Cstars

Cstars, commonly referred to as carbon stars, are a class of late-type luminous giant stars whose atmospheres contain more carbon than oxygen. This carbon enrichment causes the formation of carbon-bearing molecules, leading to characteristic spectral features such as strong C2 and CN bands. A defining property is a surface carbon-to-oxygen ratio (C/O) greater than unity, which alters the chemistry of the stellar atmosphere and the appearance of the star.

Most Cstars are on the asymptotic giant branch (AGB) of stellar evolution. During thermal pulses in the helium-burning shell, convective dredge-up can bring freshly produced carbon from the interior to the surface. Repeated dredge-up episodes can transform stars that started with near-solar compositions into carbon-rich giants. The mass loss associated with the AGB phase creates circumstellar dust shells, rich in carbonaceous grains, which produce significant infrared excess and influence the star’s observed brightness over time.

Observationally, Cstars are red and often variable. Many are Mira-type or semi-regular variables with periods ranging from tens to several hundred days. They commonly occur in the disks of galaxies and in the halos of dwarf galaxies, including the Milky Way and the Magellanic Clouds. Infrared surveys are especially effective at detecting them due to their dusty envelopes.

Subtypes include classical N-type carbon stars, which are the brightest and most well-studied, and other classes such as J- and R-type stars that exhibit variations in isotopic ratios or atmospheric chemistry. Cstars are valuable tracers of intermediate-age stellar populations and contribute to the chemical enrichment of the interstellar medium through their substantial mass loss.