Mostly Hydrogen

About 92% of the atoms of the Sun are Hydrogen with about 9% Helium. The remainder consist of a variety of elements such as Oxygen and Carbon. How do we know? By using spectroscopic analysis of it's light, this is where the light from the Sun is split into a rainbow and dark lines in the rainbow represent the elements in the Sun's atmosphere. Ok, so that is a bit simplified, but it is the basic process. The same procedure is used to determine what distant stars are made of. A philosopher in the late 18th or early 19th century said that of all the things that man could know the one thing he could never determine was the constitution of the stars! How silly does he feel now!!!