 The science of stellar spectroscopy was pioneered by Joseph von <unk> and Angelo <unk> . By comparing the spectra of stars such as Sirius to the Sun , they found differences in the strength and number of their absorption lines — the dark lines in a stellar spectra caused by the atmosphere 's absorption of specific frequencies . In 1865 <unk> began classifying stars into spectral types . However , the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s . 
