Spectroscopic analyses of hydrogen-rich WN5–6 stars within the young star clusters NGC 3603 and R136 are presented , using archival Hubble Space Telescope and Very Large Telescope spectroscopy , and high spatial resolution near-IR photometry , including Multi-Conjugate Adaptive Optics Demonstrator ( MAD ) imaging of R136 . We derive high stellar temperatures for the WN stars in NGC 3603 ( T _ { \ast } \sim 42 \pm 2 kK ) and R136 ( T _ { \ast } \sim 53 \pm 3 kK ) plus clumping-corrected mass-loss rates of 2 – 5 \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } which closely agree with theoretical predictions from Vink et al . These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters . Indeed , R136a1 alone supplies \sim 7 % of the ionizing flux of the entire 30 Doradus region . Comparisons with stellar models calculated for the main-sequence evolution of 85 – 500 M _ { \odot } accounting for rotation suggest ages of \sim 1.5 Myr and initial masses in the range 105 – 170 M _ { \odot } for three systems in NGC 3603 , plus 165 – 320 M _ { \odot } for four stars in R136 . Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC 3603 A1 . We consider the predicted X-ray luminosity of the R136 stars if they were close , colliding wind binaries . R136c is consistent with a colliding wind binary system . However , short period , colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity . Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high density environment . From simulated star clusters , whose constituents are randomly sampled from the Kroupa initial mass function , both NGC 3603 and R136 are consistent with an tentative upper mass limit of \sim 300 M _ { \odot } . The Arches cluster is either too old to be used to diagnose the upper mass limit , exhibits a deficiency of very massive stars , or more likely stellar masses have been underestimated – initial masses for the most luminous stars in the Arches cluster approach 200 M _ { \odot } according to contemporary stellar and photometric results . The potential for stars greatly exceeding 150 M _ { \odot } within metal-poor galaxies suggests that such pair-instability supernovae could occur within the local universe , as has been claimed for SN 2007bi .