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What happened to NS after OBAFGKM? The full mnemonic was "O Be a Fine Girl Kiss Me Now Sweetheart" a few decades ago as I remember. I think it was that way in "The Larousse Encyclopaedia of Astronomy" which I once had. The Cambridge Encyclopaedia of Astronomy (which I still have, first published in 1977, Reprinted 1979) has "OBAFGKMRNS", this seems to stem from work done by Annie J. Cannon resulting in the classification of over 225,000 stars in catalogues published between 1918 and 1924, suggesting that "OBAFGKMRNS" is the accepted list "now" (1977?). So what happened to the "R" also? Of course you can't quote these mnemonics now, because of "Political Correctness", though you can always explain that G could be either Guy or Girl!

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They still exist, but are perhaps less used because they don't indicate a particular temperature, or mass.

The stellar classification is based on the presence of particular lines in the spectrum: A type stars have a strong hydrogen line. B type stars have Helium. F and G and K stars have various ionised or neutral metals. M type stars have compounds like TiO2 in their atmosphere.

But these are associated with temperature, and B type stars are hotter than A type (and O type stars are hotter still) hence OBAFGKM.

Asymptotic branch supergiants can have processes that dredge up carbon from their interior, producing carbon lines. These are the R and N type stars, but R type stars are similar in temperature to K type stars. and N type are like M type. S type stars have heavy elements produced by s-process neutron capture stars. So if you are ordering the stellar classification by temperature, these don't have a natural place to go, and since the purpose of "Oh Be A Fine Girl Kiss Me (Right Now Sweetheart) is to remember the order of the letters then RN and S don't fit, since they are not ordered by temperature.

As for political correctness, many alternatives have been suggested, but none has caught on, perhaps because a good mnemonic should be a bit "naughty".

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