What should my girlfriend do if she is obsessed with constellations?

You tell her that "if you believe, there is, if you don't believe, there is nothing", as long as you like each other, don't care about this.

Otherwise, there is something wrong with her mood, and she mistakenly thinks that the constellation is inappropriate, but it is actually artificial association. It has nothing to do with the constellation.

So let her know that the constellation is just entertainment, don't take it seriously.

Nothing is absolutely suitable or unsuitable, but depends on mutual understanding and tolerance to the end.