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A Spectral Classification associated with giant or supergiant stars whose temperature is comparable to those in the more familiar K- (orange) and M- (red) spectral types. In common with more typical red giants, these are ancient stars whose original stores of hydrogen in the core have been exhausted. While a typical M-type red giant is still fusing hydrogen into helium in its outer shell, an S-type star is undergoing more complex processes, and while stars of this type are relatively rare, their nuclear processes are important in the production of heavier elements (including those necessary for life).

There are no S-type stars permanently visible to the naked eye. The brightest is Chi Cygni, a highly variable star that can occasionally reach a magnitude of +5 (placing it just on the edge of naked eye visibility, though it can sometimes be as faint as magnitude +13).

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