Wilson-Bappu Effect

The Ca II K line in cool stars is among the strongest of emission lines, of electromagnetic radiation, which originates 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 the aforementioned emission line and the absolute visual magnitude of the star. This is 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, the brighter the star, which is correlated with distance empirically.

The main interest of the Wilson–Bappu effect resides in the following facts:

The first calibration of the Wilson–Bappu effect using distance from Hipparcos parallaxes was made in 1999 by Wallerstein et al. A later work also used W₀ measurements on high-resolution spectra taken with CCD, but a smaller sample.

According to the latest calibration, the relation between absolute visual magnitude (M_v) expressed in magnitudes and W₀, transformed in km/s, is the following:

${\displaystyle M_{V}=33.2-18.0log(W_{0})}$

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