OBAFGKM sequence
/ oh-bee-ay-eff-jee-kay-em /
If you line up stars from hottest to coolest, they fall into seven big families, each with its own letter: O, B, A, F, G, K, M. Generations of astronomy students remember the order with the mnemonic 'Oh Be A Fine Girl/Guy, Kiss Me'. This odd, out-of-alphabetical sequence is the backbone of how we sort the stars.
The letters look scrambled because the classification was first organised by hydrogen line strength (which is why the labels were originally alphabetical A, B, C...), and only later reordered by temperature when astronomers realised that was the deeper variable, dropping and merging redundant classes. Now O stars are the hottest, above 30,000 K and blazing blue; B stars are blue-white; A stars white; F yellow-white; G yellow like the Sun (about 5,800 K); K orange; and M the coolest, around 3,000 K and glowing red. Each letter is finely subdivided 0 to 9, so the Sun is G2, slightly hotter than G3.
The sequence is far more than a colour chart. It also tracks how common stars are: hot O and B stars are extremely rare and short-lived, while cool M dwarfs are by far the most numerous stars in the galaxy, making up roughly three-quarters of all stars even though none is bright enough to see with the unaided eye. Reading a star's letter immediately suggests its temperature, colour, typical mass, and likely lifespan — a remarkable amount of physics packed into one symbol.
Three famous stars span the sequence: Spica (B-type, scorching blue), the Sun (G-type, yellow), and Betelgeuse (M-type, cool red). Walk down O-B-A-F-G-K-M and you are walking from roughly 40,000 K down to 3,000 K.
Seven letters, ordered by temperature, not the alphabet.
The letters are not in alphabetical order and not in brightness order — they are in temperature order, a leftover from the system's history. O is hottest and M is coolest, which is the reverse of how it first looks.