Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century . In 1913 , the Hertzsprung @-@ Russell diagram was developed , propelling the astrophysical study of stars . Successful models was developed to explain the interiors of stars and stellar evolution . Cecilia Payne @-@ Gaposchkin first proposed that stars was made primarily of hydrogen and helium in her 1925 PhD thesis . The spectra of stars was further understood through advances in quantum physics . This allowed the chemical composition of the stellar atmosphere to be determined .