριών".