To differentiate between the two scenarios, an age determination of the various sources is essential. Let's see how the analysis with the BHK diagram combined with nebular data on the physical properties of the gas resolves part of this problem. Their data shows surprisingly uniform oxygen abundances at about a third solar. The logarithmic extinction differences at H between the various regions they studied are of the order of 0.2 - 0.3. The bright regions are identified based on the convention of Wynn-Williams et al.
North is up and east is left.
The total field of view is 1 1. The solid curve is the theoretical model at 0.25 solar metallicity. For these colors, the extinction vector is parallel to the old branch. 299 labeled with the conventional symbols of Wynn-Williams et al. All the stars have a metallicity of 0.25 solar.