Deep red stars rich in carbon. C stars, known as carbon stars, are classically asymptotic giant branch red giants with more carbon in their outer layers than oxygen. This gives them a distinctive red colour, since the carbon forms a particulate 'sooty' layer which absorbs shorter wavelengths. Most classical carbon stars are long period variables.
Non-classical carbon stars have more complex histories. Some are red stars (usually, but not always, a red giant) in a binary pair with a white dwarf that was previously itself a classical carbon star. Since classical carbon stars can lose as much as half of their mass through stellar outflow, this material may be gathered by the companion star which then becomes a carbon star - then the original carbon star collapses into a white dwarf. Other types of carbon stars have even more complex histories.
C stars of whatever type are often surrounded by thick dusty carbon-rich nebulae, which is often mined for carbon to construct computronium nodes or habitable megastructures.