In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its photosphere, giving an objective measure of the photosphere's temperature. Light from the star is analyzed by splitting it with a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colors interspersed with absorption lines, each line indicating a certain ion of a certain chemical element. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature conditions are suitable for a certain excitation of this element. If the star temperature has been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the photosphere. 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. Useful mnemonics for remembering the spectral type letters are "Oh Be A Fine Girl Kiss Me" or "Oh Boy An F Grade Kills Me". According to informal tradition, O stars are called "blue", B stars are called "blue-white", A stars are called "white", F stars are called "yellow-white", G stars are called "yellow", K stars are called "orange", and M stars are called "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. Wikipedia [ - ]

In astronomy, stellar classification is a classification of stars based on their spectral... [ + ]