Colossal, as is star's study? (12)
I believe the answer is:
astronomical
I believe this is a double definition.
'colossal' is the first definition.
(thesaurus)
'as is star's study?' is the second definition.
(astronomy is the study of stars)
(Other definitions for astronomical that I've seen before include "Of the study of stars" , "Massive" , "Enormous" , "Really, really big" , "Inconceivably large" .)