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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.〔p. 77, ''Observing Variable Stars'', Gerry A. Good, Springer, 2003, ISBN 1-85233-498-3.〕 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.〔(S star ), ''The Internet Encyclopedia of Science'', David Darling. Accessed on line September 9, 2008.〕 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.〔(page 284 ) in (Robert D. McClure: The carbon and related stars )〕 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. ==See also== * Carbon star * Technetium star * Barium star 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「S-type star」の詳細全文を読む スポンサード リンク
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