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