If you have the opportunity I would appreciate any help you might have. The area you are interested in, the composition of stars, is a very important and interesting field for astronomers. Stars begin their life when an ordinary dense cloud of interstellar matter becomes unstable and begins to collapse. The composition of such a cloud of matter determines the composition of the star which results from the collapse. Let's start by looking at the chemical composition of the star we know the most about, our Sun.
Astronomers study the spectrum of the Sun to determine it's chemical composition.
These lines have been cataloged, and tell us that there are 67 chemical elements identified in the Sun. There are probably even more elements in the Sun that are present in such a small amount that our instruments can't detect them. 0.014 Sulfur 0.015 0.040 You see that hydrogen is by far the most abundant element in the Sun, followed by helium. This is also what we find in the composition of the Universe as a whole. When other stars are studied spectroscopically it is found that most stars are composed of around 70 percent hydrogen and 28 percent helium, very similar to what we see in the Sun.