There is now general agreement that the presupernova star for 1987a was indeed SK-69-202, a star that had visual magnitude 12.36 (Walbornet al. 1987), bolometric correction 1.15m(Humphreys and McElroy 1984), and extinction 0.5m(Woosleyet al. 1987). The distance modulus to the LMC is presently controversial, but most astronomers prefer a value near 18.5 (e.g., Walker 1985; Chiosi and Pigatto 1986) with an uncertainty of perhaps 0.2m. Putting it together, one arrives at a bolometric magnitude for

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... SK-69-202 of −7.8 with a probable range −7.5 to −8.2, or a luminosity of 4 × 1038erg s−1with a range of 3 to 6 × 1038erg s−1. At the time of the supernova the hydrogen burning shell contributes negligible energy generation so the critical quantity determining the luminosity is the helium core mass. Inspection of a variety of current stellar models indicates that SK-69-202 had a helium core mass of 6 ± 1 M⊙from which we may infer that, on the main sequence, it had a mass of 19 ± 3 M⊙. A B3-I supergiant has a surface temperature of ∼16,000 K (Humphreys and McElroy 1984). For the above range of luminosities and assuming a temperature in the range 15,000 to 18,000 K one obtains a radius for SK-202-69 of 3 ± 1 × 1012cm.