On the accuracy of globular cluster distances
Robert C. Roeder · The Astronomical Journal · 1965
It is shown that, because the helium content of population II stars is not known very precisely, theoretical deviation curves with marked differences in shape can be constructed for use in determining the distance moduli of globular clusters. This is deemed to imply that an uncertainty of perhaps 0.2 mag. can exist in the distance modulus of any particular cluster, in addition to any uncertainties in the observations, if the ob- servations are interpreted with the aid of a theoretical deviation curve.