The most striking feature of the globular cluster catalogued as NGC 2108 is the gleaming ruby-red spot to the lower left of the cluster’s center (on the right side of the image). It’s a carbon star. Carbon stars are almost always cool red giants with atmospheres containing more carbon than oxygen—the opposite balance of those elements in our Sun. Carbon monoxide forms in the outer layer of a carbon star through a combination of these elements until there is no more oxygen available. Then, the carbon atoms are free to form a variety of other carbon compounds, such as C2, CH, CN, C3 and SiC2, scattering blue light within the star and allowing red light to pass through undisturbed.
Image Credit: ESA / NASA