Physics is only about the objectively, reproducible observable facts of nature. Concerning the Sun we have some ideas about what happens inside from applying the known laws though we cannot directly observe his interior directly (at least not with our today's technical abilities). As long as nothing observable about the Sun disproves our model about what's going on in its interior, we can at least conclude that the known laws work for the matter in the interior of the Sun as far as observable facts are concerned. Of course, we cannot deduce that our model is right in all non-observable details, but that doesn't invalidate these models nor is there anything special with the application of QT, which we inevitably have to use to describe what's going on in the Sun. It doesn't matter whether we really observe it.