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Solving Olin Wilson’s Mystery

Wilson showed that G8–M1 dwarfs have a range of 0.30 mag in P−V color whereas the color range for earlier stars is 0.12 mag. He attributed the increased range for the later stars to differing hydrogen‐to‐metal ratios in the temperature region where most of the electrons come from the metals. I explored those stars using Hipparcos parallaxes and photometry and with MK types. His effect can be explained without resorting to abundance differences, which are not confirmed elsewhere, by poorer spectral classification from unwidened spectra for the fainter stars and the use of maximum color ranges, rather than dispersions. I also considered, with negative results, the possibility that the Ca ii H and K emission lines or chromospheric activity could cause the large ranges in colors. Different amounts of chromospheric activity do not affect the colors significantly.