Type I supernovae.

Wavelength coincidences and comparison of observed and synthetic spectra support the hypothesis that Type I supernova spectra contain blueshifted absorption features (Fe II, Ca II, Na I, Si ii, Mg Ii) characteristic of spectra of novae near maximum light. On the assumptions that the light curve is due to thermal emission from an expanding, optically thick photosphere and that the temperature may be derived from the normal stellar colour-temperature relation, a modified Baade method is applied to a composite Type I light curve. The mean absolute visual magnitude at maximum light is found to be - 20m.8+o0:79. Combined with the observed apparent magnitude-redshift relation for Type I supernovae, this absolute magnitude leads to an estimate of the Hubble constant, Ho = 40+ 2153 km s ' Mpc-'.