Step-by-step explanation:
To determine the composition of the star, scientists takes the help of the atomic physics .
As Different ions or atoms emits light waves at different combinations of wavelength that is particular of that very atom or element . These light waves are knows as the emission lines and each atom or ion has unique emission spectra which allows them to distinguish which kind of element light is sent out from the star and hence distinguish whether the star is made up of hydrogen or helium .