# How is the first detonation in Supernove type Ia triggered?

Ok, I read about the Supernova of type Ia and I found out that there are two detonations happening. First one is in helium shell around the white dwarf and second one seems to be triggered by the first one and it is the main part of whole supernova. I did not find how this helium shell explodes at all so that`s what I am looking for. Thanks in advance!

Why does the He "explode"? Well, the accreted He will form an electron-degenerate layer at the surface. A fundamental property of this degenerate gas is that the pressure is independent of the temperature. Thus, if the He ignites then at least initially, the temperature goes up but the pressure does not. Since the He fusion rate depends on something like $T^{40}$ this allows a runaway reaction to develop that could be characterised as an "explosion" in the surface layers.