A red giant of spectral type K5–M showing bands of zirconium oxide (ZrO) in its spectrum; sometimes also called a heavy-metal star or, rarely, a zirconium star. Oxides of lanthanum, yttrium, and barium are also found in place of the usual scandium and vanadium. These elements must have been created in the core of the star by nucleosynthesis during its later evolutionary stages. The usual sequence of temperature subtypes is applied, S0–S9, but luminosity classes are difficult to assign to S stars. An alternative classification based on the carbon–oxygen ratio is often used, and given as a number in the range 1–10 following a separator symbol; for example, the long-period variable star Chi Cygni has spectral type S6/1 at maximum brightness. The majority of S stars are irregular or long-period variables. Related subtypes are MS stars (M stars with zirconium oxide in their spectra) and SC stars, which are intermediate between S stars and carbon stars.
Subjects: Astronomy and Astrophysics.