  Extended main sequences (eMSs) and extended main-sequence turnoffs (eMSTOs)
are fascinating phenomena that are routinely observed in star clusters. These
phenomena strongly challenge the current canonical "simple stellar population"
picture of star clusters, which postulates that star clusters are coeval and
chemically homogeneous and can thus be described by a single, unique isochrone.
Detections of eMSs and eMSTOs provide valuable insights into stellar physics
and the evolution of star clusters. This comprehensive review delves into the
observational characteristics, underlying mechanisms, and astrophysical
implications of the eMSs and eMSTOs observed in young (less than 600 million
years) and intermediate-age (600 to 2000 million years) star clusters. Several
scenarios or hypotheses have been proposed to explain these phenomena,
including the presence of an age spread, binary interactions, variable stars,
and differences in stellar rotation rates. This review discusses the advantages
and limitations of current models. Among contemporary models and hypotheses,
stellar rotation has been demonstrated as the most plausible mechanism to
explain the occurrence of eMSs and eMSTOs. Research on stellar rotation and its
connection to eMSs has opened up a myriad of fascinating avenues, such as
investigations of the magnetic braking mechanism in stars, searches for tidally
locked binary systems in star clusters, and investigations as to whether binary
mergers can give rise to massive magnetars. These endeavors have yielded
valuable insights and significantly enriched our understanding of stellar
astrophysics.
