carbon star

A general name for a group of peculiar, red giant stars whose spectra show strong bands of molecular carbon, CN, CH or other carbon compounds, and not the more typical TiO. In the original Harvard classification system of 1918, the carbon stars were allocated to spectral types R and N. It was found that they have temperatures similar to those of the more common K stars and M stars and that the differences in the spectra arise from differences in the abundance of carbon and oxygen. The term "carbon star" was introduced in the 1940s by Morgan and Keenan, who proposed a new sequence of classes from C0 to C7, paralleling the decreasing temperature sequence in normal stars from G4 to M4. Though known carbon stars are rare in our own Galaxy, many thousands have been discovered in the Large and Small Magellanic Clouds. Some carbon stars contain the unstable element technetium, the longest-lived isotope of which has a half-life of only 210,000 years, a short period on astronomical timescales. A few (less than twenty) of the coolest carbon stars show an extremely strong line of lithium in their spectra. It is also possible to measure the proportions present of two isotopes of carbon, 12C and 13C. In the carbon stars, particularly the hotter ones, these proportions differ significantly from those encountered in the solar system. A group in which the ratio 12C /13C is unusually low are known as J stars.