Comparison of HR diagrams for Globular Clusters, relating luminosity with color. Due to the old age of these clusters, most stars left the main sequence. Only a few hot blue stars are still visible. All clusters pictures were taken with the same scope (RCX 400 16") same camera (SBIG 8Xme with AOL), same exposure to allow comparison. The different high on the horizont has made a small bias in the color. M13 seems to have the least number of blue stars left in the main sequence.