The lowest-metallicity type II supernova from the highest-mass red supergiant progenitor

Red supergiants have been confirmed as the progenitor stars of the majority of hydrogen-rich type II supernovae¹. However, while such stars are observed with masses >25 M_⊙ (ref. ²), detections of >18 M_⊙ progenitors remain elusive¹. Red supergiants are also expected to form at all metallicities, but discoveries of explosions from low-metallicity progenitors are scarce. Here, we report observations of the type II supernova, SN 2015bs, for which we infer a progenitor metallicity of ≤0.1 Z_⊙ from comparison to photospheric-phase spectral models³, and a zero-age main-sequence mass of 17-25 M_⊙ through comparison to nebular-phase spectral models^4,5. SN 2015bs displays a normal `plateau' light-curve morphology, and typical spectral properties, implying a red supergiant progenitor. This is the first example of such a high-mass progenitor for a `normal' type II supernova, suggesting a link between high-mass red supergiant explosions and low-metallicity progenitors.