S stars(zirconium stars) Stars of spectral type S, about half of which are irregular long-period variables. They are red giants similar to M stars but distinguished spectroscopically by the presence of molecular bands of zirconium oxide (ZrO) rather than the oxides of titanium and vanadium. Pure S stars have very strong ZrO bands with either very weak or no TiO bands. MS stars are M-type stars showing ZrO bands. See also technetium stars.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006