Carbon Stars

A carbon star is a giant star in a late phase of evolution. It is somewhat similar to a red giant but its atmosphere contains more carbon than oxygen. Visually, M-class red giants such as Antares and Betelgeuse appear more orange with a hint of red. Carbon stars have a more obvious red hue.

Why are carbon stars redder? There are two reasons for this. Firstly, their surface temperature is relatively cool at less than 4000 degrees Kelvin. In comparison, white to blue stars have surface temperatures of 7,500 to more than 30,000 degrees K. Secondly, their atmospheres are full of carbon “soot” in the form of carbon and carbon compounds. This soot reflects and scatters the shorter wavelengths of blue light but allows the longer red wavelengths to pass through (much the same as what happens at sunset).

Most classical carbon stars are long-period variable stars. It is not uncommon for visual magnitudes to vary by 5 or more (e.g. R Leporis whose magnitude range is 5.9 to 11 over 432 days).

Finder charts are provided for all the carbon stars. Visual magnitude range is from AAVSO data. Spectral type Cm,n, the B - V colour index and the period in days are mostly obtained from SIMBAD. 

The difference B-V is known as the B-V colour index. The smaller the colour index, the bluer or hotter the star is. Conversely, the larger the B-V, the redder the star is. The carbon star DY Crucis next to Mimosa has a colour index of 5.8 and is one of the reddest known stars. In comparison the red giants Antares and Betelgeuse have a colour index of 1.9. The white star Sirius (spectral type A1) and the star Rigel (type B8) have a colour index of zero. The blue star Naos (type O4) has a colour index of -0.3.