Out of curiosity and not related to this thread (sorry) what are some of the levels that you are having finding it difficult with? I have 77 stars currently and have not beaten the final bowser battle.
I've had the game since its release. It took me about a month to get 120 stars with Mario. Galaxy is a wonderful game full of surprises and immerses you in imaginative, creative worlds that fit so well in Mario's whacky world. SMG is now one of my favourite games and I continue to play a little bit every week and always find something new. I'm currently completing Luigi's quest. I have 42 stars and am looking forward to the rumored (spoilers) "bonus galaxy" that you unlock with 120 stars in Luigi's game.
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I've had this since release, but I put the game down after getting my first 50 stars. My wife and I had our first child, so I was busy with that. I just started playing again today, and I got 3 more stars. So I'm at 53 going on 120. Hopefully, I can get all 120 in the next week or two.
You can collect 60 and open the main battle and win to finish the game but if you want to open new characters and a final hidden star you will have to collect 120 stars.