ACN star has unusually strongcyano radical bands in its spectrum compared to other stars of itsstellar class.^[1] Cyano radical is a simple molecule of onecarbon atom and onenitrogen atom, withabsorption bands around wavelengths 388.9 and421.6 nanometer.^[2] This group of stars was first noticed in certain G and K-type giants byJ. J. Nassau andW. W. Morgan in 1949,^[3] then a further 4,150 were identified byNancy G. Roman in 1952.^[4] They can be distinguished frombarium stars by the lack ofs-process elements, and from other types of luminous stars by the general weakness of features other than the CN lines.^[3]

The excess strength of the CN bands is classified by a positive index with increments of 0.5. A value of zero indicates a normal star and is not listed in the stellar class, while the peak value of 4 is essentially similar to acarbon star. Stars classified in theMK system with a CN suffix are considered "strong" CN stars. Hence,42 Librae is a strong CN star with a class ofK3-III CN2. A value of 0.5 is also termed a marginal CN star, which corresponds to the typical giant stars in theHyades cluster.^[5]

• CN stars
• Cyanides

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