How can we determine the composition of a star from it's spectrum? How can we determine the composition of a star from it's spectrum? Use a spectrometer to disperse the emission spectrum of the star to identify the elements within the star by the specific wavelengths which are extra energetic. Use a spectrometer to disperse the emission spectrum of the star to identify the elements within the star by the specific wavelengths missing. We cannot determine the composition of a star by looking at the spectrum Use a spectrometer to disperse the absorption spectrum of the star to identify the elements within the star by the specific wavelengths which are extra energetic. Use a spectrometer to disperse the absorption spectrum of the star to identify the elements within the star by the specific wavelengths missing