Wilson–Bappu effect

The Ca II K line in cool stars is a strong photospheric absorption line that exhibits an emission line at its core originating in the star's chromosphere. In 1957, Olin C. Wilson and M. K. Vainu Bappu reported on the remarkable correlation between the measured width of this emission core and the absolute magnitude of the star. This relationship is now known as the Wilson–Bappu effect. The correlation is independent of spectral type and is applicable to stellar classification main sequence types G, K, and Red giant type M. The greater the emission band width, the brighter the star, which provides an empirical correlation with distance.

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