A carbon star is a red giant (or occasionally red dwarf) star whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide and other carbon compounds. The abundance of carbon is thought to be a product of helium fusion within the star, a process which red giants reach towards the end of their lives.

Many carbon stars are really binary stars, where one star is a giant star and the other a white dwarf. The giant star accretes carbon onto the surface of the white dwarf, thus the spectrum of the star shows that it's carbon enhanced.

Through spectral analysis, many carbon compounds have been detected at high levels, such as cyanogen and HCN. These compounds have circulated from the core of the star into its upper layers, dramatically changing the layers' composition. Other elements formed through helium fusion are also "kicked up" in this way, including lithium and zirconium. In "normal" stars (such as the Sun), whose atmospheres are rich in oxygen and relatively poor in carbon, the upper layers are conversely abundant in helium and nitrogen.

These late stars are cool, with typical surface temperatures ranging from 2000-3000 kelvin; they emit most of their energy in the form of infrared radiation with higher energies (i.e., the blue part of the visible spectrum) being absorbed by the atmospheric carbon. Although very large, carbon stars are visibly dim and hard to spot without specialized equipment; the stars are a distinctly deep red or brown colour described as "smoky". All carbon stars are variable stars, with irregular or semiregular variables; that is, they vary in brightness and appear to pulsate, an indication of these stars' internal instability.

Owing to its low gravity, as much as half (or more) of the total mass of a carbon star may be lost by way of powerful stellar winds. The star's remnants, carbon-rich "dust" similar to graphite, therefore become part of the interstellar dust. This dust is believed to be a significant factor in providing the raw materials for the creation of subsequent generations of stars. The ablated material surrounding a carbon star may blanket it to the extent that the dust absorbs all visible light.

See also

• Hind's Crimson Star - an example of a carbon star
• Marc Aaronson - an American astronomer and noted researcher of carbon stars
• List of astronomical topics

External link

• List of known carbon stars with classification explanation