The type II supernova 1979c in M100 and the distance to the Virgo cluster

Optical spectra of supernova 1979c in M100 obtained between 1979 April 22 and 1979 November 24 are presented and discussed in detail. Interstellar lines originating from both our Galaxy and M100 are examined. Synthetic spectra computed using a blackbody photosphere and a pure scattering expanding atmosphere successfully model the continua and P Cygni profiles of the supernova spectra up to May 28. The H..cap alpha.. profile is not explained by pure resonance scattering, but collisional excitation may be responsible for the net emission in this line as well as the Mg II lambda2798 emission seen in IUE spectra. Velocities and temperatures derived from the spectra are combined with photometric data to determine the distance to the supernova by the Baade method. We find the distance to be 23 +- 3 (rms) Mpc, corresponding to a Hubble parameter of about 60 km s/sup -1/ Mpc/sup -1/. The precursor star seems to have had an extended circumstellar envelope, and the mass of the precursor is estimated to have been at least 9 M/sub sun/. The chemical composition of the supernova atmosphere is discussed both from the standpoint of H II region observations and the synthetic spectra.