A cool giant star of basic spectral type K or M, which shows in its spectrum distinct absorption bands of the molecule zirconium oxide ZrO. S stars also often have bands of lanthanum oxide LaO, yttrium oxide YO and vanadium oxide VO. In normal M stars, the most prominent molecular bands are those of titanium oxide TiO. The dominance of ZrO in S-type stars reflects a higher ratio of carbon to oxygen and a high abundance of zirconium. The zirconium and other heavier elements are the products of nuclear reactions in the star’s interior which have been brought up to the surface.