Over the past five years evidence has mounted that long-duration (> 2 s) γ-ray bursts (GRBs) the most brilliant of all astronomical explosionssignal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova 1 , but now includes the association of GRBs with regions of massive star formation in distant galaxies 2,3 , the appearance of supernova-like 'bumps' in the optical afterglow light curves of several bursts 4-6 and lines of freshly synthesized elements in the spectra of a few X-ray afterglows 7 . These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that

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