What does a spectrograph help astronomers to determine?
A spectrograph splits up the light according to wavelength
A first determination as to general colour allows us to classify stars into groups.
In the spectrogram you will find dark lines, each of which points to a specific element. So the spectrogram can tell us the composition of stars, like how is the ration of hydrogen vs helium, what other elements are present? And this gives us an idea of the "age" of a star.