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Wilson-Bappu effect

(a relation between a star's spectral line and luminosity)

The Wilson-Bappu effect is a relation between the luminosity of cool stars (G-type star, K-type star and red-giant M-type star) and the width (line broadening) of a particular emission line: the Ca II K line (ionized calcium, 393.366nm). As such, it provides a means of stellar distance determination.

393.366nm762THz3.2eVWilson-Bappu effect