(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.
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