What happened to NS spectral types after OBAFGKM? What happened to NS after OBAFGKM? The full mnemonic was &quot;O Be a Fine Girl Kiss Me Now Sweetheart&quot; a few decades ago as I remember. I think it was that way in &quot;The Larousse Encyclopaedia of Astronomy&quot; which I once had. The Cambridge Encyclopaedia of Astronomy (which I still have, first published in 1977, Reprinted 1979) has &quot;OBAFGKMRNS&quot;, 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 &quot;OBAFGKMRNS&quot; is the accepted list &quot;now&quot; (1977?). So what happened to the &quot;R&quot; also? Of course you can't quote these mnemonics now, because of &quot;Political Correctness&quot;, though you can always explain that G could be either Guy or Girl!
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 &quot;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 &quot;naughty&quot;.