6.4 Uncertainties and Results
At present the self-calibration by means of radioactivity yields MB = -19.6 ± 0.5, while the other three calibrations give -20.0 ± 0.5, -19.77 ± 0.45, and -20.4 ± 1.0. Each calibration carries an external error that is difficult to quantify. The self-calibration perhaps should be regarded as the primary calibration, and the others as consistency checks (none is inconsistent with the self-calibration). Then the mean apparent magnitude of SNe Ia in the Virgo cluster of mB = 11.9 ± 0.1 (Leibundgut and Tammann 1990), combined with a typical Burstein and Heiles (1984) foreground extinction measure of AB = 0.06, yields a Virgo distance modulus of 31.44 ± 0.5 (D = 19.4 ± 5 Mpc), and equation (10) above yields H0 = 56 ± 14.
6.5 Future Needs and Directions
With modern search techniques, SNe are being identified more frequently than ever before (e.g., Muller et al. 1992). This increased rate offers the promise of validating Type Ia SNe across Hubble types. Furthermore, newly identified SNe will be observed with CCDs instead of photographic plates, thereby providing photometry of unprecedented accuracy against the background of their host galaxies. Extensive observational coverage of a few individual SNe Ia together with detailed modeling of spectra and light curves will allow the methods discussed above under the separate headings of thermal-emission and radioactivity to merge into one physical method which should lead to a tightly constrained peak luminosity. An independent check can be obtained by calibrating SN 1937C via Cepheids in IC 4182; perhaps a way to determine a good distance to NGC 5253 also can be found. Measurements of peak apparent magnitudes of SNe Ia well out in the Hubble flow will then yield a Hubble constant independent of the Virgo cluster and undisturbed by peculiar velocities.