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. :)

They are very similar, true is simply saying that they really are stars. Truly is more intensifiying and adds more passion and meaning to the fact that they are stars.

Thanks.

There's very little difference. "True" is an adjective that modifies "stars" and "truely" is an adverb that modifies "are." You could probably use whichever sounds nicer.