One of the most notable scientific advances in recent decades has been in the field of the chemistry of stars. It has been almost definitely established that the stars are made up of the same elements as those of the earth, although the relative proportions of the elements differ substantially. Thus, more than 50% of the mass of a star is usually Hyrdrogen; and Helium may account for almost 40% of its mass. All the other metallic and non-metallic elements account for only several per cent of the mass of the star.
These results, as well as a great deal of other information about the stars, are derived from careful analysis of the electromagnetic radiation, in the form of light, that reaches us from the stars. The branch of science that deals with such analysis is called "Spectroscopy", and its basic instrument is the Spectroscope.