Variable star designation

Variable stars are named using a variation on the Bayer designation format of Identifier <genitive of constellation>. In previous centuries few variable stars were known, so it seemed reasonable to use the letters of the Roman alphabet, starting from the letter R so as to avoid confusion with letter spectral types. (This system of nomenclature was developed by Friedrich W. Argelander, who noted that many variable stars were red and so began the naming with the letter R for rot (Red).) By 1836, even the letter S had only been used in one constellation, Serpens. Unfortunately the advent of photography made the number of variables pile up quickly and variable star names soon fell into the Bayer trap of reaching the end of the alphabet while still having stars to name. After two subsequent supplementary double-lettering systems hit similar limits, numbers were finally introduced.

The current naming system is:

• Start with the letter R and go through Z
• Continue with RR...RZ, then use SS...SZ, TT...TZ and so on until ZZ.
• Use AA...AZ, BA...BZ and so on until reaching QZ.
• Finally give up on the Roman alphabet after 334 combinations of letters and start naming stars with V335, V336, and so on.

Sample designations are R Coronae Borealis, YY Geminorum , and V348 Sagitarii .