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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]

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.



Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track


Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]


White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary


Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova


Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism


Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system


Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard


Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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