Massive stars in clusters cannot grow without bound because a combination of radiation feedback, gas fragmentation, and stellar dynamics imposes practical and theoretical ceilings on their mass. Observations and models together define a landscape where environment and formation pathway determine the upper limit.
Physical limits: radiation pressure, accretion, and fragmentation
As a protostar gains mass its luminosity rises and radiation pushes outward on the infalling gas. This radiation pressure and the related Eddington limit make further accretion increasingly difficult. Donald Figer, UCLA, analysed the Arches cluster and argued for an observational upper mass near 150 solar masses based on the mass distribution of massive stars. Theoretical work by Mark Krumholz, Monash University, and Christopher McKee, University of California Berkeley, has developed models showing how radiation-hydrodynamic feedback and ionizing irradiation can halt or redirect accretion flows, setting effective ceilings on how large a single star can grow in isolation. Simultaneously, the parent molecular cloud tends to fragment into multiple objects, distributing mass among companions rather than into a single extreme star.
Environmental and dynamical factors
Environment matters. Paul Crowther, University of Sheffield, reported very massive stars in the R136 cluster of the Large Magellanic Cloud with initial mass estimates up to about 300 solar masses, suggesting that lower metallicity and extreme cluster density can permit larger initial masses. In very dense clusters, stellar collisions and mergers provide an alternate route to produce supermassive stars; Simon Portegies Zwart, Leiden University, has shown through N-body simulations that runaway collisions in young dense clusters can assemble stars far more massive than those formed purely by steady accretion.
The consequences of these limits are significant. Upper-mass thresholds influence the intensity of stellar feedback that sculpts star-forming regions, the chemical enrichment of galaxies, and the types of stellar deaths that follow. Extremely massive stars may lose mass through powerful winds, produce pair-instability supernovae in low-metallicity environments, or collapse to massive black holes. Culturally and observationally, studies of clusters in neighboring systems like the Large Magellanic Cloud reveal how local metallicity and cluster density—territorial factors on a galactic scale—shape the population of the most massive stars.
Together, observational surveys and multidimensional simulations are narrowing uncertainties, but the maximum mass in any given cluster remains a product of competing processes: radiative feedback, cloud fragmentation, metallicity-dependent winds, and dynamical mergers, each modulated by the cluster’s environment.