The standard stellar evolution theory predicts that stars ina cluster should form a narrow and well-defined main sequence, marking a single cluster age. However, recent observations from the Hubble Space Telescope and Gaia have challenged this view. Many young and intermediate-age star clusters exhibit broadened or even multiple turnoffs. Similarly, blue straggler stars also seem to defy this prediction by occupying bluer and brighter regions above main-sequence turnoffs. Various theories have been put forward to explain these anomalies, including stellar rotation, multiple stellar interactions, and star-disk interactions during the pre-main-sequence phase. In this talk, I will present recent findings on the role of stellar rotation and multiple stellar interactions in shaping cluster color–magnitude diagrams and discuss how these discoveries compel us to rethink the concept of a “simple” stellar population.
