What do the letters of stellar classification stand for?

O, B, A, F and so on… What do they all mean?

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Asked by Christopher Attwood

Astronomers have always searched for ways to separate stars into different groups. These letters of spectral classification started off as an alphabetical ordering of the types of stars (e.g. A, B, C, D etc) but have become reordered and minimised as more is discovered about the stars they represent. The full order of classification is O, B, A, F, G, K, M – more easily remembered by using the mnemonic “Oh Be A Fine Girl/Guy, Kiss Me”.

This also orders adult (main sequence) stars from hot to cool; the hottest (and largest) main sequence stars are O and B type, while the coolest (and smallest) stars are M dwarfs. Astronomers can infer the temperature of a star by looking at the type of light it emits. Very hot stars (O and B stars) look blue to our eyes, and as stars get cooler they become more white, then yellow, then orange, then red (M dwarfs). Our Sun is a yellow/orange G type star on this classification; pretty average by all accounts.

Answered by Megan Whewell from the National Space Centre

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