Carbon star

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Carbon stars are cool, bright red giants, usually on the asymptotic giant branch, whose atmospheres contain more carbon than oxygen. Because carbon and oxygen quickly form carbon monoxide, most of the oxygen is locked up, leaving carbon to form many other molecules. This gives the star a sooty atmosphere and a deep ruby-red color.

There are classical carbon stars (giants) and non-classical ones (dwarfs or supergiants). Most classical carbon stars vary in brightness over long periods. The non-classical carbon stars are often in binary systems where a companion white dwarf has transferred carbon-rich material to the observed star; these are extrinsic carbon stars. Some hydrogen-deficient carbon stars are rare and not fully understood.

In classical carbon stars, the carbon comes from inside the star, produced during helium fusion in shell burning on the asymptotic giant branch. It is brought to the surface by deep convection in episodes called dredge-up, especially after helium-shell flashes. This process also brings up other elements formed by slow neutron capture. The stars lose a lot of mass in strong winds, creating a surrounding shell of carbon-rich dust and molecules.

Their spectra show strong carbon-bearing molecules (like C2, CN, CH, and C3), and many have rich chemistry in their surroundings; for example, CW Leonis hosts many different molecules.

Because they are very red and bright in the infrared, carbon stars can be seen in nearby galaxies and can help estimate distances when studied in large samples.

In short: carbon stars are evolved, carbon-rich stars with distinctive chemistry and dusty envelopes that play an important role in enriching space with carbon-containing material.