Wilson–Bappu effect

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 magnitude of the star.

It can be studied using nearby stars, for which independent distance measurements are possible, and it can be expressed in a simple analytical form.

The first calibration of the Wilson–Bappu effect using distance from Hipparcos parallaxes was made in 1999 by Wallerstein et al.[2] A later work also used W0 measurements on high-resolution spectra taken with CCD, but a smaller sample.

Another limitation comes from the fact that the measurement of W0 in distant stars is very challenging, requires long observations at big telescopes.

In 1977, Stencel published a spectroscopic survey that showed that the wing emission features seen in the broad wings of the K line among higher luminosity late type stars, share a correlation of line width and Mv similar to the Wilson–Bappu effect.

K line spectrum of KW 326, a dwarf star in the Praesepe open cluster. The line is very wide and very deep, and it originates in the photosphere, just like any other absorption line. Several other lines are superimposed on it. In the center, the emission due to the K line itself, which takes place in the chromosphere.
Zoom on the emission core. W 0 is defined as the difference in wavelength between the points on either side of the emission at an average intensity between the K1 minimum and the K2 maximum