Viewer's following celebrity astronomer (9)
I believe the answer is:
stargazer
'astronomer' is the definition.
(synonyms)
'viewer's following celebrity' is the wordplay.
'viewer' becomes 'gazer' (someone viewing or gazing at something).
'following' is a charade indicator (letters next to each other).
'celebrity' becomes 'star' (a star is a celebrity or famous person).
'gazer' after 'star' is 'STARGAZER'.
(Other definitions for stargazer that I've seen before include "One looks up" , "I look at the Sun" , "Astronomer or astrologer" , "Looker-up at night" , "Astronomer, or dreamer" .)