Harvard classification system

The Harvard classification system is a method used in astronomy to categorize stars based on their surface temperatures, as determined by their spectra. This system, developed at Harvard College Observatory, initially used letters in alphabetical order, but was later rearranged into a non-alphabetic sequence from hottest to coolest stars: O, B, A, F, G, K, M.1

This sequence is often remembered by the mnemonic "Oh Be A Fine Girl [or Guy], Kiss Me." Additional letters (L, T, Y) have been added to classify brown dwarfs, which are objects that form like stars but do not undergo thermonuclear fusion. Each spectral type is further subdivided by numbers from 0 to 9, with higher numbers indicating cooler stars. For example, the Sun is a G-type star. The MK system, an extension of the Harvard system, also incorporates luminosity classes.12