$\star$ Absorption and intrinsic dispersion

As we saw, the correction of absorption apart from our galaxy uses the color of the supernovæ to the maximum, it thus seems difficult to have access to the intrinsic color of this one.

However, phillips1999 showed that the lightcurves of the supernovæ had very similar evolutions out of B and V beyond 30 days after the maximum of luminosity, some is their behavior to the maximum (law of Will read).

The measurement of the color after 30 days thus makes it possible to make a direct measurement of absorption and to measure reddening indépendemment color to the maximum.

From this law, nobili2003 showed, with a batch of forty supernovæ close which there was an intrinsic dispersion of the color of the supernovæ independent of an unspecified extinction. This dispersion implies that the measurement of the extragalactic extinction at the time of the maximum of luminosity is limited. The authors find that the dispersion of color to the maximum implies an uncertainty of about 0.1 magnitude to the measure of absorption in the filter V.