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A CN star has unusually strong cyanogen bands in its spectrum compared to other stars of its stellar class.[1] Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around wavelengths 388.9 and 421.6 nanometer.[2] This group of stars was first noticed in certain G and K-type giants by J. J. Nassau and W. W. Morgan in 1949,[3] then a further 4,150 were identified by Nancy G. Roman in 1952.[4] They can be distinguished from barium stars by the lack of s-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 a carbon star. Stars classified in the MK system with a CN suffix are considered "strong" CN stars. Hence, 42 Librae is a strong CN star with a class of K3-III CN2. A value of 0.5 is also termed a marginal CN star, which corresponds to the typical giant stars in the Hyades cluster.[5]
References

Keenan, P. C.; Heck, A. (July 1994). "SMR stars, strong-CN stars, and R stars". Revista Mexicana de Astronomía y Astrofísica. 29: 103–110. Bibcode:1994RMxAA..29..103K.
Schmitt, John L. (3 June 1970). "Stars with Strong Cyanogen Absorption". Astrophysical Journal. 163: 75. Bibcode:1971ApJ...163...75S. doi:10.1086/150747.
Keenan, Philip C. (August 1987). "Spectral types and their uses". Publications of the Astronomical Society of the Pacific. 99: 713–723. Bibcode:1987PASP...99..713K. doi:10.1086/132036.
Roman, Nancy G. (1 March 1952). "The Spectra of the Bright Stars of Types F5-K5". Astrophysical Journal. 116: 122. Bibcode:1952ApJ...116..122R. doi:10.1086/145598.

Keenan, Philip C.; et al. (July 1987). "Recognition and classification of strong-CN giants". Publications of the Astronomical Society of the Pacific. 99: 629–636. Bibcode:1987PASP...99..629K. doi:10.1086/132025.

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