Abstract
The absorption lines of Mg I at λ 5167–84A have been measured photoelectrically in 539 G0–K5 stars. The results show an increasing absorption with advancing spectral type and a negative absolute luminosity effect. No significant differences are found between the strong and weak line groups of Miss Roman, or between weak-CN and strong-CN stars. High velocity class III stars have slightly stronger Mg than normal. It is suggested that the Mg b lines can be used as a good criterion of the MK luminosity class for stars with B−V between 0.70 and 1.30, and as a fairly good criterion of MK spectral type between G8 and K5.

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