These sentences are very similar but there can be a slightly different nuance depending on context.
1. They are true stars - this can almost be like saying “they are true stars compared to other people” depending on context. Like “Beyoncé thinks she’s a star, but fire fighters are the true stars” haha something like that.
2. They are truly stars - you can use this just as a statement or to say something like “they don’t look like it, but they are truly stars” haha I can’t think of a good example.
Really these sentences are pretty much the same, it just depends how you use them in different situations. :)