Two physical methods for determining luminosities and distances of type Ia supernovae, the spectral fitting expanding atmosphere method and the ${}^{56}\mathrm{Ni}$ radioactivity method, depend on the interval between the times of explosion and maximum brightness, but in differing ways. By requiring consistency between the two methods we derive blue and visual absolute magnitudes $M_B\simeq M_V\simeq -19.74\mathrm{}\pm 0.45\mathrm{mag}$ which, together with the ridge line of type Ia supernovae in the magnitude-redshift diagram, constrain the Hubble constant to be ${50}_{-10\mathrm{}}^{+12\mathrm{}}\mathrm{km}{\mathrm{s}}^{-1\mathrm{}}{\mathrm{Mpc}}^{-1\mathrm{}}$.

• Received 16 January 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.394