Carbon Stars

Carbon stars are fascinating objects that most people have never heard of. These are stars whose atmospheres contain more carbon than oxygen. That might not sound dramatic, but it completely transforms how the star looks and behaves.

Normal stars like our Sun have more oxygen than carbon. Because carbon monoxide (CO) is extremely stable, all the available carbon bonds with oxygen to form CO. But in carbon stars, the carbon-to-oxygen ratio is reversed: after all the oxygen is tied up in CO, there’s still carbon left over. This excess carbon forms molecules such as C₂, CN, CH, and C₃. These molecules strongly absorb blue and green light, giving carbon stars their distinctive deep red or orange appearance. Some of the reddest stars visible from Earth are carbon stars.

Carbon stars aren’t born this way. They begin life as ordinary stars, usually slightly more massive than the Sun. As they age and expand into red giants, helium in their interiors fuses into carbon through the triple-alpha process. Later in their evolution, convection dredge-ups mix this newly created carbon from the star’s interior into the outer layers. When enough carbon reaches the surface, the star becomes a carbon star.

If you’ve ever wondered where the carbon in your body came from, this is a big part of the answer. In their final stages, carbon stars lose mass through slow, dense stellar winds, filling space with carbon-rich material. Over millions of years, this material becomes part of new stars, planets, and eventually living things like us. We are literally made of the ashes of ancient carbon stars and other evolved giants.

Carbon stars are indeed dying. They occupy the final, thermally pulsing phase of the asymptotic giant branch—only a few hundred thousand years from expelling their outer layers entirely and becoming white dwarfs. For a star that has lived billions of years, this is the last flicker.

They are also among the most beautiful sights in the night sky. Through a telescope, carbon stars appear as intensely red or orange points of light, noticeably different from ordinary stars. R Leporis—Hind’s Crimson Star—is one of the most famous examples. In the 19th century, John Russell Hind described it as “a drop of blood on a black field,” a description that observers still echo today.