Type Ia supernovae are very good, but not perfect, standard candles, because their observed brightness shows a little diversity. The origin of this dibersity needs to be understood for the application to cosmology.
In close binary systems, a white dwarf must be rotating faster and faster as it gains angular momentum from the accretion disk. Its rapid rotation affects its final mass and strucure just before a supernova expolosion. Brightness of supernovae can be changed if mass of their progenitors have some diversity.
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