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The Question

(Submitted January 25, 2007)

While self-teaching in astronomy (amateur stargazer class), I've come accross spectra marked R,N,C & S.(The reference is in an online version of Hipparcos). While I've become well-aquainted with the usual OBAFGKM system, these leave me a little puzzled. After a look on the web, I've found about 7,000 files that describe the details (lots of research papers), but not the basics (such as where they are on the HR diagram or even their names such as "an R star is better known as a Jones Star") without which the later information isn't much help!

The Answer

Thanks for your question. The R, N, C & S classification for stars are reserved for a special class of stars known as carbon stars, stars with a rather high carbon content. Most carbon stars are variable stars and many are thought to be in the red giant/AGB phase on the H-R diagram. A very good description can be found at the following website:
http://en.wikipedia.org/wiki/Carbon_star

Hope this helps,
Georgia & Mike
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