As we saw in chapter 5 , the supernovæ of the Ia type reach their maximum of luminosity a score of days after their explosion. However, this maximum is not reached at the same time in all the filters, it is reached later in the red bands and infra-red that in the blue bands.
We thus take as convention to define the day of the maximum of luminosity as being the day of the maximum of the lightcurve in the filter B in the reference frame of the supernova.