Why does a stellar spectrum tell us about the surface layers but not the deeper layers of the star?

Question

Is this because one can only observe the surface?

Answer 1

A stellar spectrum does tell us something of what's going on beneath. Specifically if the star is a main-sequence hydrogen fusing star, an old star that's fusing helium or heavier elements for energy, or a star that collapsed and became a white dwarf. Mainly what a stellar spectrum tells us is things like the star's surface temperature, chemical composition, age, size, rotation rate, luminosity and mass. As stars get old, spectra will tell us what heavier elements have been fused because convection in the outer envelope dredges them to the surface, and thus makes them observable to a spectrograph. They can also reveal whether the star is flaring or not, and provide clues to the structure and nature of their magnetic fields. A spectrum will also reveal the presence of a companion too close to be seen directly through telescopes, and show clues as to the masses of the stars along with their effects upon each other. For example, some double stars undergo mass exchange, with material flowing from one star to the other, which induces a telltale signature in a spectrum. However a visible light spectrum will not show things that one in the X-ray, ultraviolet or infrared specturm will. Many elements are largely invisible to a spectrograph in the visible, but prominent in other regions of the spectrum. We cannot observe directly what's going on in the interiors of stars, but astronomers can see their internal activity's effects upon stellar surfaces. Spectroscopy is the mainstay of astrophyisics for a very good reason, because energy emitted from stars carries information about that star along with it.

Answer 2

Any electromagnetic radiation from deep in the star must pass through the surface layer of the star to escape into space. The surface material would absorb some of the wavelengths of the deep material as it passes through. That means we would not get the entire spectra of the deep materials which pass to the surface and could not be sure of their identity.

Answer 3

What we see as the surface of a star, the photosphere, is opaque, so radiation from deeper inside the star does not come through. The spectrum we observe is from the photosphere and the above.