I have heard that whatever we know about stars experimentally is through only what we can see from its surface since the light from the interior is "hidden." However, when we look at the spectrum of a star, we do see absorption lines for heavy elements. I think the reason why that is is because when an electron of a heavy element's atom inside the star is excited and it falls back again, it emits a photon which can only be absorbed by an atom of the same element. And since there are no heavy elements (usually) in the outer shells of a star, the photon doesn't get absorbed and we see it in our detectors. But if the latter part is true (we can see photons from inside of a star), then why is it said that the information from the interior of a star is hidden?