Carbon star
A luminous red giant with a carbon-rich, sooty atmosphere and a strikingly ruby red appearance.
A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a 'sooty' atmosphere and a strikingly ruby red appearance. There are also some dwarf and supergiant carbon stars, with the more common giant stars sometimes being called classical carbon stars to distinguish them.
- type
- Stellar classification
- first_identified_by
- Angelo Secchi
- first_identified_in
- 1860s
- common_type
- Asymptotic giant branch star
- key_feature
- Atmosphere contains more carbon than oxygen
- spectral_classes
- C-N, C-R, C-H, C-J, C-Hd
Lore & Background
Carbon stars were discovered in the 1860s when spectral classification pioneer Angelo Secchi erected the Secchi class IV for the carbon stars, which in the late 1890s were reclassified as N class stars. Using the Harvard classification, the N class was later enhanced by an R class for less deeply red stars sharing the characteristic carbon bands of the spectrum. Later correlation of this R to N scheme with conventional spectra showed that the R-N sequence approximately runs in parallel with about G7 to M10 with regards to star temperature. The Morgan–Keenan C system replaced the older R-N classifications from 1960 to 1993. A new revised Morgan–Keenan classification was published in 1993 by Philip Keenan, defining the classes: C-N, C-R and C-H. Later the classes C-J and C-Hd were added.
Reader's Guide
By definition carbon stars have dominant spectral Swan bands from the molecule C2. Many other carbon compounds may be present at high levels, such as CH, CN (cyanogen), C3 and SiC2. Carbon is formed in the core and circulated into its upper layers, dramatically changing the layers' composition. In addition to carbon, S-process elements such as barium, technetium, and zirconium are formed in the shell flashes and are 'dredged up' to the surface. Carbon stars also show a rich spectrum of molecular lines at millimeter wavelengths and submillimeter wavelengths. In the carbon star CW Leonis more than 50 different circumstellar molecules have been detected. Most classical carbon stars are variable stars of the long period variable types. Due to the insensitivity of night vision to red and a slow adaption of the red sensitive eye rods to the light of the stars, astronomers making magnitude estimates of red variable stars, especially carbon stars, have to know how to deal with the Purkinje effect in order not to underestimate the magnitude of the observed star.
Did You Know?
- Carbon stars were first recognized by their spectra by Angelo Secchi in the 1860s.
- The Morgan–Keenan C classification for Y Canum Venaticorum was determined to be C54, where 5 refers to temperature dependent features and 4 to the strength of the C2 Swan bands.
- Owing to its low surface gravity, as much as half (or more) of the total mass of a carbon star may be lost by way of powerful stellar winds.
- Silicon carbide outflow from carbon stars was accreted in the early solar nebula and survived in the matrices of relatively unaltered chondritic meteorites.
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