Spectroscopy (Stars)

Spectrscopy is a scientific tool used to gauge the composition of an object by the wavelengths of light absorbed.

The Argument
The Round Earth claim is that we can determine the composition of all the stars, and see that they are made of primarily hydrogen and helium. This might be used to say that they must be the size RET states otherwise they would not be alight, or to specifically contradict models that state the stars are something else.

The Response
There are three important concepts to how spectroscopy works. First is the emission spectra; these are specific patterns of wavelengths of light that correspond uniquely to each element. You can detect elements such as hydrogen and iron this way.

Second is the absorption spectra. This is the most important to spectroscopy; according to the argument, the gas that composes the Sun absorbs certain wavelengths of light, and so these will be absent from the spectra emitted by the Sun. The Sun is not the only object to have an absorption spectra however, even empty space does. Light passes through pockets of gas, and is thus affected.

The crucial concept here is the continuous spectra, however. This is the full spectrum of light, without any lines absorbed. This is the initial state, and it is emitted by anything that radiates heat. Any solid object will have a continuous spectra.

Spectroscopy can only tell you the composition of wholly gaseous objects, or that those gases are between us and a solid source of light and heat. The physical object itself emits a continuous spectrum, the absorption spectra tells us only the composition of what surrounds the light source.