Star Clusters in the Ultraviolet

Abstract Ultraviolet (UV) observations provide a uniquely powerful window into the hot and evolved stellar populations that shape the structure, evolution and integrated light of star clusters. Because UV wavelengths are highly sensitive to massive main-sequence stars, blue straggler stars (BSS), extreme-horizontal-branch (extreme-HB) stars, post-AGB objects, interacting binaries and compact remnants, UV studies enable constraints on fundamental processes that are inaccessible at optical and infrared wavelengths. This review synthesises five decades of UV investigations of star clusters across the Milky Way, the Magellanic Clouds (MCs) and nearby galaxies, drawing on results from early space telescopes, wide-field surveys, high-resolution imagers and the recent capabilities of AstroSat /UVIT and Swift /UVOT. In Galactic open clusters, UV photometry has revealed diverse compact companions—including white dwarfs, hot subdwarfs and stripped stars—and has been essential for establishing the mass-transfer origins of BSS and related populations such as Blue Lurkers and yellow stragglers. In globular clusters, UV imaging is indispensable for identifying multiple stellar populations through UV-sensitive molecular bands, probing helium enrichment and mapping HB morphologies from cluster cores to tidal radii. Wide-field UVIT surveys have provided homogeneous catalogues of HB and post-HB stars, extending earlier Hubble Space Telescope results to full-cluster scales. In the MCs, UV observations have transformed our understanding of multiple populations in intermediate-age clusters, rotation-driven extended main-sequence turn-offs and the recently identified UV-dim phenomenon. UV spectroscopic surveys have advanced constraints on massive-star evolution, winds and binarity at low metallicity. Meanwhile, UV mapping of the Magellanic Bridge demonstrates ongoing massive-star formation in low-density tidal environments. Beyond the Local Group, UV studies of extragalactic clusters reveal star-formation histories, stellar feedback and population synthesis constraints in diverse galactic environments. Collectively, UV observations now form a cornerstone of star cluster astrophysics and will continue to do so with upcoming missions.