Towards the thermonuclear explosion...

Consequently, a first approach would consist in saying that all the stars of more than $1.5 \rm \, M_\odot$ are dedicated to becoming the dwarf white ones with masses higher than the mass of Chandrasekhar. However the stellar winds for the periods of dilation of star make him lose the essence of its mass. In practice, only the stars of more than 8 to 9 solar masses with the birth will be able to reach the mass of Chandrasekhar. Moreover, these stars are so massive that the internal temperatures reach sufficient values to enable them to burn their carbon and their oxygen. As we saw earlier, these stars will explode in supernova of the gravitational type (type II, $M<8-9 \rm \, M_\odot$) thus finish their life in the form of dwarf white of mass lower than the mass of Chandrasekhar.

It is thus necessary to bring mass of outside to dwarf white so that it reaches this limiting mass. The most natural solution is to consider that this mass comes from a stellar companion. The case of binary system is not, indeed, rare, it accounts for about 60% of the stellar population.

If the system is sufficiently tight, then the dwarf white one can accréter of the matter of his/her companion to reach the mass of Chandrasekhar. The pressure of the electrons is not then sufficient any more to support the gravitational pressure. At this time, the same phenomenon as at the time of the helium flash occurs: the temperature can increase with constant pressure. Once the melting point of carbon reached, fusions sets out again and packs in this extremely degenerated medium. This time, energy is so significant that it blows star literally and disperses it in the interstellar environment. Contrary to the gravitational supernovæ, there does not remain compact residual after the explosion.

This model has the advantage of explaining why the supernovæ of the Ia type present neither hydrogen, nor helium in their spectrum: dwarf the white C/O are deprived by it.

The systems progéniteurs belong to stellar populations having long lifespans, which explains why one observes these explosions in elliptic galaxies. Moreover, the presence of the decay products of ${ }^{56}$Ni: cobalt and iron, come from the fusion of carbon and oxygen, just as silicon.

Lastly, successive disintegrations of nickel then cobalt explain the double exponential decay of the lightcurve after the maximum.

Finallement the existence of the mass of Chandrasekhar like threshold of ' explosion, microscopic threshold purely of origin depending only little on the composition of star, makes it possible to give a robust theoretical argument to explain the great homogeneity of this family of supernovæ.

We gave only broad outline here and as we will see it in the continuation of many difficulties remain.