Carbon star
Carbon-rich red giants with sooty atmospheres and distinctive spectra.
A carbon star, also known as a C-type star, is usually a luminous red giant on the asymptotic giant branch. Its atmosphere holds more carbon than oxygen. These two elements combine in the star’s upper layers to form carbon monoxide, which uses up most of the available oxygen. The leftover carbon atoms then create other carbon compounds, giving the star a sooty atmosphere and a deep ruby red color. Some carbon stars are dwarfs or supergiants, while the more common giant variety is often called a classical carbon star to set it apart. In most stars, like the Sun, oxygen is more abundant than carbon. Ordinary stars that are cool enough to form carbon monoxide but lack carbon-star traits are referred to as oxygen-rich stars. Carbon stars have very distinct spectral features, and their spectra were first identified by Angelo Secchi in the 1860s, during the early days of astronomical spectroscopy.
By definition, carbon stars show strong spectral Swan bands from the C₂ molecule. Many other carbon-based molecules, such as CH, CN (cyanogen), C₃, and SiC₂, can also be abundant. Carbon is produced in the star’s core and then carried up to the outer layers, drastically changing their composition. Along with carbon, elements from the S-process—like barium, technetium, and zirconium—are created during shell flashes and dredged up to the surface. When astronomers developed a spectral classification for carbon stars, they struggled to link the spectra to the stars’ effective temperatures. The problem was that all the atmospheric carbon hid the absorption lines normally used as temperature indicators. Carbon stars also display a rich set of molecular lines at millimeter and submillimeter wavelengths. In the carbon star CW Leonis, more than 50 different circumstellar molecules have been detected, and this star is often used to search for new ones.
Carbon stars were discovered in the 1860s by spectral classification pioneer Angelo Secchi, who created the Secchi class IV for them. In the late 1890s, these were reclassified as N class stars. Using the new Harvard classification, the N class was later expanded with an R class for less deeply red stars that still showed the characteristic carbon bands. When this R-to-N scheme was compared with conventional spectra, it turned out that the R-N sequence roughly runs parallel to temperatures from about G7 to M10. The later N classes
- 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 them, later reclassified as N class stars in the late 1890s. Using the Harvard classification, the N class was later enhanced by an R class for less deeply red stars sharing the characteristic carbon bands. The Morgan–Keenan C system replaced the older R-N classifications from 1960 to 1993, introducing a two-dimensional classification based on temperature and carbon abundance. A revised Morgan–Keenan classification was published in 1993 by Philip Keenan, defining the classes C-N, C-R, and C-H, with later additions of C-J and C-Hd.
Reader's Guide
Carbon stars are significant because they represent a distinct stage in stellar evolution, particularly for asymptotic giant branch stars, and they contribute heavily to the interstellar dust that forms future stars and planetary systems. Their spectra, dominated by Swan bands from C2 and other carbon compounds, provide a rich laboratory for studying molecular chemistry in space. The star CW Leonis alone has yielded more than 50 different circumstellar molecules. The classification of carbon stars has evolved from Secchi's early work through the Harvard and Morgan–Keenan systems to the current revised Morgan–Keenan system, reflecting ongoing refinement in understanding their temperature and carbon abundance. Classical carbon stars produce carbon internally via helium fusion and dredge-up, while non-classical types (C-J, C-H) are believed to be binary systems where the observed giant accreted carbon from a companion white dwarf. The enigmatic hydrogen-deficient carbon stars (C-Hd) remain poorly understood, with only five known and no known binaries. Carbon stars also play a key role in the origin of presolar silicon carbide grains found in meteorites, offering direct isotopic analysis of their circumstellar environments.
Did You Know?
- Carbon stars were first recognized by their spectra by Angelo Secchi in the 1860s.
- The carbon star CW Leonis has more than 50 different circumstellar molecules detected.
- 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 chondritic meteorites.
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