TYPE IA SUPERNOVAE AND THE HUBBLE CONSTANT

The focus of this review is the work that has been done during the 199 0s on using Type Ia supernovae (SNe Ia) to measure the Hubble constant (H-0) SNe Ia are well suited for measuring H-0. A straightforward max imum-light color criterion can weed out the minority of observed event s that are either intrinsically subluminous or substantially extinguis hed by dust, leaving a majority subsample that has observational absol ute-magnitude dispersions of less than sigma(obs)(M-B) similar or equa l to sigma(obs)(M-V) similar or equal to 0.3 mag. Correlations between absolute magnitude and one or more distance-independent SN Ia or pare nt-galaxy observables can be used to further standardize the absolute magnitudes to better than 0.2 mag. The absolute magnitudes can be cali brated in two independent ways: empirically, using Cepheid-based dista nces to parent galaxies of SNe Ia, and physically, by light curve and spectrum fitting. At present the empirical and physical calibrations a re in agreement at M-B similar or equal to M-V similar or equal to -19 .4 or -19.5. Various ways thar have been used to match Cepheid-calibra ted SNe Ia or physical models to SNe Ia that have been observed out in the Hubble flow have given values of H-0 distributed throughout the r ange of 54-67 km s(-1) Mpc(-1). Astronomers who want a consensus value of H-0 from SNe Ia with conservative errors could, for now, use 60 +/ - 10 km s(-1) Mpc(-1).