The immediate progenitor of a Type I supernova (SN I) is thought to be a mass-accreting carbon-oxygen (C-O) white dwarf in a binary system. When the mass of the white dwarf approaches the Chandrasekhar mass (1.4 M⊙) the C-O nuclear fuel ignites, part of the star is incinerated to radioactive 56Ni, and the thermonuclear energy completely disrupts the star. The optical luminosity results from the trapping and thermalization of the γ rays and positrons emitted by the decay of 56Ni through 56Co to stable 56Fe. The amount of 56Ni synthesized, MNi, and the corresponding peak luminosity, L max, can be used with the observed Hubble diagram for SN I to determine the value of Hubble's constant, H0. We argue here that if this model is correct, MNi is in the range 0.4-1.4 M ⊙, the best estimate being 0.6 M⊙, and that H0 is in the range 39-73 km s-1 Mpc-1 with a best estimate of 59 km s-1 Mpc-1. This line of reasoning does not require knowledge of the temperature of the supernova and, therefore, is not subject to the uncertainties associated with attempts to determine supernova luminosities and distances by the Baade method1. It relies on the physical correctness of the model, which is subject to independent tests.
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