What is a medium size yellow star composed mostly of hydrogen and helium?

All stars are composed mostly of hydrogen. The star fuses hydrogen into helium. The helium cannot escape the start until the end of the star's life cycle. Thus a middle-aged, medium sized star will be comprised of hydrogen and helium. As the star ages, it may start to fuse helium at its core into carbon. At this point it will likely expand into a red giant star, and thus would no longer be yellow.