Hi Stacy,

I think scientists first figured out the composition of the sun by studying its spectra. Spectra show all the different colors the Sun emits. Different elements make different lines in the spectra, like a fingerprint. You can use those lines to figure out what the Sun is made of. It turns out it is mostly hydrogen, but there is also a fair bit of helium, and small amounts of other elements. In fact, helium was discovered in spectra fro the sun before it was discovered on the Earth!

Here is an image of the visible solar spectrum. It is so long it is wrapped around in many different rows. the dark lines ar mostly due to hydrogen.

http://content.answers.com/main/content/wp/en/thumb/3/3c/350px-High_Resolution_Solar_Spectrum.jpg

I don't know off the top of my head exactly when all this happened, but sources on the web indicate that lines were first discovered in solar spectra around 1802. Spectral lines and their relationship to elements were not yet understood until later that century, though, and helium was not discovered until 1868.

Terry