Annu. Rev. Astron. Astrophys. 1998. 36: 17-55 Copyright © 1998 by Annual Reviews. All rights reserved

2.3. Summary

SNe Ia can be divided into a majority of "normal" events that have highly homogeneous properties and a minority that are "peculiar." When normals and peculiars are considered together, correlations among their observational properties are obvious. To a certain extent, the correlations hold among normal SNe Ia, and the concept of a sequence of SNe Ia ranging from those having high-excitation spectra, high blueshifts of spectral features, slow light curves, and high luminosities to those having low-excitation spectra, low blueshifts, fast light curves, and low luminosities is useful. The former tend to occur in blue, late-type galaxies and the latter in red, early-type galaxies. A one-dimensional sequence of SNe Ia is not, however, the whole story.

A simple B - V cut that eliminates events that are observationally red at maximum light yields a sample of nearly standard candles having _obs(M_B) (M_V)_obs less than 0.3. Correlations between absolute magnitude and one or more SN Ia or parent-galaxy observables can be used to further standardize the absolute magnitudes to better than 0.2 mag. As far as determining the value of H₀ by means of Cepheid-calibrated SNe Ia is concerned, the accuracy of the result will depend not only on the tightness of the intrinsic correlation among normal SNe Ia but also on the degree to which the relevant distance-independent observables happen to be known for the Cepheid-calibrated SNe Ia.