Wesenheit function

The Wesenheit function is a particular function of a star's passband magnitudes that produces a measure unaffected by distance and reddening. A star's Wesenheit magnitude is the result of this function, which is an estimate of its apparent visual magnitude. The function was developed for the analysis of Cepheid variable period-luminosity relations (though making use of a color index, the result can be considered a period-luminosity-color relation), but some versions of the function are not specific to Cepheid variables. Such a version:

W = V - R (B-V)

• W - Wesenheit magnitude.
• V - V filter apparent magnitude.
• B - B filter apparent magnitude.
• R - ratio of the absorption of V to that affecting B-V.

R needs to be determined based on observation of similarly-distant objects, such as stars within the same galaxy or stellar cluster. It is presumed that the reddening of the target object and these other objects is similar.

Note: Wesenheit is a German word, example translations being essence and true nature. I presume this is to suggest a function that reveals something about the star's true nature despite the reddening.