What is the origin of the letters used in star classification? Why didn’t astronomers use ABCD? J. HaldimanChicago, Illinois Stellar spectral types originally followed alphabetical order. But as astronomers observed more stars and obtained more detailed spectra, they consolidated some categories and reordered the remaining ones. This left seven basic stellar types, designated OBAFGKM, which Continue reading "What is the origin of the letters used in star classification?" The post What is the origin of the letters used in star classification? appeared first on Astronomy Magazine .