Wilson-Bappu Effect

The Wilson-Bappu effect is a physical-astronomical effect that is used to determine the distance between stars.

In 1957, Olin C. Wilson and MK Vainu Bappu reported the remarkable correlation between the width of the emission line at the center of Ca II K and the absolute brightness of a star . This effect is called the Wilson-Bappu effect. The effect is independent of the star's spectral type and applies to the G, K and M spectral types.

This method is one of many ways to determine extragalactic distances. As soon as you know the absolute brightness, you can use the formula

${\displaystyle \ M-m=-2{,}5\cdot log_{10}(F_{1}/F_{2})\,.}$

determine the distance. Although in theory this method has the ability to perform reliable distance calculations for stars up to a distance of 7 megaparsecs , it is generally only used for stars up to a few hundred kiloparsecs away. It is also important that this only works for stars with a brightness greater than 15 mag.

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• Wilson, OC, and MK Vainu Bappu: H and K Emission in Late-Type Stars: Dependence of Line Width on Luminosity and Related Topics. In: Astrophysical Journal , Vol. 125, 1957, pp. 661-683. bibcode : 1957ApJ ... 125..661W