carbon stars

carbon stars

(C stars) Red giant stars of low temperature that have an over-abundance of carbon relative to oxygen in their surface layers. In cool stars carbon and oxygen atoms combine to form stable carbon monoxide, and in carbon stars the excess carbon can then form other molecules. Their spectra therefore show strong bands of carbon compounds, including C₂, CN, and CH. All carbon stars undergo mass loss, enriching the interstellar medium with considerable carbon, some nitrogen and oxygen, and also s-process elements. In the earlier Harvard classification (see spectral types) carbon stars were divided into R stars and N stars : N stars are the ‘classical’ carbon stars, discovered spectroscopically by Angelo Secchi (1868). They are very cool and very luminous and many have been discovered in the Magellanic Clouds and other galaxies. They are observed to be losing mass rapidly and are much further evolved than the hotter less luminous R stars. The R stars are enriched in the isotopes ¹³C and ¹⁴N, but unlike most N stars, show no enhancement in s-process elements. See also R Coronae Borealis stars; S stars.