A star with spectral type S is a late-type giant star (similar to class K5–M) whose spectrum displays bands from zirconium oxide, in addition to the titanium oxide bands characteristically exhibited by K and M class giant stars. Other s-process elements, for example yttrium oxide and technetium, are also enhanced, clearly indicating neutron capture elements in period 5 in the periodic system of chemical elements. An S star may also have spectrographic features from cyanogen and lithium. Most of these stars are long-period variables. The class was first defined by astronomer Paul W. Merrill in 1922.
It has been proposed that most S stars are an intermediate stage in which asymptotic giant stars (AGB stars) transform from ordinary M class giants to carbon stars of class C-N. AGB stars are usually powered by fusion in a shell of hydrogen surrounding an inert core, but during "thermal pulses" helium shell fusion can come to dominate. Other S stars ("extrinsic" S stars) may be evolved, cooler versions of barium stars, where the enhancements of carbon and s-process elements seen in the stars' spectra are relics of mass transfer between members of a binary star system. In these systems, the presently-observed S star has not synthesized the excess carbon and s-process elements itself; that material was produced at an earlier time by its binary companion, which was at the time a carbon star. We are observing these systems long after the mass transfer event, when the companion has evolved to become a white dwarf and is usually not directly visible.
S-type stars are generally redder than their K- or M-type counterparts of the same photospheric temperature. The mira variable Chi Cygni is (when near maximum light) the sky's brightest S-type star, with a variable late type spectrum about S7 to S10, with features of zirconium, titanium and vanadium oxides, sometimes bordering to the intermediary MS type. S Ursae Majoris is another example. BD Camelopardalis is an example of an "extrinsic" S star.
- p. 77, Observing Variable Stars, Gerry A. Good, Springer, 2003, ISBN 1-85233-498-3.
- S star, The Internet Encyclopedia of Science, David Darling. Accessed on line September 9, 2008.
- Merrill, Paul W. (1922). "Stellar spectra of class S". Astrophysical Journal 56: 457–82. Bibcode:1922ApJ....56..457M. doi:10.1086/142716.
- page 284 in Robert D. McClure: The carbon and related stars