Carbon star

A cool star whose atmosphere contains more carbon than oxygen, producing prominent molecular carbon bands in its spectrum. Most are evolved stars whose atmospheres have been enriched with carbon.

Carbon-to-oxygen ratio: The ratio of carbon atoms to oxygen atoms in a material. When this ratio exceeds one in a stellar atmosphere, leftover carbon forms distinctive molecules.
Asymptotic giant branch: A late evolutionary phase of low- and intermediate-mass stars with an inert core and burning shells. Many carbon stars become carbon-rich during thermal pulses on this giant branch.
N-type carbon star: A cool, luminous carbon star whose spectrum is dominated by carbon-bearing molecules. This traditional class includes many intrinsic carbon stars on the asymptotic giant branch.
Carbon dust: Solid carbon-rich grains formed in cool, carbon-rich environments such as stellar outflows. Carbon stars condense soot-like grains that absorb light and influence their winds.
Molecular spectroscopy: The study of how molecules absorb and emit electromagnetic radiation. Carbon-star bands arise when molecules in the cool atmosphere absorb particular wavelengths.
Third dredge-up: An episode in which a giant star’s convective envelope carries products of helium burning to its surface. It brings newly made carbon into the atmosphere, raising the carbon-to-oxygen ratio.
R-type carbon star: A relatively warm carbon star, often classified by its strong carbon features and lack of s-process enrichment. Its chemistry and evolutionary status distinguish it from typical cool N-type stars.
Circumstellar envelope: Gas and dust surrounding a star, often created by material expelled from its atmosphere. A carbon star’s mass loss builds an envelope rich in molecules and carbonaceous dust.
Swan bands: Prominent molecular emission or absorption bands produced by the C₂ molecule. These bands provide a recognizable spectral signature of carbon-rich stellar atmospheres.
Helium burning: Stellar fusion that converts helium into heavier elements, including carbon and oxygen. The triple-alpha process creates the carbon that dredge-up can deliver to a star’s surface.

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