It helps us find out the make-up of the stars because each element has an unique emmission spectrum
Why?
When the electrons of an element are excited they rise to higher energy levels, and the emit radiation when they go back to their original level.
Since each element has an unique electronic configuration, the emmision spectra of the elements inside a star has unique lines for each element, and analyzing the emmission spectrum (the light) from a star will help us find out which elements can be found in it.
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