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Originally Posted by Eroica
By the way, the spectral classes R and N have been replaced by C (for carbon stars). It was once thought that R and N stars were cooler than M stars, but it turns out that carbon in their upper atmospheres was absorbing some of the light from their photospheres, making them appear cooler. They're actually as hot as G, K and early M - which rather spoils those longer mnemonics.
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Not true. Classes R and N have not been replaced by C, astronomers just refer to R and N collectively as C. Except for the additional spectral features due to carbon compounds, R stars resemble K stars, N stars resemble M stars. The carbon compounds absorb blue light, so that the stars look redder.