The Mv-W(H-gamma) calibration of main-sequence stars of Millward and Walker (1985) is evaluated by observing bright supergiants in clusters. Fifty-two spectra of 39 O, B, and A supergiants combined with 19 higher dispersion spectra of 18 stars are studied. Wing profiles were generated by fitting a two-component, absorption-coefficient model to the H-gamma profiles. The calculated Mv data are compared to photometric values. It is observed that there is a difference in the present calibration and the Millward and Walker calibration due to the manner in which the wing profiles are defined; however, the nonlinear relation between the spectral types and W(H-gamma) found by Millward and Walker is confirmed.
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Hill et al. (1986) studied this question.
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