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…Jump Cannon from 1918 to 1924. The original scheme used capital letters running alphabetically, but the subsequent revisions have reduced this as stellar evolution and typing has become better understood. The work was published in the Henry Draper Catalog and Henry Draper Extension which contained spectra of 225,000 stars down to ninth magnitude. The scheme is based on lines which are mainly sensitiveto stellar surface tempertures rather than actual composition differences,gravity, or luminosity. Inportant lines Word Count: 418