Carbon star

A carbon star, also known as a C-type star, is usually a large, luminous red giant star in its final stages of life (Asymptotic giant branch). These stars have more carbon than oxygen in their atmospheres. In the upper layers of the star, carbon and oxygen combine to form carbon monoxide. This process uses up most of the oxygen, allowing carbon atoms to form various compounds, resulting in a "sooty" atmosphere and a distinct ruby red color for the star. There are also dwarf and supergiant carbon stars. The more common giant stars are sometimes referred to as classical carbon stars to set them apart.

Most stars, like the Sun, have atmospheres with more oxygen than carbon. Stars that aren't carbon stars but are cool enough to form carbon monoxide are known as oxygen-rich stars.

Carbon stars have unique spectral characteristics, and they were identified by their spectra in the 1860s by Angelo Secchi during the early days of astronomical spectroscopy.