carbon star

carbon star A cool red-giant star in an advanced stage of evolution, displaying strong carbon features in the form of CN, CH, and C₂ (Swan) bands in its spectrum; also known as spectral type C. In carbon stars, the abundance of carbon is greater than that of oxygen. The additional presence of lithium indicates that these elements have been produced by nuclear reactions in the star's core and are now being transported by convection to its surface. Since carbon can be produced only by the triple-alpha process at a very high temperature, these stars must be highly evolved. These rare but luminous objects include the former types R (K-type giants with temperatures of 4000–5000 K) and N (M-type giants which are cooler, about 3000 K) which were introduced in the Harvard classification. The N-type carbon stars can be up to 10 times as luminous as the R type. Many are either semiregular variables, such as Y Canum Venaticorum (La Superba), U Hydrae, and UU Aurigae, or Mira stars, for example R Leporis (Hind's Crimson Star) and V Coronae Borealis. Visually they appear deep orange or red. At maximum, U Hydrae and TX Piscium are the brightest carbon stars.