Barium stars are spectral class G to K stars whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at λ 455.4 nm. Barium stars also show enhanced spectral features of carbon, the bands of the molecules CH, CN and C2. The class was originally recognized and defined by William P. Bidelman and Philip Keenan. Initially, after their discovery, it was thought the category was limited to red giants, but the same chemical signature has been observed in main-sequence stars as well.