Not-so-simple stellar populations in nearby, resolved massive star clusters

TL;DR

This paper reviews recent advances in understanding complex stellar populations in nearby star clusters, highlighting the roles of binaries, rotation, and multiple populations, and suggests a shift back to simpler models for certain clusters.

Contribution

It synthesizes recent findings on stellar populations in clusters, emphasizing the importance of binaries, rotation, and multiple populations, and proposes a paradigm shift for intermediate-age clusters.

Findings

Binary systems significantly influence cluster properties.

Rapid stellar rotation affects observed cluster features.

Some intermediate-age clusters may be simpler than previously thought.

Abstract

Around the turn of the last century, star clusters of all kinds were considered "simple" stellar populations. Over the past decade, this situation has changed dramatically. At the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. In this review of where the field has moved to in recent years, we place particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Magellanic Cloud star clusters across the full age range. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate-age (~1--3 Gyr-old) star clusters, opening up exciting avenues for future research efforts.