Page "Stellar classification" Paragraph 1

Most stars are currently classified using the letters O, B, A, F, G, K, and M, where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class.

A useful mnemonic for remembering the spectral type letters is " Oh Be A Fine Girl Kiss Me ".

According to informal tradition, O stars are called " blue ", B " blue-white ", A stars " white ", F stars " yellow-white ", G stars " yellow ", K stars " orange ", and M stars " red ", even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed.

The current non-alphabetical scheme developed from an earlier scheme using all letters from A to O ; the original letters were retained but the star classes were re-ordered in the current temperature order when the connection between the stars ' class and temperatures became clear.

A few star classes were dropped as duplicates of others.