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