IS THE DUST OBSCURING SUPERNOVAE IN DISTANT GALAXIES THE SAME AS DUSTIN THE MILKY-WAY

Previous attempts to correct type Ia supernovae (SN Ia's) for host gal axy extinction have given strange results: increased dispersion on the Hubble diagram or impossibly low values of the reddening ratio for du st in distant galaxies. The cause is the incorrect assumption that SN Ia's have a uniform intrinsic luminosity and color at maximum light. O ur multicolor light-curve shape (MLCS) method establishes the relation between intrinsic luminosity and color for SN Ia's using information in lightcurve shapes. Here we estimate the B-V, V-R, and V-I color exc ess for 20 SN Ids using MLCS and estimate the reddening ratios of dust in distant galaxies. The ratios of selective to total absorption from dust in distant galaxies hosting SN Ia's are consistent with the gala ctic extinction law. The SN Ia's in late-type galaxies are often obscu red by dust, while those in early-type galaxies are dust free. This su ggests that SN Ia extinction is caused by interstellar (not circumstel lar) dust, with similar optical properties as the dust in the Milky Wa y.