The degree to which you are bound to a promise made to a star is exactly the same as that of any promise made to anyone or anything. Basically, it is a balance between how much not fulfilling the promise will cause you personal anguish as to the state of your morality, good-faith etcetera, against how much actually fulfilling the promise will cause you inconvenience, hassle, grief etcetera. It’s a call only you can make.