The broad swath extending diagonally from the lower right is the cluster's main sequence . A sharp turn toward the upper right hand corner follows the red giant branch while the blue giants are found grouped in the upper left. M55's stars were formed at the same time and at first were all located along the main sequence by mass, lower mass stars at the lower right. Over time, higher mass stars have evolved off the main sequence into red, then blue giants and beyond . The exact position of the sharp turn-off from the main sequence to the red giant branch measures the cluster's age.
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