First, a comment about definite articles. If you say the stars, you are talking about a particular group of stars that you referred to in a previous sentence. The sentence
The stars are two million miles away.
is therefore perfectly valid: The meaning, though, is not that the stars exist, but that you are talking about some stars you mentioned earlier and explaining that their physical location is two million miles away (the DESCRIPTION meaning).
It is true that be has an EXIST meaning, but: it is intransitive, very formal and generally only used in the infinitive. It doesn't really work in your sentence, because your sentence sounds like the DESCRIPTION meaning, where be is a linking verb rather than intransitive. You could, however, say:
The stars ceased to be two million years ago.
If stars there be, they must be a long way from here.
When using exist, it only really make sense if you talk about non-specific stars, stating that stars do exist two million miles away.
Stars exist two million miles away.
On a technical note, if there were stars two million miles away, we would be toast. Our sun is 93 million miles away.