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graphene is a pretty big field in solid state physics. It has applications in the implementation of quantum computation.
air probably isn't necessary and can be merged with one of the fluid tags
nebulae this isn't really a specific object as nebulae are quite a vast topic in astrophysics. If this tag were to be deleted, it would be because we aren't getting many questions about this (not always a good reason to delete a tag imo)
asteroids ditto. And in this case we are getting questions about it.
aircraft A rather vast topic in fluid dynamics.